FIGURE 1.1. LOCATION MAP OF THE PROJECT AREA Location, Topography and Accessibility..... 3 1.3.2...

i

Contents

Contents ............................................................................................................................................. i

List of Figures.................................................................................................................................. iii

List of Tables ................................................................................................................................... iv

1. .................................................................................................................... 1 INTRODUCTION

1.1 Background to the project........................................................................................................ 1

1.2 Objectives ................................................................................................................................ 2

1.3 The project area........................................................................................................................ 3

1.3.1 Location, Topography and Accessibility .......................................................................... 3

1.3.2 Physiography and Climate ................................................................................................ 4

1.3.3 Regional setting ................................................................................................................ 8

1.4 Methodology, Data, Materials and Software used................................................................. 10

1.4.1 Methodology................................................................................................................... 10

1.4.2 Data and Material Used .................................................................................................. 11

1.4.3 Software used.................................................................................................................. 11

2. REMOTE SENSING AND GIS ANALYSIS .......................................................................... 13

2.1 Remote Sensing Analysis ...................................................................................................... 13

2.1.1 Drainage and Catchments Extraction.............................................................................. 13

2.1.2 Image processing and Interpretation............................................................................... 15

2.2 GIS Analysis .......................................................................................................................... 17

3. INTEGRATION OF REMOTE SENSING AND GIS ........................................................... 17

3.1. Introduction........................................................................................................................... 17

3.2 Factors Controlling Groundwater Occurrence in the Project Area........................................ 18

3.2.1 Drainage density ............................................................................................................. 18

3.2.2 Slope Steepness............................................................................................................... 22

3.2.4 Land Use/Land Cover ................................................................................................... 24

3.2.6 ....................................................................................................................... 27 Structure

3.2.7 Geomorphology .............................................................................................................. 32

3.2.8 Geology........................................................................................................................... 38

4. INTEGRATED ANALYSIS IN GIS ENVIRONMENT ........................................................ 51

4.2 GIS Modeling......................................................................................................................... 52

4.3 Weighting............................................................................................................................... 52

ii

5. RESULT..................................................................................................................................... 54

6. CONCLUSIONS AND RECOMMENDATION..................................................................... 58

6.1 Conclusion ............................................................................................................................. 58

6.2 Recommendation ................................................................................................................... 59

REFERENCES............................................................................................................................... 60

iii

List of Figures

FIGURE 1.1. LOCATION MAP OF THE PROJECT AREA........................................................................................................ 3

FIGURE 1.2 TOPOGRAPHIC MAP OF THE PROJECT AREA.................................................................................................. 4

FIGURE 1.3 MEAN ANNUAL RAINFALL CHART OF MEGA STATION .................................................................................. 5

FIGURE 1.4 MEAN ANNUAL RAINFALL CHART OF MOYALE STATION STATION................................................................ 6

FIGURE 2.1 ARC HYDRO DEMHYDROPROSESSING PROCESSES FOR DRAINAGE EXTRACTION ....................................... 14

FIGURE 2.2 PROJECT AREA IMAGES OF YEAR 2000 G. C., FALSE COLOR COMPOSITE OF BANDS 432 ........................... 16

FIGURE 2.3 PROJECT AREA IMAGE OF YEAR 2000 G. C., BAND RATIO OF BAND 4 AND 3.............................................. 16

FIGURE 3.1 DRAINAGE NETWORK OF THE PROJECT AREA ............................................................................................. 19

FIGURE 3.2 COMPARISON OF DRAINAGE AND LINEAMENT ORIENTATION ................................................................... 20

FIGURE 3.3 DRAINAGE DENSITY OF THE PROJECT AREA ................................................................................................ 20

FIGURE 3.4 RECLASSIFIED DRAINAGE DENSITY MAP...................................................................................................... 21

FIGURE 3.5 SLOPE MAP OF THE PROJECT AREA ............................................................................................................. 22

FIGURE 3.6 RECLASSIFIED SLOPE MAP OF THE PROJECT AREA ...................................................................................... 24

FIGURE 3.7 LAND USE/COVER MAP OF THE PROJECT AREA........................................................................................... 26

FIGURE 3.8 RECLASSIFIED LAND USE/COVER MAP OF THE PROJECT AREA.................................................................... 27

FIGURE 3.9 EDGE ENHANCEMENT OF PANCHROMATIC IMAGE (A) MAGNIFIED LINEAMENTS (B) DIGITIZED

LINEAMENTS ......................................................................................................................................................... 28

FIGURE 3.10 STRUCTURAL MAP OF THE PROJECT AREA ................................................................................................ 28

FIGURE 3.11 PROXIMITY TO GEOLOGICAL STRUCTURE MAP......................................................................................... 31

FIGURE 3.12 GEOMORPHOLOGY MAP OF THE PROJECT AREA ...................................................................................... 33

FIGURE 3.13 RECLASSIFIED GEOMORPHOLOGIC MAP OF THE PROJECT AREA .............................................................. 37

FIGURE 3.14 GEOLOGICAL MAP OF THE PROJECT AREA ................................................................................................ 38

FIGURE 3.15 RECLASSIFIED GEOLOGICAL MAP OF THE PROJECT AREA ......................................................................... 50

FIGURE 5.1 GROUND WATER POTENTIAL ZONES ANALYZED ON THE BASIS OF STRUCTURE, GEOLOGY, SLOPE,

GEOMORPHOLOGY, DRAINAGE AND LAND USE/COVER ...................................................................................... 54

FIGURE 5.2 DISTRIBUTION OF BOREHOLES IN GROUND WATER POTENTIAL ZONES..................................................... 56

iv

List of Tables

TABLE 1.1 SUMMARY OF MEAN RAINFALL DATA OF MEGA STATION, YEAR 1985‐2005 ................................................. 5

TABLE 1.2 SUMMARY OF MEAN RAINFALL DATA OF MOYALE STATION, YEAR 1986‐2005 ............................................. 5

TABLE 1.3 MEAN MONTHLY TEMPERATURE OF MEGA STATION..................................................................................... 6

TABLE 1.4 MEAN MONTHLY TEMPERATURE OF MOYALE STATION ................................................................................. 6

TABLE 3.1 THE CONTINUOUS RATING SCALE DEVELOPED BY SAATY (1977).................................................................. 18

TABLE 3.2 WEIGHT FOR DRAINAGE DENSITY.................................................................................................................. 21

TABLE 3.3 SLOPE AMOUNT CLASS IN THE PROJECT AREA.............................................................................................. 23

TABLE 3.4 WEIGHT FOR SLOPE THE PROJECT AREA ....................................................................................................... 23

TABLE 3.5 ARIAL EXTENT OF VARIOUS LAND USE/COVER CATEGORIES......................................................................... 25

TABLE 3.6 WEIGHT FOR LAND USE/COVER MAP............................................................................................................ 26

TABLE 3.7 WEIGHT FOR PROXIMITY TO GEOLOGICAL STRUCTURE MAP ....................................................................... 31

TABLE 3.8 WEIGHT FOR GEOMORPHOLOGY OF THE PROJECT AREA ............................................................................. 37

TABLE 3.9 GEOLOGIC UNITS GROUPED BASED ON THEIR GROUND WATER IMPORTANCE........................................... 49

TABLE 3.10 WEIGHT FOR GEOLOGY OF THE PROJECT AREA .......................................................................................... 50

TABLE 4.1 WEIGHT FOR ALL FACTOR MAPS ................................................................................................................... 53

TABLE 5.1 BOREHOLE DATA FROM DIFFERENT LOCALITIES OF THE PROJECT AREA ...................................................... 55

GROUNDWATER POTENTIAL ZONE MAPPING USING GIS AND REMOTE SENSING - MOYALE-TELTELE SUB BASIN -

DIRE, ARERO, YABELO AND TELTELE WOREDAS, BORENA ZONE OF OROMIA REGIONAL STATE

MAB CONSULT – CONSULTING HYDROGEOLOGISTS AND ENGINEERS 1 LAY VOLUNTEERS INTERNATIONAL ASSOCIATION (LVIA)

1. INTRODUCTION

1.1 Background to the project

Consultancy service contract agreement for the research in groundwater resource mapping using

Remote sensing and GIS Multi-Criteria decision technique in Moyale-Teltele sub basin at Dire,

Arero,Yabelo and Teltele Woredas, Borena Zone of Oromia Regional State was signed on April,

15 2009 between MAB Consult – Consulting Hydrogeologist and Engineers and LAY Volunteers

International Association. The project started the study activities as per the work program of the

Contract Agreement and vehicles were timely assigned for the project by the Client, collection of

previous data and reconnaissance fieldwork into the project area was undertaken.

Review of previous studies and field visit to the project area shows that a few studies have been

conducted in groundwater resource assessments using GIS and Remote Sensing Technique.

Previous works includes by (Getachew A. 2007), Integration of Remote Sensing and GIS for

Groundwater Resources Assessment in Moyale-Teltele Sub-basin, South Ethiopia

In Moyale-Teltele Groundwater Potential Assessment Project, due emphases was given in

investigation of different groundwater controlling factors. These factors were carefully analyzed

and integrated to produce groundwater potential map of the project area. The parameters used in

this project were:

i. Drainage Density

ii. Slope Steepness

iii. Land use/Land cover

iv. Geological Structures/Lineaments

v. Landforms/Geomorphology

vi. Lithology/Geology

Remotely Sensed data by its wide area coverage and multispectral nature has helped in

identification and mapping of most of the above factors with selective ground checks in a cost-

effective manner. An integrated analysis of these factors together with the available well and

ancillary data in the GIS environment was carried out in identifying the potential groundwater




zones. Target areas for conducting detailed hydrogeological and geophysical surveys on the

ground are narrowed to locate the site for drilling.

1.2 Objectives

General objective:

To delineate groundwater potential areas, in Yabelo, Arero and Teltele woredas (Moyale-Teletele

Sub Basin). Systematic groundwater studies utilizing Remote Sensing, field studies, Digital

Elevation Models (DEM) and Geographic Information Systems (GIS).

The specific objectives:

Prepare thematic maps of the area such as lithology, lineaments, landforms and slopes from

remotely sensed data and other data sources like DEM.

Assess groundwater controlling features by combining remote sensing, field studies and

DEM.

Identify and delineate groundwater potential zones through integration of various thematic

maps with GIS techniques.

Validate the result using secondary hydrogeological data

Recommendations for future work and provide guidelines for groundwater prospecting.



1.3 The project area

1.3.1 Location, Topography and Accessibility

The project area is located in the Moyale-Teltele sub basin at Dire, Arero,Yabelo and Teltele

Woredas, Borena Zone of Oromia Regional State in the southern extreme of Ethiopia (Figure 1.1

). The area is internationally bordered with Kenya in the south and with Somalia in the east. It is

bounded by 441795N – 595650N and 243956E - 591061E covering a total area of 30,086 km2.

The topographic elevation ranges from 437 meter above sea level on the western part to 2344

meter above sea level on the central part (Figure). The area is accessible by four-wheel drive

vehicles. The 775 Km Addis - Moyale international asphalt road runs north-south across the

central part of the part of the project area.

