WATER HARVESTING & MANAGEMENT (WHM) FOR

CROP & PASTURE PRODUCTION

FFA Regional Training March 22 – 24, 2011

Mombasa, Kenya.

Presented By: Kimeu P. M & Mutiso J.W.

What is Water Harvesting (WH)• Water harvesting is the collection of runoff for productive

purposes. It creates an opportunity to improve crop & fodder production in ASALs.

• Runoff is harvested and concentrated or retained at the point of fall in order to fill the water gap need for crop/fodder giving a reasonable yield

Catchment (e.g. roofs, ground surfaces, road surfaces, rock catchments, intermittent or ephemeral water courses)

Cultivated Area

RUNOFF

Classification of Water Harvesting Techniques

Water Harvesting

Rainwater Harvesting(Local Source)

Floodwater Harvesting(Channel Flow)

Rooftop Harvesting (collection from rooftops)

Runoff Harvesting (overland/rill flow)

Deep Ponding (storage)

Water Supply

Deep Ponding (storage)

Water Supply

Soil Storage

Plant Production

Runoff Farming**

Micro-Catchment Systems (Short slope catchment techniques)

External Catchment Systems (Long slope catchment techniques

Deep Ponding(storage)

Water Supply

Soil Storage

Plant Production

Floodwater farming =

Water spreading

1. 2.

3.

Sub Divisions

Main plant production categories

Productive use*

Storage

Category of WH system by source

NB: * Water supply systems (i.e. ponded water) used for a variety of purposes, mainly domestic and stock water but also some supplementary irrigation** ‘Farming’ in ‘’Runoff farming’’ broadly used to include trees, agro-forestry, rangeland rehabilitation, crops etc.

DEFINITIONS & CLASSIFICATION• Water Harvesting (WH): Collection ,concentration and

storage of runoff for its productive use;• Rainwater Harvesting Systems: Water harvesting

techniques which harvest and store runoff from roofs or ground surfaces;

• Floodwater harvesting Systems: Systems which collect discharges from watercourses;

Basic Categories of Water Harvesting Systems for Crop Production

Three basic categories to be discussed:1. Microcatchments (Within-Field catchment systems) -

RWHMain characteristics:• overland flow harvested from short catchment length• catchment length usually between 1 and 30 metres• runoff stored in soil profile• ratio catchment: cultivated area usually 1:1 to 3:1• normally no provision for overflow• plant growth is even

Typical Examples:• Negarim Microcatchments (for trees)• Contour Bunds (for trees)• Contour Ridges (for crops)• Semi-Circular Bunds (for range and fodder)• Zai Pits (for crops)

2. External Catchment Systems (Long Slope Catchments) – RWHMain Characteristics:• overland flow or rill flow harvested• runoff stored in soil profile• catchment usually 30 - 200 metres in length• ratio catchment: cultivated area usually 2:1 to 10:1• provision for overflow of excess water• uneven plant growth unless land levelled

Typical Examples:• Trapezoidal Bunds (for crops)• Contour Stone Bunds (for crops)

3. Floodwater Farming (Floodwater Harvesting)(often referred to as "Water Spreading" and sometimes "Spate Irrigation")Main Characteristics:• turbulent channel flow harvested either (a) by diversion

or (b) by spreading within channel bed/valley floor• runoff stored in soil profile• catchment long (may be several kilometres)• ratio catchment: cultivated area above 10:1• provision for overflow of excess water

Typical Examples:• Permeable Rock Dams (for crops)• Water Spreading Bunds (for crops)

Negarims

Examples of main WH systems

Contour Bunds

Semi-Circular Bunds Contour Ridges

Trapezoidal Bund Contour stone bunds

Permeable rock dams Water spreading bunds

OVERVIEW OF MAIN WH SYSTEMSStructure Classification

Main UsesDescription Where Appropriate Limitations

Negarim Micro-catchment

Trees & Grass

Closed grid of diamond shapes or open ended V’s formed by small earth ridges within infiltration pits

- Tree planting in areas where land is uneven or only a few trees are planted

• Not easily mechanized thus limited to small scale.