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Adindan_UTM_Zone_37N

Figure 1.1. Location map of the project area





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Woreda Boundary

Elevation

Value in m

High : 2344.84

Low : 473.122

Figure 1.2 Topographic map of the project area

1.3.2 Physiography and Climate

The project area is included in the Moyale, Dire, Arero, and Teltele Woredas, Borena Zone of

Oromia Administrative Regional State and Moyale Woreda, Liben Zone of Somali Administrative

Regional State (Figure 1.1 ). The area is characterized by hot semi-arid climate experiencing high-

temperature, low rain fall and high evapotranspiration. This climatic condition is favorable to

sustain grasses and acacia trees. (Zenaw et al, 2000).

In the project area, there are two meteorological stations, which are located in Mega (38020'E,

4005'N) and Moyale (39004'E, 3031'N) 21 years of rainfall data was collected from the National



Meteorological Agency for the stations from 1985 to 2005. Summary of the mean rainfall

(mm/month) data is given in the table 1.1 and 1.2 below.

Table 1.1 Summary of mean rainfall data of Mega station, year 1985-2005

Month Jan Feb Mar April May June July Aug Sept Oct Nov Dec Total

Rainfall 16.7 41.3 80.6 142 74.7 17.1 14.7 5.0 7.3 74.5 57.9 41 572.8

Table 1.2 Summary of mean rainfall data of Moyale station, year 1986-2005

Month Jan Feb Mar April May June July Aug Sept Oct Nov Dec Total

Rainfall 17.5 22 42 148.9 65.6 13.4 7.4 4.9 13.5 83.5 82.2 30.2 531.1

From the rainfall data of the stations two short rainy seasons are observed in the project area. The

first rainy period lasts from March to May. The second rainy period lasts from October to

November, which is torrential in October. The main cause of the rainfall in this region is the

southward migrating Inter Tropical Convergence Zone (ITCZ) and westward propagating

disturbance from the Indian Ocean (Zenaw et al, 2000).

Rainfall in Mega Station

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Time (Month)

Rainfall

Figure 1.3 Mean annual rainfall chart of Mega station




Rainfall in Moyale Station

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Figure 1.4 Mean annual rainfall chart of Moyale station Station

The above charts clearly show that the area has uniform rainfall pattern which is relatively low.

Since there is no variation in the mean annual rainfall of the two stations, it is not possible to

consider rainfall as input factor layer for the analysis.

ii) Temperature

The temperature of the project area is typical of tropical monsoon lands. In most cases the mean

monthly temperature exceeds 20oC. The daily average minimum temperature registered is 11.4oC

in the month of July in Mega Station and the daily average maximum temperature is 35.3oC

registered in Moyale Station. The hottest season extends from December to late March.

The summary of mean monthly air temperature of the stations is given below.

Table 1.3 Mean monthly temperature of Mega station

Month Jan Feb Mar April May June July Aug Sept Oct Nov Dec

T (oC) 20.6 21.0 20.3 19.5 18.3 16.8 16.1 16.9 18.3 18.9 19.2 19.8

Table 1.4 Mean monthly temperature of Moyale station

Month Jan Feb Mar April May June July Aug Sept Oct Nov Dec

T (oC) 24.9 25.5 25.0 23.0 21.8 20.6 19.9 20.5 21.6 22.0 22.5 24.9





ii) Potential Evapotranspiration

In tropical regions, where there is little change in temperature or duration of sunlight, potential

evapotranspiration is likely to be constant through out the year. The peneplained part of the area is

characterized by tall savannah grass with shrubs acacia and thorn bushes. The southern rugged

terrain bordering Kenya is composed of continuous dense shrubs with scattered small trees. The

vegetation cover over some part of the project area has contributed to increasing

evapotranspiration.

The average evapotranspiration of the project area is 1633 mm/yr at 1000 m.a.s.l. (Zenaw et al,

2000). In this report, the evapotranspiration has been estimated according to penman method. High

potential evapotranspiration values known in the lowlands exceed 1500mm/year (up to 2300

mm/year) with no water surplus throughout the area. The moisture deficits in the area reach

1047mm.




1.3.3 Regional setting

In southern Ethiopia, three major chrnostratigraphic units are known to occur namely Precambrian

crystalline basement, Mesozoic sediments, Cenozoic volcanics with sporadic sediments and

superficial deposits (Kazmin 1972; Merla 1973; Davidson 1983; and Teffera et al., 1996). The

metamorphic complex and associated plutonic rocks makeup the crystalline basement which are

the northern and southern continuation of the Mozambique belt (MB) and the Arabian Nubian

Shield (ANS), respectively (Kazmin, 1978; Chewaka and de Wit, 1981). The extensive

sedimentary rocks are result of marine transgression and regression of the Indian Ocean during

Mesozoic era (Purcel, 1981; Mohr 1986). The voluminous volcanic rocks with subordinate

lacustrine and fluviatile sediments overlying unconformably the crystalline basement are resulted

from extensional tectonics responsible for the formation of the East African Rift (Davidson, 1983;

Mohr, 1986; WoldeGebriel, 1987; and Ebinger, et al., 2000). Although three major time

stratigraphic units are known in southern Ethiopia, sedimentary rocks are virtually absent in the

area. The confine of the project area is partly within the East African Orogen, fossil fragments of a

Neoproterozoic Wilson cycle, (Stern, 1994) and partly at the southwestern ''broad rift zone''

(Davidson, 1983) of East African Rift system .

The Precambrian crystalline rocks of southern Ethiopia exhibit similar lithological associations and

structural features characteristic to both Mozambique belt and Arabian Nubian Shield to the south

and north, respectively. However, lithological association belonging to Mozambique belt, which

forms the local basement by far, exceeds the discontinuous lens like bodies of Arabian Nubian

Shield. Therefore, this region is situated in the transitional zone, where both are inter-fingering

mainly with imprints of Pan-African tectnothermal events (Vail, 1976; Kazmin et al., 1978;

Davidson, 1983). The Mozambique belt consists of high-grade, poly-deformed, metamorphosed

gneisses, and migmatites, psammo-pellitic schists and amphibolites that are commonly intruded by

felsic to mafic intrusions.

The high grade gneisses are juxtaposed with the low grade metavolcano-sedimentary rocks

suggesting a tectonic contact where considerable vertical movements took place, which are

commonly referred as orogen parallel shear zones (Abdelsalam and Stern, 1996; Worku, 1996). In




general, the low-grade rocks of southern Ethiopia defining linear belt plunge under Cenozoic

volcanics in the north and continue as discontinuous lenses southward up to the Ethio-Kenyan

boarder.

Granitoids ranging in composition from intermediate to acidic are widespread in southern Ethiopia,

especially in the gneissic terrain ranging in shape from elliptical to circular. It is worth mentioning

the outstanding en echelon arranged plutonic bodies of Gariboro, Ranu, and Arero represented by

distinct massif having considerable aerial coverage. Based on field relationships and geochemical

criteria, they are subdivided into pre-, syn-, and post- collisional (Worku, 1996; Woldehaimanot

and Brehmann, 1995; Piccerillo et al., 1998).

In the sector between the Ethiopian and Kenyan domal uplifts, the East African Rift system is

represented by more than 300 Km ''broad rift'' zone (Davidson and Rex, 1980; Davidson, 1983;

Woldegebriel and Aronson, 1987). It is characterized by overlapping either north-south, northeast-

southwest, and northwest-southeast trending two or more rift systems which is more than three

times the breadth of the Main Ethiopian Rift or Gregory Rift away from the zone of overlap. These

rift systems in southwestern Ethiopia mainly encompass the branches of Turkana rift in the west,

Chew Bahir rift, Ririba rift (Davidson, 1983; Woldegebriel, 1987) at the center, and Mega rift in

the east (this study).

The surface expression of the northeast trending Main Ethiopian Rift splays into north-south

trending Chamo and Segan basins separated from the narrow Gelana basin by Amaro horst. Until

recently, the southern terminus of the main Ethiopian Rift was considered to be a few Km south of

Amaro horst, around 5ºN latitude (Zanetten et al., 1978; Davidson, 1983; Woldegebriel et al.,

1991; Ebinger, 1993). However, both field mapping and satellite image interpretation

unequivocally demonstrated the Main Ethiopian Rift continue southward to join the Ririba rift.

Moreover, around Mega a series of northwest-southeast trending high scarp steep normal faults

with considerable strike length defining outstanding horsts and graben were recognized for the first

time in this study. Unlike the MER, which is characterized by north south trending faults, the

northwest southeast trending define the Mega rift in the broadly rifted zone of southern Ethiopia.

Proceeding further southeast in Kenya and northwest in southern Sudan Anza graben (Ebinger and

Ibrahim, 1994; Reeves et al., 1987; Hetzel and Strcker, 1993) and White Nile rift (Ebinger and

Ibrahim1994) are situated on same strike line, respectively.




Most sections exposed along the Rift margins of the Main Ethiopian Rift and southwestern

Ethiopian Rift are predominantly Tertiary and Quaternary volcanic rocks (Davidson, 1983;

Woldegebriel, 1987; Woldegebriel et al., 1991; Genzebu et al., 1994), except few localities where

crystalline basement is unconformably overlain by Mesozoic sedimentary and/or Tertiary volcanic

rocks (Woldegebriel, 1987). The Tertiary-Quaternary volcanic rocks are subdivided into two broad

categories based on whether erupted before or after the rifting. The presence of lacustrine and

fluviatile sediments intimately associated with basalt sheets of Miocene age overlying the tilted

blocks of pre-rift volcanics suggested the development of rift related basin during Early to Mid-

Miocene (Woldegebriel et al., 1991; Davidson, 1983). Therefore, volcanic rocks around and/or

within the rift are classified based on absolute age determinations, their spatial distribution, and

association with rift related sediments in to pre rift- and post rift succession. Thus basalt, salic

flows, pyroclastic rocks as well as hypabysal intrusions represent the Pre-rift successions. Where

as: rapidly built volcanic piles erupted from chains of vents controlled by fractures, widely spread

flood basalt sheets, cluster of vents and collapse caldera with much ejected material and little build

up of volcanic edifices, salic pyroclastic material incorporated into sediments as airborne tuffs

represent post-rift successions (Davidson, 1983).

1.4 Methodology, Data, Materials and Software used

1.4.1 Methodology

The research in groundwater resources was undertaken by well-programmed and integrated

approach set up on reliable methodology for data collection and review, carrying out field survey,

identification, selection and evaluation of well data it was completed in three phases:

1. Data collection and review of previous work

2. Data Analysis and Interpretation

3. Validation of results

Methodology for the investigation and study is summarized in flow chart (Fig. 1.5). It involves

catchment and drainage extraction using ARCHYDROG module, digital image processing for the

extraction of geomorphology, lithological, linear features, land use/cover etc... The field studies




were comprised of hydrogeological, structural and geomorphological investigations. DEM, which

is produced from SRTM, was used to extract lineaments and for landform mapping. All data were

integrated in a Geographic Information System (GIS) and analyzed to assess the groundwater

controlling features. Finally groundwater potential map was prepared based on GIS analysis.