• Not easy to cultivate btn the tree lines

Contour Bunds

Micro-catchment

Trees & Grass

Earth bunds on contour spaced at 5-10m apart with furrow upslope and cross-ties

- Tree planting on a large scale esp when mechanized

- Not suitable for uneven terrain

Semi-circular bunds

Micro-catchment

Rangeland & fodder (also trees)

Semi-circular shaped earth bunds with tips on contour. In a series with bunds in staggered formation

-Useful for grass reseeding, fodder or tree planting in degraded rangeland

- Cannot be mechanized hence limited to areas with available hand labour

Contour ridges

Micro-catchment

Crops Small earth ridges on contours 1.5 – 5m apart with furrow upslope and cross ties. Uncultivated catchment btn ridges

- For crop production in semi-arid areas esp where soil is fertile and easy to work

- Requires new technique of land preparation & planting thus may be problem with acceptance

Structure Classification

Main Uses

Description Where Appropriate Limitations

Trapezoidal bunds

External catchment

Crops Trapezoidal shaped earth bunds capturing runoff from external catchment and overflowing around wing tips

- Widely suitable (in a variety of designs) for crop production in ASALs

- Labour intensive and uneven depth of runoff withing plot

Contour stone bunds

External catchment

Crops Small stone bunds constructed on contour at 15-35m apart slowing & filtering runoff

• Versatile system for crop prodn in wide variety of situations;

• Easily constructed by resource poor farmers

-Only possible where abundant loose stones are available

Permeable rock dams

Floodwater farming technique

Crops Long low rock dams across valleys slowing and spreading floodwater as well as healing gullies

- Suitable for situation where gently sloping valleys are becoming gullies and better water spreading is required

- Very site specific and needs considerable stones as well as provision of transport

Water spreading bunds

Floodwater farming technique

Crops & rangeland

Earth bunds set at a gradient, with a ‘’dogleg’’ shape, spreading diverted floodwater

-For arid areas where water is diverted from water course onto crop or fodder block

-Does not impound much water and maintenance high in early stages after construction

DESIGN CRITERIA

A. WATER REQUIREMENTS FOR CROPS• Design of WH systems requires an assessment of water

requirement of crop to be grown• In absence of any measured climatic data, its often

adequate to use estimates of water requirements for common crops as shown in table below (Source: FAO Manual)

Crop Crop water need (mm/total growing period)

Beans 300 - 500

Citrus 900 - 1200

Cotton 700 - 1300

Groundnut 500 - 700

Maize 500 - 800

Sorghum/Millet 450 - 650

Soybean 450 - 700

Sunflower 600 - 1000

Factors influencing CWR1. Influence of climate

-Highest crop water needs found in hot, dry, windy & sunny areas-Crop grown in diff climatic zones will have diff water needs-Grass is usually taken as reference crop for calculating water needs i.e. the amount of water the crop needs in the various climatic conditionsii. Influence of crop type on crop water needs-Crop type has influence on daily water needs of a fully grown crop e.g. the peak daily water needs of a fully developed maize crop will need more water per day than a fully developed crop of onions-Crop type has influence over duration of the total growing season of the crop e.g. short duration crops (peas: 90-100 days), longer duration crops (melons: 120-160 days) & perennials (fruit trees)

Calculation of crop water requirements (CWR)• Basic formula: ETcrop = Kc X Eto

Where: ETcrop = water requirement of a given crop in mm/unit time e.g mm/dayKc = the ‘crop factor’Eto = the ‘reference crop evapotranspiration’’ in mm/day etc.