1.4.2 Data and Material Used

The remote sensing data used for the study are Landsat Thematic Mapper (ETM+) (28.5 m.

resolution), Year 2000 with 7 bands and orthorectified. Primary data derived were land cover/use,

geomorphology, drainage density, lineaments and slope. Secondary data, which was modified and

used was lithology. GPS for point and rout data collection was used.

1.4.3 Software used

ARCHydro module was used for catchments delineation and drainage extraction.

Arcview 3.2a, Mapinfo 7 and ArcGis9 were used for GIS analysis.

ERDAS Imagine 8.6 and ENVI 4.2 were used for georeferencing, image analysis and coordinate

transformation of all the data used in to UTM 37 Zone, Adindan Datum.

DNRGARMIN, which is extension of arcview 3.2a, was used for transferring GPS data in to

computer.

Globalmapper 8 was used for analysis of landforms/Geomorphology,

IDRISI 32 for calculation of weight.

GROUNDWATER POTENTIAL ZONE MAPPING IN MOYALE-TELTELE SUB BASIN AT DIRE, ARERO, YABELO AND TELTELE WOREDAS, BORENA ZONE OF OROMIA REGIONAL STATE USING GIS AND

REMOTE SENSING

MAB CONSULT – CONSULTING HYDROGEOLOGISTS AND ENGINEERS 12 LAY VOLUNTEERS INTERNATIONAL ASSOCAITION

DEM

Thematic Maps Derived

GROUNDWATER

POTENTIAL MAP

GIS ANALYSIS

(Spatial Analysis)

DEMHYDROPROCESSING

SRTM V-3

SATELLITE DATA

Land sat ETM+)

Digital image

Processing

Interpretation of lithology,

Lineament, Land Use Land Cover

TOPO MAP

Hydrogeology

Well logs

Pump tests

Site information

FIELD STUDIES and AUXILLARY

DATA

SRTM

Drainage and Catchments Extraction

GIS Processing

(Building Database)

Drainage

Density

Slope

Steepness

Land Use/Cover Lineaments Geomorphology Geology

Interpretation of

Geomorphology

Figure 1.5 Flow chart showing data and methods employed for the study.

GROUNDWATER POTENTIAL ZONE MAPPING IN MOYALE-TELTELE SUB BASIN AT DIRE, ARERO, YABELO AND TELTELE WOREDAS, BORENA

ZONE OF OROMIA REGIONAL STATE USING GIS AND REMOTE SENSING


2. REMOTE SENSING AND GIS ANALYSIS

2.1 Remote Sensing Analysis

2.1.1 Drainage and Catchments Extraction

Drainage and catchments were extracted using ARC HYDRO module after eight consecutive

processes (Fig 2.1) on 4 SRTM data, which are described below:

IMPORTING of DEM derived from in to SRTM ArcGIS:

INTEPOLATION of the raster map in kriging methods to fill sinks (areas with no data) for

the undefined values, which are weighted average values, similar to a moving average

operation.

FLOW DIRECTION determination to determine into which neighboring pixel any water in

a central pixel will flow naturally.

FLOW ACCUMULATION that performs a cumulative count of the number of pixels that

naturally drains into outlets. The operation can be used to find the drainage pattern of a

terrain.

DRAINAGE NETWORK EXTRACTION of drainage based on user defined drainage length.

DRAINAGE ORDERING for assigning drainage order for each drainage line.

CATCHMENT MERGING for the extraction of catchments.



Raw DEM Catchments Definition

Fill Sink Catchments Extraction

Flow Direction Drainage Extraction

Figure 2.1 Arc Hydro DEMHYDROPROSESSING processes for drainage extraction





2.1.2 Image processing and Interpretation

i ) Pre-Processing

Satellite image of Landsat 7 sensor ETM with a map projection of UTM zone 37, spheroid and

datum WGS 84) has been used for most of the processing and mapping activities after re-

projection into Adindan datum, directional filter was done in order to digitize the existing

geological structures and the result was compared with the previous structural map of the area.

Printouts of different band combinations were used to identify features during field survey.

Digital elevation model (DEM) was derived from SRTM where slope data layer were produced.

Pre-processing of satellite images was done to correct distorted or degraded image data to create

a more faithful representation of the original scene and geometric distortion was calibrated using

this technique.

ii) Processing

Image enhancement such as contrast stretching, density slicing, edge enhancement, and spatial

filtering were done to enhance linear features and subdue other features extract more detail

information such like for visual interpretation

Image Transformation

Image transformation is done in order to differentiate between the various brightness values,

which are obtained from identical surfaces due to topographic slope and aspect, shadows, or

seasonal changes in sunlight illumination angle, and intensity.

For vegetation discrimination in land use/cover mapping band ratio of 4 to 3 band and false color

combinations in RGB order of bands 432 for Landsat 7 of ETM+ image was done. In the ration

image vegetation cover has shown light color where as in false color composite open forest has

brown to red color (Fig. 2.2).




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Figure 2.2 Project area images of year 2000 G. C., false color composite of bands 432 240000

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Figure 2.3 Project area image of year 2000 G. C., band ratio of band 4 and 3




Image Classification

Image classification that involves the analysis of multispectral image data and the application of

statistically based decision rules for determining the land cover identification of each pixel in an

image. Unsupervised classification was performed in order to have a general idea of the area.

Supervised classification was performed for final land use/cover mapping.

2.2 GIS Analysis

The different inputs taken for GIS analysis were from topographic maps, different available

maps, Landsat satellite images and SRTM data. GIS analyses such as distance from geologic

structures, density of drainage, interpolation of rainfall point data, derivation of slope from

SRTM and overlay analysis for producing the groundwater potential area were done; moreover

weights and multi-criteria evaluation were done for analysis of the different parameters that

control groundwater occurrences.

All data layers derived were converted to raster data sets having the same pixel size. Each data

sets in a single map were given weight by pair-wise comparison in addition the factor maps were

compared each other in pair-wise comparison. Reclassification of each map was done based on

the weights produced. To produce groundwater potential zone map multi-criteria evaluation was

used.

3. INTEGRATION OF REMOTE SENSING AND GIS

3.1. Introduction

The present study has attempted to apply integrated remote sensing and GIS for generating new

thematic data layers as well existing data for delineating potential groundwater zone in Dire,

Arero, Yabelo and Teltele Woredas, Borena Zone of Oromia Regional State. The six thematic

layers taken for the determination of potential groundwater were drainage density, slope

steepness, land cover/use and distance from lineaments, geomorphology and lithology.




Prior to integration of the data sets, individual class weights and map scores were assessed based

on Satty’s Analytic Hierarchy Process (AHP) (Table 3.1); in this method the relative importance

of each individual class with in the same map were compared by each other by pair-wise and

eight importance matrices were prepared for assigning weight to each class.

Table 3.1 The continuous rating scale developed by Saaty (1977)

1/9

1/7

1/5

1/3

1

3

5

7

9

Extremely Very

strongly

Strongly Moderately Equally Moderately Strongly Very

strongly

Extremely

Less Important More Important

3.2 Factors Controlling Groundwater Occurrence in the Project Area

Factors that have significant influence in groundwater distribution and occurrence that are used

for integration to demarcate potential groundwater zones are:

3.2.1 Drainage density

The drainage network of the project area was derived from SRTM data and on screen digitization

from topographic map, the major rivers present in the project area Segen river which runs from

west to east, also the drainage is more denser in western part.

All the small river and large rivers, which are found in the project area drain from central part to

east and west direction.

Comparison of the drainage system of the area and structure has shown that the drainage system

of the area is structurally controlled following lineaments directions. Dendritic and parallel

drainage pattern are recognized, which are indicative of the presence of structures that act as

conduits or storage for sub-surface water. Structurally controlled drainage patterns are observed

in western and eastern part of the project area.



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DraiageLine

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Woreda Boundary


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Figure 3.1 Drainage network of the project area

The drainage density was calculated directly in Arcmap using spatial analyst extension. In the

project area, mainly 4 drainage density categories have been identified and mapped as shown in

(Fig.3.3). Very high drainage density is found in the western, eastern and northeastern part of the

project area whereas high drainage density is found scattered in allm parts of the project area.

Moderate and low drainage density concentrates in the southern and central part of the project

area.




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Geologic Structure

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Figure 3.2 Comparison of drainage and lineament orientation

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Drainage Density

0 - 0.1

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Figure 3.3 Drainage density of the project area




With respect to groundwater occurrences the higher drainage density is related to less infiltration

of water to the ground, which in turn leads to higher run off and vice versa. The pair-wise

comparison done based on this fact has shown that for areas with low drainage density higher

weight was calculated (Table 2) and vice versa and the reclassified map of drainage density

(Fig.3.3) was produced based on these weight.

Table 3.2 Weight for drainage density

(Km/Km2) Very

High

High Moder

ate

Low Weight

Weight

*


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Low 1 0.5232 52 Moderate 1/3 1 0.2976

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High 1/9 1/7 1/3 1 0.0570 6

Consistency ratio = 0.03

Figure 3.4 Reclassified drainage density map



3.2.2 Slope Steepness

Slope Analysis

The slope amount map has been prepared using contours produced from SRTM 90m data and. In

relation to groundwater flat areas where the slope amount is low are capable of holding rainfall,

which in turn facilitates recharge whereas in elevated areas where the slope amount is high, there

will be high run-off and low infiltration. The method of producing the slope amount map is

described below

Method

Steps followed to prepare the slope amount of the project area are described below:

i. Derive DEM from SRTM.

ii. Importing in to ArcGIS

iii. Derivation of Slope Amount using Spatial Analysis and reclassification in to appropriate

classes.


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Figure 3.5 Slope map of the project area



The slope amount derived have shown that elevation is low in SE and Eastern part

Table 3.3 Slope amount class in the project area

Slope Class Slope in Degree

1 (0-2)

2 (2-6)

3 (6-13) 4 (13-21) 5 (21-55)

The mountain located on both side of Yabelo in the central part has an elevation which ranges

from 2249 m.a.s.l to 2344 m.a.s.l with slopes from 21% to 55%. The SE and SW part is

characterized by flat generally the area has high elevation in the NE central and NW which can

as well be confirmed by the drainage system flow direction. The slope amount map classified in

to five classes (Fig 3.6) has been prepared.

Pair-wise comparison done and the weight calculated (Table 3.4) for slope angle was based on

the fact that the flatter the topography (low slope angle) is the better are the chances for

groundwater accumulation. The reclassified map was produced based on the weight calculated

(Fig. 3.4).

Table 3.4 Weight for slope the project area

Flat

Gentle

Moderate

Steep

Very

Steep

Weight Weight

*

100

Flat 1 0.4978 50

Gentle 1/3 1 0.2680 27

Moderate 1/4 1/3 1 0.1362 14

Steep 1/7 1/5 1/3 1 0.0642 6

Very

Steep

1/9 1/7 1/5 1/3 1 0.0337

3





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Figure 3.6 Reclassified slope map of the project area

3.2.4 Land Use/Land Cover

One of the parameters that influence the occurrence of sub-surface groundwater occurrence is

the present condition of land cover and land use of the area. The effect of land use / cover

is manifested either by reducing runoff and facilitating, or by trapping water on their leaf.

Water droplets trapped in this way go down to recharge groundwater. Land use/cover may also

affect groundwater negatively by evapotranspiration, assuming interception to be constant.

The land use/cover map of the area was readily interpreted from Landsat image by using

visual interpretation, unsupervised classification, supervised classification and print outs of

band 453,432 543 in RGB combinations.

For identification of vegetation cover band ration of band 4 to band 3 was done. After detailed

analysis the result was compared and corrected by data collected from different locations of




the area. Spot map with spatial resolution of 5m was also used to digitize settlements.

Comparison of Landsat image and topographic map of year 1975 has shown that there

is remarkable expansion of settlements since 1974, which negatively affect the groundwater

recharge of the area. The original vegetation is more preserved in area far from the main road .

This is due to the fact that people had made the natural forest to disappear on the flat lands near

to the main road and cultivate the land.

Classification of land use/cover for analysis was done based on their character to infiltrate water

in to the ground and to hold water on the ground. Generally settlements are found to be the least

suitable for infiltration and after pair-wise comparison of each class weight for each class was

calculated (Table 3.5). Reclassified map was produced based on the weight calculated (Fig. 3.8).

Table 3.5 Arial extent of various land use/cover categories

LandUC AreaSize

1 Water Body 236944301

2 Swamps 403955793

3 Sand Surface 85233691 4 Dense forest 172040954

5 Dense Bushland 7257709447

6 Open Bushland 312547789

7 Bush Shrub Grass Land 11015492699

8 Wood Grass land 1630677836

9 Agriculture Land 502752844

10 Open GrassLand 8312474247

11 Rock Surface 96827220

12 Settlements 5725396

Total 30,032,382,217




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Legend

Water Body

Swamps

Sand Surface

Dense forest

Dense Bushland

Open Bushland

Bush Shrub Grass Land

Wood Grass land

Agriculture Land

Open GrassLand

Rock Surface

Settlements

Figure 3.7 Land use/cover map of the project area

Table 3.6 Weight for land use/cover map

Water

Body

Swamps

Sand

Surface

Dense

forest

Dense

Bushland

Open

Bushland

Bush

Shrub

Grass

Land

Wood

Grass

land

Agriculture

Land

Open

Grass

Land

Rock

Surface

Settle

ments

Weight Weight

*

100

Water Body 1 0.2411 24

Swamps 0.9 1 0.1926 19

Sand Surface 1/2 0.9 1 0.1488 15

Dense forest 1/3 1/2 0.9 1 0.1130 11

Dense

Bushland

1/4 1/3 1/2 0.9 1 0.0849 8

Open

Bushland

1/5 1/4 1/3 1/2 0.9 1 0.0633 6

Bush Shrub

Grass Land

1/6 1/5 1/4 1/3 1/2 0.9 1 0.0470 5

Wood Grass

land

1/7 1/6 1/5 1/4 1/3 1/2 0.9 1 0.0349 4

Agriculture

Land

1/8 1/7 1/6 1/5 1/4 1/3 1/2 0.9 1 0.0261 3

Open

GrassLand

1/9 1/8 1/7 1/6 1/5 1/4 1/3 1/2 0.9 1 0.0198 2

Rock Surface 1/10 1/9 1/8 1/7 1/6 1/5 1/4 1/3 1/2 0.9 1 0.0156 2

Settlements 1/11 1/10 1/9 1/8 1/7 1/6 1/5 1/4 1/3 1/2 0.9 1 0.0129 1





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Figure 3.8 Reclassified land use/cover map of the project area

3.2.6 Structure

Mapping of lineaments in the project area was done by visual interpretation of various digitally

enhanced single band (Fig.3.9) and multi band images that involves standard band combinations,

principal component analysis and directional filtering, since lineaments are the results of faults

and fractures they infer that they are the zone of increased porosity and permeability, which in

turn has greater significance in groundwater studies occurrence and distribution. Structural

features were interpreted from satellite imagery. In the imagery they were identified on the basis

of break of slope, truncation of terraces knick points, abrupt change in stream course, lithology,

vegetation, texture, drainage density etc.

The lineaments were identified by visual interpretation and interactive digitization (Fig.3.16c) in

the images. A final lineaments map was constructed from the digital enhancement of individual

single band and multiband images together with previous work.





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a b

Figure 3.9 Edge enhancement of panchromatic image (a) magnified lineaments (b) digitized

lineaments

Rocks of the map sheet record major tectonic activities distinctly separated in space and time: the

first one is deformation accompanied by metamorphism in the Precambrian rocks while the

second was extensional tectonics with volcanism responsible for the formation of the rift.

Figure 3.10 Structural map of the project area




In the project area, the basement rocks are subjected to poly-phase deformation events including

folding, thrusting, and sheering, which impart a pervasive north south oriented regional and local

structures (Genzebu et al., 1994; Worku 1996; Yihunie and Tesfaye 1998 and reference therein).

The various structural elements including foliations, folds, lineations, shear zones, faults, and

lineaments characterizing the metamorphic rocks are merely the result of deformation and/or

metamorphism. The variations in size, shape, and orientation of these structural elements are

mainly attributed to style, nature of deformation, and rheological behavior of the rocks.

Cenozoic structures due to extensional tectonics in the rift valley terrain are largely primary

layering, flow direction, normal faults of variable magnitude, orientation, and strike length.

While the associated morpho-tectonic features predominantly comprise alluvial fans,

escarpments, triangular facets, and fabulous volcanic vents (i.e., crater, cinder-, spatter cones,

maars, and volcanic ramparts).

Primary layering ranging from few centimeters up to few meters are nicely represented in the

pre-rift succession of Jirarsa uplands and pyroclastic surge deposit of post rift volcanics. Both

textural and compositional inhomogeneuity define the primary layering, which can be readily

identified from aerial photographs as physiographic break. Besides, the pyroclastic surge deposit

commonly exposed at the crater rims and surroundings exhibit primary layering, which gradually

thins and dies out away from the crater mouth. Weather the disposition of this surge deposit is

symmetrical or directional could not be established due to absence of available section. It is

poorly sorted containing principally angular fragments of various rock types. The beds measures

up to few centimeters and differential weathering gave rise the pile a saw-and-tooth appearance

while holes are noted where rock fragments are released. In areas covered by recent lava, flow

directions depending upon the paleomorphology are readily discerned from aerial photographs

and satellite imagery.

Low- and high scarp faults with north south, northwest southeast, and northeast southwest trends

are exhibited in the volcanic terrain, despite the prevalence of low scarp faults over high scarp

faults. In the northwestern corner of the project area, the westerly tilted (up to 30°) Jirarsa

uplands with a strike length of 30 km in the map sheet is the southern continuation of Teltele

plateau. Triangular facets and alluvial fans indicatives of neo-tectonics decorate this westerly




tilted block covered by pre-rift succession standing more than 1000 m from the floor. Moreover,

the wide Arbala graben is situated south of the Sagan basin. It is worth noting the strong linear

trend of Ririba stream in the southwestern corner of the project area is due to low scarp fault

extending south in to Kenya beyond the map sheet. The wide Arbala graben covered by Tertiary

Bulal basalt is largely affected by north south and seldom by NNE-SSW trending low scarp

normal faults, which continue and merge with low scarp faults defining the Ririba Rift. The

pyroclastic surge deposits overlying the Bulal basalt commonly interrupt the strike continuation

of most of these low scarp normal faults. Moreover, the recent lava flow southwest of Goraye

village presumably of Quaternary age is impeded by low scarp fault at the southwestern corner

of the project area. The faulting has affected the Bulal basalt of probable Pliocene age (Davidson

1983) suggesting recent volcanism and continued tectonic activity. Unlike the surface expression

of the MER, which is characterized by north south trending high scarp steep faults, a series of

northwest southeast principal normal faults gave rise to a series of horst and graben morpho-

tectonic features around Mega. These northwest southeast trending high scarp steep faults have

considerable strike length in excess of 100 km defining the Mega Rift in the broadly rifted zone.

The spatial distribution of majority of the volcanic vents (that is, craters, maars, scoria-, and

spatter cones) appear to be controlled both by these low- and high scarp faults as most volcanic

centers are situated close and/or on these faults. A striking feature of the post rift volcanic

products is the presence of mantle nodules, essentially composed of pyroxene and olivine, of

variable size blanketed by thin aphyric basalt. It can be thus concluded that most of the normal

faults have considerable depth extension reaching at least up to upper mantle.

For analysis of lineaments in relation to groundwater prospective zones, distance analyses were

carried out and 4 classes were produced (Fig 3.17). Reclassified map of lineament (Fig.3.18) was

then produced based on the weight calculated (Table 11) after a pair-wise comparison done

based on the fact that areas closer to lineaments are the highest zone of increased porosity and

permeability which in tern have greater chance of accumulating groundwater.




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Figure 3.11 Proximity to geological structure map

Table 3.7 Weight for proximity to geological structure map

Distance Very

Close

Close

Far

Very

Far

Weight

Weight

*

100

Very

Close

1 0.5812 58

Close 1/3 1 0.2599 26

Far 1/6 1/3 1 0.1195 12

Very

Far

1/9 1/7 1/5 1 0.0394 4





3.2.7 Geomorphology

About half of the project area is situated in the ''broad rift zone'' of southwestern Ethiopia and the

remaining half to southeastern plateau. In this sector of the ''broad rift zone'' of southern

Ethiopia, north south trending low scarp faults characterize the Ririba rift presumably the

southern extension of the Main Ethiopian Rift. Whereas the transverse northwest-southeast

trending high scarp, steep faults around Mega resulted a series of outstanding horst and subdued

grabens due to abundant volcanic vents and accumulation of their products. Precambrian

crystalline rocks dominate the region falling in the southeastern plateau and granitic rocks

present outstanding physiographic features. In general elevation declines from north to south

except the Gamadu- and Mega ranges where step faults uplifted more than 1000 meters

crystalline rocks and gave rise to the highest known elevation in the map sheet. The lowest and

highest elevations are 740 and 2495 meters above sea level, respectively. Two main drainage

systems characterize the area, the Dawa and Ririba catchements. Tributaries of Dawa river drain

the eastern part, whereas the western part including the Rift valley terrain belongs to the Ririba

catchement.

The physiographic expressions of the area reflect type and structure of the bed rock, accordingly

physiographic forms vary greatly in bed rock type and structure: for example, between basalts

and acidic intrusive rocks and notably between Precambrian crystalline rocks and Cenozoic

block faulted mountain ranges. Contrasting landforms are noted on volcanic rocks of different

ages, for example basalt platform with stony soils overlying unconformably the metamorphic

rocks and recent aa type lava flows devoid of soil and vegetation. Moreover, the various volcanic

landforms (such as craters, cinder- spatter cones, maars, and volcanic ramparts) exhibit

spectacular physiographic expressions. Owing to these variations, the area is subdivided into

seven physiographic regions and presented from west to east as follows (Fig. 4).

i. Rift valley terrain

ii. Yabelo massif

iii. Rolling topography

iv. Alona plain

v. Ridge and valley terrain

vi. Arero massif

vii. Southern lowlands and associated inselbergs



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Rift valley terrain

Yabelo massif

Rolling topography

Alona plain

Ridge and valley terrain

Arero massif

Southern lowlands and associated inselbergs

Figure 3.12 Geomorphology map of the project area

i) Rift valley terrain

In this subdivision a combination of type and structure of the rocks discern spectacular

physiographic expressions. The Rift valley terrain encompasses west of a line passing through

Soda crater and western flank of Yabelo massif. The rocks characterizing the wide physiographic

expressions are mainly horizontally piled up pre-Rift volcanic rocks, post-Rift sheets of basaltic

flows, and cluster of vents as well as craters with ejected material and edifices, Precambrian

crystalline basement. While, the structures are essentially boundary or subsidiary normal faults

which are often steep high scarp faults and rarely low scarp normal faults.





In the north western corner of this physiographic sub-division Jirarsa upland made up of

horizontally piled up pre-Rift volcanic rocks rises above 2100 meters above sea level standing

over 1000 meters from the surrounding plain. Principal normal fault on the eastern part gave rise

to westerly tilted block with triangular facets. The eastern boundary fault passing through

western flank of Yabelo massif and Soda crater is a low scarp normal fault, thus defining half

graben like structure. Further south north of Dilo this steep high scarp fault splays in to an array

of low scarp faults resulting typical steps. Around Mega a series of NNW-SSE trending steep

high scarp faults gave rise to basin-and-range like province of crystalline basement rocks.

Surface elevation in the horst blocks increases markedly westward giving rise to steep mountain

ranges and highlands. For instance, Gamedu mountain range stands over 1000 meters from the

floor and with strike length in excess of sixty Km. These horst blocks continue southward, while

to the northwest it abut intermittently into the rock floored and sediment filled half graben. Apart

from these outstanding mountain ranges, there are isolated elongated ridges and inselbergs with

erosinally-modified escarpment. The grabens in this basin-and-range like region are relatively

narrow 2-8 Km wide and subdued due to cluster of volcanic vents and accumulation of their

products mainly pyroclastic material.

The rocks exposed in the grabens are predominantly volcanic, except basement inselbergs. The

base of these horst blocks is commonly decorated by cone shaped colluvium and alluvial fans at

the mouth of main streams emanating from these physiographic features. Pysiographic features

in this part of the Rift owe their origin to be controlled both by rock types and structures i. e.,

faults associated with rifting.

Apart from the aforementioned physiographic feature, the various volcanic vents (i. e., cinder-,

spatter cones, craters, and maars) present notable landforms mainly situated on and/or close to

these normal faults. Most craters are single and circular, however, there are overlapping craters

such as Tinshu Dilo and Goraye craters, where a number of subsidiary collapsed vents define

swarm of craters. The size of cinder- and spatter cones varies markedly from few tens of meters

to few hundreds of meters. The shape varies from circular to elliptical either convex upward or

downward and breached to intact.

The Ririba low scarp fault controls the Ririba stream to flow almost in straight course of

considerable length for more than fifty Km. Most of the intermittent stream emerging from the

horsts disappear after draining shortly in the grabens or form temporary swamps.




ii) Yabelo massif

The Yabelo massif is a northwesterly southeasterly-aligned elliptical mountain mass with rugged

topography with maximum elevation rising up 2300 meters rising above sea level with forty and

eighteen Km in the longest and shortest dimensions, respectively. In the south the massif dies out

intermittently by isolated hills and ridges in the flat lying area. Whereas, in the north it is

characterized by deeply incised valleys with terraced morphological features and anomalously

high standing peaks. In this mountain range rare high standing widely jointed blocky rock

masses devoid of vegetation forming steep cliff are noted. Incorporation of older crustal material

within this massif is evident from the localized flat lying topography and deeply incised valleys.

It is worth mentioning the plain where the town of Yabelo is situated, where older crystalline

rocks are exposed.

iii) Rolling topography

West of the ridge and valley terrain gently sloping relatively small chain of ridges and broad

immature river courses characterize this physiographic feature. Topographically, these small

chains of ridge gradually slope southward and merge with the low lands and associated

inselbergs. This physiograpjic subdivision has developed partly on crystalline basement rock and

partly on stocks of acidic intrusive rocks.

iv) Alona plain

The Alona plain is characteristically a flat lying plain with stony soil capping the crystalline rock

covering considerable area and extends north into Agermariam sheet. The flat nature of this plain

is presumed to be a function of the horizontal attitude of the underlying Tertiary flow. The

thickness of this flow might not exceed a few meters as erosion exposed the underlying

crystalline rock at some localities as windows to look through older material.




v) Ridge and valley terrain

The ridge and valley terrain stretches linearly across the map sheet represented mainly by deeply

dissected “V” or “U” shaped valleys and continuous chains of mountains and erosinal remnants

of discontinuous ridges in the north and south, respectively. Whereas in the north. Most of the

isolated hills and elongated mountain chains in the southern and northern sector of this

physiographic sub-division exhibit smooth textures on aerial photographs and satellite imageries

attributed to grass cover and absence of trees. Unlike other physiograhic sub-divisions of the area

streams in the northern sector of ridge and valley terrain have matured courses and drains

northeasterly to join Dawa river.

vi) Arero massif

A striking landform is provided by Arero plutonic body in the northeastern part of the map sheet,

rising above 1800 meters above sea level. This north south aligned elliptical body with strike

length of more than sixty km. and breadth of 20 km. terminates gradually in the flat lying

lowlands in the south, whereas it continues to Ageremariam (NA37-10) sheet in the north. This

mountain range is characteristically rugged with deeply dissected narrow valleys whose eastern

and western flanks are moderately dipping. In the eastern part high outstanding peaks

characterized by widely spaced joints, blocky rock masses forming cliff and devoid of vegetation

are noted. Convex sheeting, that is, progressive rounding as successive sheets, another form of

exfoliation weathering, is noted in the western flank. Two outstanding post-tectonic granites

situated with in Arero massif exhibit circular physiographic features with central depression and

craggy peripheral part. Stream courses in this physiographic subdivision are commonly "V"-

shaped characterized by radial convergent or divergent drainage pattern draining at different

direction to join Dawa river.

vi) Southern lowlands and associated inselbergs

This physiographic subdivision covers vast area next to Rift valley terrain and predominantly

characterized by extensive flat lying plain with thick soil cover except ubiquitous steep sided

inselbergs and hills. In general elevation in this physiographic subdivision decreases southward



with an average elevation 1350 and 1800 meters above sea level for the plain and ubiquitous

inselbergs, respectively. Stream courses in this physiographic sub-division are broad and mostly

disappear shortly in the flat lying plain.

Table 3.8 Weight for geomorphology of the project area

Rift

valley

terrain

Southern

lowlands and

associated

inselbergs

Rolling

topography

Alona

plain

Ridge

and

valley

terrain

Arero

massif

Yabelo

massif

Weight

Weight

*

100

Rift valley terrain 1

0.3722

37

Southern

lowlands and

associated

inselbergs

1/2 1

0.2410

24

Rolling

topography

1/3 1/2 1

0.1534

15

Alona plain 1/4 1/3 1/2 1

0.0999

9

Ridge and valley

terrain

1/5 1/4 1/3 1/2 1

0.0652

7

Arero massif 1/8 1/5 1/4 1/3 1/2 1

0.0406

4

Yabelo massif 1/9 1/8 1/5 1/4 1/3 1/2 1

0.0276

3



270000

270000

300000

300000

330000

330000

360000

360000

390000

390000

420000

420000

450000

450000

480000

480000

510000

510000

540000

540000

570000

570000

3900

00

3900

00

420

000

420

000

4500

00

4500

00

480

000

480

000

5100

00

5100

00

5400

00

5400

00

5700

00

5700

00

6000

00

6000

00


1:1,350,000


Legend

3

4

7

9

15

24

37

Figure 3.13 Reclassified geomorphologic map of the project area



3.2.8 Geology

The project area comprises two major litho-stratigraphic units ranging in age from Precambrian

to Quaternary, that is, Precambrian crystalline rocks and associated intrusives, Tertiary to

Quaternary volcano-sedimentary rocks and Quaternary superficial deposit.

270000

270000

300000

300000

330000

330000

360000

360000

390000

390000

420000

420000

450000

450000

480000

480000

510000

510000

540000

540000

570000

5700003600

00

3600

00

3900

00

3900

00

420

000

420

000

450

000

450

000

480

000

480

000

510

000

510

000

54

0000

54

0000

57

0000

57

0000


1:1,350,000


Legend

Banded gneiss (Pbg)

Augen granitic gneiss (Pagng)

Biotite bearing quartzofeldspatic gneiss (Pbqfg)

Graphite bearing marble and amphibolite (Pgms)

Undifferentiated mafic-ultramafic (Pmus)

Amphibole gneiss (Pag)

Megacrystic granite (Pmgt)

Granite (Pgt2)

Monzonite (Pmo)

Granodiorite (Pgd)

Arero granitoid (PAgt)

Weakly to distinictly foliated granite (Pgt1)

Bulale basalt (QBb)

Pyroclastic deposit (Qps)

Scoriaceous basalt (Tsc)

Undifferentiated volcanic rocks (Tuv)

Olivenphyric basalt (Tob)

Alluvial soil (Qa)

Elluvial soil (Qe)

Calcrite (Qcf)

Limestone (Jh)

Figure 3.14 Geological map of the project area

i) Precambrian crystalline rocks

The Precambrian crystalline rocks of the area comprise high-grade gneisses, schists, weakly to

moderately metamorphosed sedimentary-, basic volcanic- and mafic-ultramafic rocks. The

Precambrian crystalline rocks mostly occupy the eastern part of the project area, that is, west of

the line passing through Soda crater and western flank of Yabelo massif.

Exposures are often extensive and continuous in the mountain ranges, blocky and fragmental in

the ridges and hills, whereas patchy and discontinuous in the flat lying areas.





Banded gneiss intercalated with sporadic undifferentiated schists and marble (Pbg)

Banded gneiss and associated undifferentiated schists are the most wide spread crystalline

basement rocks in the project area. It is mainly exposed in the southwestern part of the project

area. Continuous and extensive outcrop is common in the horsts while in the inselbergs outcrop

is mainly discontinuous and patchy.

The fundamental feature of this map unit is compositional and textural heterogeneous lithologies

(which are evident even on outcrop scale) comprising mainly a suite of quartzofeldspathic rocks

that are variably interlayered with mafic gneisses, and intercalated with sporadic schists. The

compositional layering/banding ranges from few mm to some tens of meters, which virtually

appears a preferred orientation (i. e., foliation) on aerial photographs as well as on the outcrops.

Wide textural variation from massive through weakly foliated to distinctly foliated and from

medium grained to coarse-grained variety is common on outcrop scale. Differential weathering

gives the rock a typical saw-and-tooth appearance, which is distinct from far distance, and

ellipsoidal cavities up to 50 cm deep. In general the thickness and intensity of these mafic layers

decrease towards east. The rock unconformably underlies the Cenozoic volcanic rocks and has

concealed contact with other crystalline rocks due to soil cover. Closely spaced vertical and

horizontal presumably tensional joints gave the rock (especially the quartzofeldspathic

component) a slaby appearance. Moreover, sporadic concordant and/or discordant late stage very

coarse grained (1cm), pink pegmatite veins ranging in thickness from few tens of millimeters to

few meters intrude this unit.

Augen granitic gneiss (Pagng)

This map unit defines strike parallel lens like north west trending prominent ridges characterized

by craggy outcrop pattern at the top part. It is buff pink, medium to coarse grained, with

accentuated foliation, and with blocky outcrop pattern characterized by two sets of joint i., e.,

parallel and perpendicular to the foliation.

Biotite bearing quartzofeldspathic gneiss (Pbqfg)

A large part of the crystalline basement in the project area mainly consists of quartzofeldspathic

rock with variable modal abundance of biotite and amphibole as well as development of

layering. That is, at places the modal abundance of biotite predominates over the amphibole




content and vice versa, however, systematic variation can not be outlined. In the northern part of

the domain, elas (water wells), road cuts as well as river sections where exposures are

continuous much of this unit is relatively uniform and poorly layered. Whereas, in the south

where outcrop is scanty, differential weathering and erosion preferentially expose the leucocratic

layer which may mistake with deformed intrusive, particularly granite. The rock is exhumed by

pre- to syn tectonic granitoids especially along the contact with the low-grade rocks and in the

northern part of the domain and unconformably overlain by Cenozoic volcanic at places. In the

south its contact is not observed and clearly understood due to thick soil cover. The unit is

invariably intruded by pegmatite and quartz veins of variable dimension (1-10 cm thick) with

different orientation, mostly exhibiting pinch-and-swell structure. Unlike to the banded gneiss,

pegmatite veins intruding this litho unit contain hydrous minerals such as amphiboles and biotite.

The rock is pinkish gray to buff gray, medium grained, distinctly foliated defined by parallel

alignment of the mafic constituents.

Graphite bearing marble, talc-tremolite schist, and amphibolite (Pgms)

These intimately associated discontinuous patchy litho-units are exposed in the Rift valley

terrain mantled by eluvial cover and its relation with the nearby high-grade rock is neither

observed nor understood. The marble is grayish white to dirty white, coarse grained, with

compositional layering defined by alternating color variation i. e., felsic and mafic minerals

dominated layer gave the rock a banded appearance ranging from 1 to 3 cm thick. Talc-

tremolite-actinolite schist is dark green, fine to medium grained and massive to weakly foliated

with occasional sulfide bearing quartz veins and veinlets. The amphibolite is dark green, fine to

medium grained and with accentuated foliation, exhibiting compositional banding defined by

felsic- and mafic rich constituents.

Undifferentiated mafic-ultramafic rocks (Pmus)

Undifferentiated mafic-ultramafic rocks are persistently exposed through out the low-grade belt

across the sheet, despite its discontinuous outcrop pattern in the southern part of the belt.

Extensive and continuous outcrop of undifferentiated mafic-ultramafic rocks are exposed on the

eastern part of the belt, however, separate small lenses are found on the western part of the belt

and ubiquitously within the metavolcano-sedimentary assemblage. Repetitions of this mafic-

ultramafic lithounit across the belt might have some tectonic implication despite the absence of




systematic documentation to unravel it. Generally it forms strike parallel outstanding chains of

mountains in the northern part of the belt whereas as small discrete lenses in the southern part of

the belt, which in most cases is characterized by smooth topography at the top part attributed to

alteration. It’s peculiar photo-characteristic, that is, smooth texture due to grass cover, and

absence of trees makes their identification very easy on aerial photographs.

Compositionally this unit comprises:

a. talc-chlorite-tremolite schist

b. chlorite-talc schist

c. silicified ultramafic (birbirite)

and separation into mappable units at this scale impossible. Undifferentiated mafic-ultramafic

rocks exhibit variegated colors (i. e., silvery gray, grayish green, light green, yellowish brown),

and are fine to medium grained.

a) talc-chlorite-tremolite schist: This litho-unit make up the second largest rock type of the

undifferentiated mafic-ultramafic rocks. It is grayish gray, fine to medium grained, feels soapy

when touched with hands.

b) chlorite-talc schist: It occurs as lens like small patchy intercalation within the

metasediment. It is greenish gray to silvery gray, fine to medium grained and weekly foliated.

c) silicified ultramafic (birbirite): This litho-unit is often ridge forming exhibiting blocky out

crop pattern and commonly found at the top part. Its peculiar photo- characteristic, (i. e., smooth

texture, due to grass cover, and absence of trees) makes its identification very easy on aerial

photographs. It is yellowish brown, cryptocrystalline rock essentially oxidized and silicified

mafic-ultramafic rocks with box work/mesh like structure resulted from criss croosing quartz-

and magnesite veinlets. These mesh like structures are filled by reddish brown clayey material

and disseminated fibrous light green serpentine. Magnesite veins ranging from few centimeters

up to tens of centimeters have been encountered in most places. Rarely large crystals of

vermiculite have been noted associated with the serpentinite. In association with this altered

ultramafic, serpentinite is commonly found characterized by greenish green color, fine grained,

massive to weakly foliated.

Amphibole gneiss (Pag)

This unit commonly exposed as discontinuous small ridges and rarely as patchy discontinuous in

the flat lying area in the southern part of the eastern gneissic domain. Its contact is concealed

with thick soil cover and thus not understood. The rock is gray green to dark green, medium




grained, massive to distinctly foliated, and silicified at places expressed as stringers parallel to

the foliation containing sulfide mineralization.

Megacrystic biotite granite (PMgt)

It occurs two plutonic bodies in the northeastern part of the project area. These plutonic bodies

are bowl shape with a depression at the central part while the peripheral part is represented by

continuous craggy and mountainous topography. This circular pervasive planar fabric cut across

the main regional fabric trending meridional to submeridional.

The contact of this megacrystic granite with the country rock is highly tectonized and distinctly

foliated dipping steeply inwards. The intensity of the deformation decreases towards the center

of the intrusion, which gradually gets massive and characterized by blocky and sheet like outcrop

pattern. The rock is pink to buff pink, coarse to very coarse grained, massive, with randomly

oriented abundant large K-feldspar crystals (up to 50 mm long), which gave the rock

pegmatoidal appearance. Biotite and amphibole (hornblende) are the common mafic minerals in

the rock. Pegmatite veinlets and veins intrude the unit.

Granite (Pgt2)

This litho unit is exposed ubiquitously in the project area occurring as small inselbergs standing

high against the flat lying plain. These stocks like bodies commonly exhibit circular to sub-

circular outcrop pattern widely jointed with blocky rock masses forming steep cliff devoid of

vegetation and with steep contacts with the host rock as deduced from the outcrop pattern. The

top parts of these bodies are commonly cliff forming, while the foothill is commonly weathered

and kaolinized at places showing spheroidal weathering. Pegmatitic and quartz veins of variable

size and strike length have been noted. Rare xenoliths of the country rock are encountered in

some of these intrusions.

The rock is buff pink to pink, coarse grained to very coarse grained (at places porphyritic with

crystals up to 4 cm long), massive, textural and compositional variation is evident among

different bodies.

Monzonite (Pmo)

This elliptical intrusion is exposed west of Das village with longest dimension (10 km) across the

regional planar fabric. It crops out in Sidola and Kormanjeli hills while the former is elliptical

and the latter is circular exhibiting blocky outcrop pattern. The rock is light grey to buff pink,




coarse to very coarse grained (at places pegmatoidal with grain size reaching up to 1cm),

sometimes porphyritic with feldspar phenocrysts, and massive with no sign of deformation.

Exfoliation weathering is characteristics of this unit. One set of joints trending NW-SE and

dipping moderately to SW is observed.

Granodiorite (Pgd)

This intrusion is exposed in the northeastern part of the Map Sheet in the western gneissic terrain

and half of the body extends into Ageremariam Map Sheet. The metavolcano-sedimentary,

gneisses and pre-rift basalt to the east, west, and south flank this sub-circular intrusion,

respectively. It has a bowl shape with smooth topographic expression and outcrops are

commonly found as patches and big blocks.

The rock is buff grey to grayish white, medium grained, massive, with internal (color, textural,

and compositional) variations but not as diverse as larger intrusions, and at places exfoliated.

Arero granitoid (PAgt)

This plutonic body is named after a small village Arero, situated on the southern terminus of the

intrusion. This north south stretched concordant (i. e., parallel to the regional fabric) body is

elliptical in shape with twenty by forty kilometer in diameter represents an outstanding massif.

Although the contact of this map unit with adjacent enclosing units is covered with soil it is

presumably moderate as deduced from the outcrop pattern. It is worth noting that this plutonic

body represent the southern most intrusion of the three en echelon arranged prominent plutons in

the Adola area of southern Ethiopia, namely Gariboro, Ranu and Arero (Kozyrev et al., 1985;

Awoke and Meshesha 1993).

In general, the intrusive form very rugged topography dissected by deeply incised valleys giving

rise to commonly ''V'' and rarely ''U'' shaped valleys, at places with towering rocky blocks

forming steep cliffs devoid of vegetation. This plutonic body exhibit convex sheeting structure,

that is, progressive rounding of the intrusion as successive thin sheets (another form of

exfoliation) with no pervasive internal fabric which mistaken with foliation. This intrusion

exhibits wide textural and compositional variations even at the same outcrop. In general, the rock

grades from gneissose variety at the periphery to massive variety at the center of the body,

however, ubiquitous pods of purely undeformed massive porphyritic granites are common.

Although systematic compositional variation cannot be established the intrusion exhibit wide

compositional variations ranging from granodiorite through granites to alkali feldspar granite.




The rock is pink, buff pink, medium to coarse grained, massive to gneissose, widely jointed (i. e.,

sub-horizontal and vertical). The ubiquitous towering blocks are commonly pink, coarse to very

coarse grained, widely jointed, rich in K feldspar, and relatively fresh. At places it is intensely

weathered giving rise to kaolin, which is used for painting houses locally, it is only quartz that

can be identified from this weathered rock. It is leucocratic with color index 1 to 10, and with

varying proportion of mafic constituents.

Granite (Pgt1)

This map unit is mainly exposed as elliptical, sub-circular small discrete bodies concordant to the

regional fabric close and/or in contact to the low-grade rocks. Moreover, ubiquitous exposures of

this map unit are encountered through out the area. These intrusions are believed to consume the

low-grade rocks as evidenced by the presence of xenoliths, roof pendants and enclaves of the

low-grade rocks within it and pegmatitic veins in the ultramafic rocks. Apart from this, lensoidal

or chocolate shaped xenoliths of mafic gneisses are common in this unit exposed in the Rift

valley terrain. Those intrusions found close and/or in contact with the ultramafic exhibit

continuous closely spaced fractures, which are readily recognizable on aerial photographs giving

rise the rock slaby appearance. In general, this map unit is weathered except the top part, which

is characterized by blocky and relatively fresh outcrop. Both concordant and discordant

pegmatitic and quartz veins with variable thickness and strike length are common in this unit.

The rock is grayish pink to buff pink, medium to coarse grained, often weakly- to distinctly

foliated defined by alignment of mafic constituents stretched felsic constituents.

ii) Cenozoic volcanic rocks

Western part of the project area, that is, west of a line passing through Soda crater, Dubuluk

village, and western flank of Yabelo massif belongs to the broadly rifted zone of southwestern

Ethiopia and predominantly covered by Cenozoic volcanic rocks. Volcanic rocks in the area are

due to both central and fissural eruptions the former include large number of volcanic cones and

craters producing volcanic rocks mainly consisting of pyroclastic fall deposit and vesicular to

scoriaceous basalt, while the latter is represented by widespread bimodal sheet flows (Fig.3.14).

The craters/maars has variable size and shape ranging from few meters to few tens of meters in

diameter and nearly from circular to overlapping swarm of vents, respectively (Fig.3.14).

Similarly the shape of the volcanic cones (i. e., cinder- and spatter cones) varies from circular to

elliptical despite the presence of both breached and intact variety. Commonly their diameter is in




the order of few tens of meters and convex- upward and downward varieties are encountered. It

is worth noting most of the aforementioned volcanic vents are situated on and/or near the

principal boundary- or subsidiary normal faults.

The Cenozoic volcanic rocks are subdivided into two broad categories based on whether erupted

before or after rifting, namely pre rift- and post rift volcanics. The pre rift succession, whose

earliest flow range in age from mid to late Eocene (Davidson 1983), is represented by sub-

horizontally piled up basaltic, and salic (trachyte and trachybasalt) rocks overlying fault bounded

tilted blocks. It occupies predominantly the uplands of Jirarsa, Sarite, and Werersu mountain

chains in the northwestern corner of the project area The post rift succession comprises

widespread sheet of basaltic flows with variable textural attribute and pyoroclastic deposit. These

rocks are mainly exposed in the grabens, in the flat lying topography in the southwestern sector

of the project area.

Bulal basalt (QBb)

This litho-unit is the most widespread post rift volcanic rock in the project area occupying the

flat lying Arbala plain extending further south beyond Dilo and Goraye villages to adjacent

Sololo map sheet. This informal lithostratigraphic unit was introduced first by Davidson, (1983)

after a vast plain northwest of the map sheet known as Bulal extending to the map sheet. This

map unit lapse up against the westerly tilted Jirarsa, Sarite, and Werersu uplands covered by pre-

rift Miocene (Davidson 1983) salic volcanics in the western part of the project area, and thus has

an unconformable relationship with these rocks. While the base of this unit is not exposed

anywhere and nothing can be said about the contact nature with underlying unit. It is not possible

to know the exact thickness of the unit as there is no available section, however, it is estimated

not more than 20m as calculated from the 1:50000 topographic map. The rock occupies

commonly flat lying topography and faulted blocks with stony soil outcrop pattern and rarely as

horizontally layered continuous sheet. The unit essentially comprises a wide variety of texturally

inhomogeneous basic rocks namely: aphanitic basalt, vesicular basalt, and amygdaloidal basalt

whose separation into mapable units at this scale impossible.

The aphanitic basalt is mainly exposed as rounded subrounded boulders/blocks with brownish

gray weathering rinds enclosingblack to dark gray fresh core and rarely as patchy sheet. Locally,

flow lamination defined by colour variation with thickness ranging from 0.5-1.5 mm is noted.

The rock is black to dark gray, fine to medium grained, massive, locally with minor vesicles




ranging in diameter from 1 mm to 4 mm. At places, these vesicles are filled with calcite and

zeolite amygdale.

Pyroclastic and Scoria deposits (Qps)

Pyroclastic and scoria deposits are mainly crop out in the southwestern sector of the project area.

The former is commonly found occupying flat lying areas surrounding the rim of most notable

craters and maars ( Soda, and Goray) with maximum thickness at the crater rim gradually dying

out away from the crater. While the later crops out intermittently at cinder-, spatter cones,

volcanic ramparts and few meters down slope, at places the scoria and pyroclastic deposit

intermingle each other and separation of these unit is difficult at this scale, however, detailed

description of each lithounit is provided.

Generally it is gray, grayish white, grayish brown, poorly sorted, with distinct primary layering

(exhibits graded bedding, cross bedding structures) ranging in thickness from few mm up to

thirty cm. It essentially consists of rock fragments of banded gneiss, granite, basalt, and mantle

nodules. The size of these rock fragments range from few cm by cm to 1m by 0.5 m, that is, from

lapilli to blocks and bombs. The shape varies from angular to subangular. At places these rock

fragments are released and gave rise the map unit a jig-saw appearance. Ash fall deposit, lapilli

tuff, and agglomerate are the predominant type of pyroclastic deposits in the area, which in some

cases are interlayered. Commonly the thickness of this unit is maximum at the crater rim (up to

50 m) gradually dying out away from the crater, however, west of Mega town in a deeply

dissected narrow stream the thickness of the ash deposit is beyond the limit of observation (>100

m). This litho-unit has sharp contact with the other volcanic units and sometimes

unconformablly overlies the crystalline rock (e. g., Soda crater).

The scoria is reddish brown, grayish black and is formed of loosely packed/agglutinated cobble,

gravel, or lapilli sized cinder, which in some cases show graded bedding.

iii) Pre Rift volcanics

Scoriaceous basalt (Tsc)

This stratigraphically higher unit covers the upper most section (greater than 1500m elevation) of

jirarsa, Sarite, and Werersu uplands in the northwestern part of the project area overlying the

undifferentiated volcanic rocks.




The scoriaceous basalt exhibits a prominent horizontal layering with topographic break readily

seen on aerial photographs overlying the undifferentiated volcanic rocks. It is not more than 40m

thick. Although thin section from this unit is not available the texture ranges from aphyric to

scoriaceous, through mildly porphyritic, and amygdaloidal. The color varies from light gray to

brownish gray through light greenish green. The vesicles are some times filled with zeolites and

calcite amygdales.

Undifferentiated volcanic rocks (Tuv)

In the western part of the project area up to 500m thick undifferentiated volcanic rocks of pre-rift

succession make up the Jirarsa, Sarite, and Werersu uplands which is tilted approximately 30º

towards westerly. It essentially comprises of basalt and subordinate trachyte/trachybasalt with

wide textural attributes at places separated by different thin (up to 70cm) backed zones.

There are at least four observable basaltic flows separated by backed zones, which at places

reach up to 70cm in thickness. The number and thickness of flows differ from place to place and

there exists considerable variation in texture among the different flows (i. e., aphyric,

porphyritic, layered, amygdaloidal, and scoriaceous). The salic (trachyte/trachybasalt) volcanic

rocks exhibit wide range of textural and compositional attributes, that is, from aphyric to

porphyritic, massive, through flow banded and glassy (with devitrified glass in some sections).

Compositional variation from trachyte to trachybasalt through volcanic ash and tuff are noted in

this package. The trachyte is light gray to gray and greenish gray on fresh outcrops, with flow

banding structure, and at places silicified and brecciated.

Olivine phyric basalt (Tob)

This litho unit overlies unconformablly the Precambrian crystalline rocks in the northern part of

the area. It is designated as Lower Basalt and radiometric age dating from this unit ranges from

36.7 to 44.9 Ma (Woldegebriel et al., 1994) despite samples were taken from similar map units

far from the olivine phyric basalt is exposed. This unit covers flat lying area with sub-horizontal

layering, with stony soil outcrop pattern. They weather characteristically to rounded boulder with

thin grayish brown weathering rinds enclosing dark gray fresh core. Some granitic hills stand

fairly above the basalt sheet in the western part of this flow. The rock is dark gray, porphyritic,

rarely vesicular and vesicles are filled by calcite amygdale.




iv) Superficial Deposits

Considerable part of the project area (i. e., 57%) was covered by superficial deposits occupying

the flat lying areas which are broadly subdivided into three namely alluvium, elluvium, and

calcrete with subordinate ferricrete.

Alluvium (Qa)

Alluvial deposit occupies streambeds, flood plains, stream banks, and rarely defines alluvial fans

at the mouth of some rivers. It mainly consists of sand, silt, clay in various proportions with

some pebbles and cobles and the color varies from grayish white through reddish brown to black.

The constituent minerals of the sand and silt are chiefly quartz, feldspar, rounded to subrounded

rock fragments, and subordinate magnetite, biotite, and white micas. Near Web village very thick

unconsolidated loose sand with thin layer of calcrete ranging in thickness from 15 to 20 meters

were noted in a water well (Ela) probably representing a paleochanel. In the rift valley terrain

some of the streams ended up in alluvial fans.

Elluvium (Qe)

This unit comprises of residual and transported soils occupying flat lying and gentle topography

represented by sand, silt, clay, and gravels in various proportions. The color varies mainly from

black, through reddish brown sometimes to grayish white and constituent minerals mainly

quartz, feldspar, rock fragments with subordinate biotite and magnetite and white micas. The

thickness of this unit is unknown as the base can be seen.

Calcrete (Qcf)

This lithounit is exposed ubiquitously in the project area occupying ridge tops, flat lying areas

and flood plains as thin sheets (few meters thick), lenticular, and elongated. It is chiefly grayish

white to buff white rarely with yellowish brown and reddish brown varieties, fine grained to

cryptocrystalline, with some clasts of rock fragments. Although this unit is mapped as calcrete

there are subordinate ferricrete and silcrete, which are characterized by reddish brown and buff

white color, respectively. They are chiefly composed of fine cryptocrystalline carbonate with

subordinate minerals and rock fragments of gravel and pebble size. At places they exhibit

horizontal layering readily recognizable from aerial photographs. Moreover, laminations defined

by color variation and convolute folding suggesting their deposition under sub aerial

environment.



Table 3.9 Geologic units grouped based on their ground water importance

lithology Rank Group Importance


Calcrite (Qcf) 1

Alluvial soil (Qa) 1

Group1

Elluvial soil (Qe) 2

Group2

Pyroclastic deposit (Qps) 3

Group3

Limestone (Jh) 4

Group4

Scoriaceous basalt (Tsc) 5

Group5

Olivenphyric basalt (Tob) 6

Undifferentiated volcanic rocks (Tuv) 6

Bulale basalt (QBb) 6

Group6

Biotite bearing quartzofeldspatic gneiss

(Pbqfg) 7

Granite (Pgt2) 7

Augen granitic gneiss (Pagng) 7

Graphite bearing marble and amphibolite

(Pgms) 7

Granodiorite (Pgd) 7

Megacrystic granite (Pmgt) 7

Decreasing

Importance

Weakly to distinictly foliated granite

(Pgt1) 7

Banded gneiss (Pbg) 7

Monzonite (Pmo) 7

Amphibole gneiss (Pag) 7

Arero granitoid (PAgt) 7

Group7

Undifferentiated mafic-ultramafic (Pmus) 7



Table 3.10 Weight for geology of the project area

Group1

Group2

Group3

Group4

G4roup5

Group6

Group7

Weight

Weight

*

100

Group1 1 0.3728 37

Group2 1/2 1 0.2442 24

Group3 1/3 1/2 1 0.1553 16

Group4 1/4 1/3 1/2 1 0.0986 10

Group5 1/6 1/4 1/3 1/2 1 0.0627

6

Group6 1/8 1/6 1/4 1/3 1/2 1 0.0398 4

Group7 1/9 1/8 1/6 1/4 1/3 1/2 1 0.0267

3


270000

270000

300000

300000

330000

330000

360000

360000

390000

390000

420000

420000

450000

450000

480000

480000

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510000

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3900

00

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000

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000

450

000

450

000

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00

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00

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00

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00


1:1,350,000


Legend

3

4

6

10

16

24

37

Figure 3.15 Reclassified geological map of the project area





4. INTEGRATED ANALYSIS IN GIS ENVIRONMENT

The main objective of the study is to generate groundwater potential zone of the based on

different thematic maps by considering their relevance to groundwater occurrence. In order to

produce the potential groundwater zone map detailed GIS analysis of eight thematic maps was

conducted. A groundwater model was constructed using ArgGIS model builder engine (Fig 4.1).

Using the model all maps were rasterized, reclassified and given appropriate weight in order to

integrate them for multi criteria evaluation (MCE).

The following steps have been followed to produce groundwater potential zone:

i. Selection of data for an input based on their groundwater controlling parameters.

ii. Using the model personal geodatabse and feature dataset was prepared and each data set

that was produced from previous work, remote sensing imagery, digital elevation model

(DEM), topographic maps and field observation were imported into geodatabase to have

the same spatial reference.

iii. All the data sets were then converted into raster grid in the model in order to perform

different GIS analysis between data layers such as overlay analysis.

iv. All the data sets were reclassified based on their importance to groundwater potentiality

(availability).

v. Prior to integration of the data sets, individual class weights and map scores were

assessed based on Satty’s Analytic Hierarchy Process (AHP) (Table 1); in this method

the relative importance of each individual class with in the same map and factor maps are

compared each other and important matrices are produced with calculated weight using

WEIGHT module of IDRSIS32. The matrices have consistence known as consistency

ratio (CR). Satty recommends that matrices with CR rating greater than 0.1 should be re-

evaluated. The weights derived from this method were normalized after multiplying them




by 100 and rounded to integer value to avoid complexities of computation in further

analysis.

vi. Eight matrices for pair-wise comparison of each data set in a single map with calculated

weight of each data set was produced and the factor maps were reclassified based on the

weight calculated.

vii. After a pair-wise comparison of each factor maps based on their influence to groundwater

occurrence a single matrix (Table 14) with calculated weight of each factor map was

produced.

4.2 GIS Modeling

In order to delineate potential groundwater site in the projec area , all the data sets were

integrated using the model constructed in ArcGIS model builder engine (Fig 4.1). The final map

was produced by Weighted Linear Combination (WLC) where each class individual’s weight

was multiplied by the map scores and then adding the results:

S = Wi Xi

Where S = Suitability

Wi = Weight for each map score

Xi = Individual map

4.3 Weighting

In order to apply multi-criteria evaluation (MCE), a set of relative weights was assigned for each

map using WEIGHT module of IDRISI 32. The procedure mentioned in section 5.1 step vii was

followed using continuous rating scale developed by Satty (1977) (Table 1). The weights

calculated for each factor map were the results of pair-wise comparisons of each factor map

based on their relative importance to groundwater accumulation.

GROUNDWATER POTENTIAL ZONE MAPPING IN MOYALE-TELTELE SUB BASIN AT DIRE, ARERO, YABELO AND TELTELE WOREDAS,

BORENA ZONE OF OROMIA REGIONAL STATE USING GIS AND REMOTE SENSING

Table 4.1 Weight for all factor maps

Lineament

Distance

Geolog

y

Slope

Steepness

Geomor

phology

Drainage

Density

Land

use/Cover

Weight

Weight

*

100

Lineament

Distance

1 0.4046 40

Geology 1/2 1 0.2468 25

Slope

Steepness

1/3 1/2 1 0.1647

16

Geomorphology 1/5 1/3 1/3 1 0.0984

10

Drainage

Density

1/8 1/5 1/3 1/4 1 0.0583 6

Land Use

/Cover

1/9 1/8 1/6 1/3 1/5 1 0.0273


3




5. RESULT

The delineation of groundwater potential zones by reclassifying into different potential zones;

Very Good, Good, Moderate, Fair and Poor (fig 5.1) was made by utilizing the model designed

using ARCGIS model builder engine. The map produced has shown that the groundwater

potential of the project area is related mainly to lineaments, geology and slope.

Arero

Teltelie

Yabelo

Dire

240000

240000

270000

270000

300000

300000

330000

330000

360000

360000

390000

390000

420000

420000

450000

450000

480000

480000

510000

510000

540000

540000

570000

570000

390

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420

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420

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00


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Legend

Groundwater Potential

Poor

Fair

Moderate

Good

Very Good

Figure 5.1 Ground water potential zones analyzed on the basis of structure, geology, slope,

geomorphology, drainage and land use/cover

The validity of the model developed was tested against the borehole data, where out of 40 with

yield borehole data collected from the study area 21 are on very good and good zones, 12 on

moderate zones, 5 on fair and 1 on poor zones (Fig. 5.2).




Table 5.1 Borehole data from different localities of the project area

NO NAME Easting Northing Discharge(L/S) Potential

1 Dhaka Kalla 320975 552429 1 Poor

2 Birindar 350065 566262 4 Fair

3 Ghendile 313937 545560 4 Fair

4 Chilanko 389737 541642 0.5 Fair

5 YABELO ETH/027 400211 543067 0.2 Fair

6 DAS ETH/041 468462 465848 0.5 Fair

7 Nekayo/Mermero 305786 513683 7.2 Moderate

8 Harodimtu 478665 525964 1 Moderate

9 Mermero 305774 514001 7.2 Moderate

10 Metagefersa 478665 525964 1 Moderate

11 Millemi 321643 559592 0.62 Moderate

12 Surupha 423502 567889 0.45 Moderate

13 Dololo Merkala1 378646 539472 0.66 Moderate

14 El-leh 400413 524294 1 Moderate

15 WACHILE ETH/005 507209 502191 0.1 Moderate

16 Y ABELO ETH/026 404255 539723 6.5 Moderate

17 Sarite/Giwesa 346964 545325 3.25 Moderate

18 Utalo 389294 520566 2 Moderate

19 kello 314774 565086 3.5 Good

20 Afura 412109 550665 1.25 Good

21 Billa 317086 550869 5.5 Good

22 Did hara 429402 532064 0.5 Good

23 Horbate 338110 523280 5.8 Good

24 QA-GOFA ETH/042 433479 470795 1 Good

25 Horbate/Ambo 338052 523061 5.8 Good

26 Haro wayu 1 398981 516768 1.6 Very Good

27 Hoboq 306254 505927 7.5 Very Good

28 walenso 496909 498110 3.2 Very Good

29 Mormora 349378 538949 2 Very Good

30 Orbati1 347744 535635 2.17 Very Good

31 ANOLE ETH/024 439801 476795 0.3 Very Good

32 DUBLUK ETH/025 420030 482730 0.5 Very Good

33 GAYU EHT/BH5 448327 466494 0.7 Very Good




34 UDET ETH/BH4 526727 525631 1.5 Very Good

35 Dubluk Town 420112 482697 6 Very Good

36 Adegelchet 378395 533283 0.66 Very Good

37 Goray/Melka Sedeka 338000 454149 4 Very Good

38 Harewayu 398972 516763 9 Very Good

39 Dubluk area 425179 477832 0.5 Very Good

40 Dubluk Town 419909 482961 0.5 Very Good

Arero

Teltelie

Yabelo

Dire

240000

240000

270000

270000

300000

300000

330000

330000

360000

360000

390000

390000

420000

420000

450000

450000

480000

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390

000

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1:1,350,000


Legend

Discharg L/sGroundwaterPotential

Poor

Fair

Moderate

Good

Very Good

0.10 - 1.00

1.01 - 2.17

2.18 - 4.00

4.01 - 6.50

6.51 - 9.00

Figure 5.2 Distribution of boreholes in ground water potential zones

Moreover out of 40 bore holes (with yield data), bore holes with yield between 4 l/s to 9 l/s

are on the very good and good zones which reflects the actual groundwater potential (Fig. 5.2).

Although some wells exist in all groundwater potential zones, the best yielding wells lie in the

very good and good groundwater prospect zone.

The model generated will help as a guideline for designing a suitable groundwater exploration

plan in the future. The spatial distributions of the various groundwater potential zones obtained




from the model generally show regional patterns of lineaments, drainage, landform and

lithology.

Spatially the very good and good categories are distributed along areas near to lineaments and

less drainage density and where the lithology is affected by secondary structure and having

interconnected pore spaces. This highlights importance of lineaments, geology and

hydrogeomorphological parameters in the project area.

Areas with moderate groundwater prospects are attributed to contributions from combinations

of the land use/cover, lithology, slope and landform. The low to poor categories of

groundwater potential zones are spatially distributed mainly along ridges where slope class is

very high, the lithology is compact/massive and far from lineaments.




6. CONCLUSIONS AND RECOMMENDATION

6.1 Conclusion

The main objective of this project is to use GIS and Remote sensing technique for the

assessment, evaluation and analysis of spatial distribution of ground water potential zones with

in an area of 30,086 km2. Ground water potential zone map have been produced using eight

thematic maps from satellites images, exiting data and field data. Produced ground water

potential zone map were compared and validated by existing discharge data obtained from

different localities of the project area. The result showed fairly significant correlation or

agreement with the discharge data.

This study has shown that large spatial variability of ground water potential. This variability

closely followed variability in the structure, geology, geomorphology and land use/cover in the

project area. The most promising potential zone in the area is related to volcanic rock of which

is affected, by secondary structure and having interconnected pore spaces, with plain

geomorphic feature and less drainage density. Most of the zones with fair to poor groundwater

potential lie in the massive basements unit which is far from lineaments.

This study generally demonstrates that GIS and Remote sensing techniques in combination

with field data could be used for the assessments of ground water potential zones in an area

with little primary porosity and low bedrock hydraulic conductivity and where hydrogeological

properties are mainly determined by secondary factors fracture zones and associated

weathering. It can be considered as a time and cost-effective tool for delineations and

identification of high ground water potential target area.




6.2 Recommendation

Remote sensing data are powerful tools to improve our understanding of groundwater systems.

Despite unable to measure hydrogeological properties directly, they provide continuous

detailed terrain information and allow the mapping of features significant to groundwater

development there fore it is important to incorporate them in the data collection stage of

groundwater exploration works.

Despite various satellite data with different spectral and spatial resolutions coupled with digital

image processing techniques help to produce detailed maps, ground verification is crucial to

increase the accuracy of the interpretation results. The result obtained from this study should

be supported by subsurface data obtained from geophysical study.

Since geology, geomorphology and lineament mainly control the distribution occurrence and

flow of groundwater, analysis of these parameters should be supported by high-resolution

terrain data and satellite imagery.




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