Eto or potential evapotranspiration (PET) is the rate of evapotranspiration from a large area covered by green grass which grows actively, completely shades the ground and is not short of water

Crop Factors (Kc)CROP Average Kc per growing season

Cotton 0.82

Maize 0.82

Millet 0.79

Sorghum 0.78

Grain/small 0.78

Legumes 0.79

Ground nuts 0.79

Calculation of Eto 1. Pan evaporation method: • Use Class A pan of US Weather Bureau

(diameter=1.21m, depth=25cm and placed 15cm above ground)

• Fill with water 5cm below rim and allow to evaporate for period of time (usually 24hrs). Measure rainfall, if any, simultaneously

• Measure water depth after 24hrs• Difference btn the two measured depths = Pan

evaporation rate (Epan)Eto = Epan X Kpan

where Kpan is ‘pan coefficient’ varying between 0.35 – 0.85 with an average of 0.70 for class A pan

2. Blaney-Criddle Method• Use when no measured data on pan evaporation is

availableEto = p(0.46Tmean + 8), Where:Eto = the ‘reference crop evapotranspiration’’ in mm/day etc.Tmean = mean daily temperature (oC)P = mean daily percentage of annual daytime hours

Indicative values of ETo (mm/day)

Climatic Zone

Mean Daily Temp

15o 15-25o 25o

Desert/arid 4-6 7-8 9-10

Semi arid 4-5 6-7 8-9

Sub-humid 3-4 5-6 7-8

Humid 1-2 3-4 5-6

EXAMPLE: Calculation of ETcropSORGHUM• Total growing season = 120 days• ETo = Avg 6.0 mm/day (refer to table)• Kc = 0.78 (seasons average for sorghum)

ETcrop = Kc X Eto

ETcrop = 0.78 x 6 = 4.68 mm/day

Also, ETcrop = 4.68 x120 day = approx. 560 mm per total growing season

Water requirements for trees, rangeland & fodder

1. Multipurpose trees• Little information available i.e. CWR for trees are more

difficult to determine than for crops• Trees sensitive to moisture stress during establishment

stage• No accurate info available on response of these

species, in terms of yields, to different irrig/water regimes

B. Fruit trees• There are some known values of water requirements for

fruit trees under WH systems (most figures derived from Israel)

C. Rangeland & Fodder• Water requirements for rangeland & fodder species

grown in ASAL under WH systems are usually not calculated…. WHY??

Because the objective of rangeland & fodder grown in ASAL under WH systems is to improve performance, within economic constraints, and to ensure the survival of the plants from season to season, rather than fully satisfying water requirements!

B. SOIL REQUIREMENTS FOR WH1. Texture (composition in terms of mineral particles):

influences infiltration rate & available water capacity thus medium textured soils (loamy) the best

2. Structure (grp of soil particles to aggregates, and the arrangement of the aggregates): Good soil structure associated with loamy and relatively high content of OM

3. Depth: Deep soils have better capacity to store harvested runoff and provide greater amount of total nutrients for plant growth. <1m deep poorly suited for WH and >2m deep more ideal but rarely found

4. Fertility5. Salinity/sodicity: Avoid sodic (high exchangeable Na+ %)

and saline (have excess soluble salts) soils for WH systems

6. Infiltration rate7. Available water capacity (AWC)8. Constructional characteristics

C. RAINFALL-RUNOFF ANALYSISDESIGN RAINFALL• Is the total amount of rain during the cropping season at

which or above which the catchment area will provide sufficient runoff to satisfy the crop water requirements.

• Usually assigned to a certain probability of occurrence or exceedance e.g. If 67 % (most commonly used) it means on average, the value will be reached in 2 out of 3 year thus CWR will be met in 2 out of 3 years.

• Determined by statistical probability analysis

Probability analysis 1. Obtain annual rainfall totals for the cropping season

from area of concern;2. Rank the annual totals with m=1 for largest and m=n for

lowest and re-arrange accordingly;3. Probability of occurrence P (%) for each ranked

observation can be calculated from the equation:P (%) = m-0.375 X 100 where,N+0.25P = Probability in % of the observation of the rank mm = the rank of the observationN = the total no. of observations made (recommended for N= 1 to 100)

1. Plot ranked observations against corresponding probabilities

• Fit curve to plotted observations in to make curve of best fit (It can be a straight line)

• From curve, obtain probability of occurrence of a rainfall value of a specific magnitude and vice versa

7. Return period T (in yrs) can be easily derived once exceedance probability P (%) is known from the equation:T= 100(years)PE.g.T67% = 100/67 = 1.5 (years)

D. FACTORS AFFECTING RUNOFF1. Soil type • Vegetation• Slope & catchment size

Runoff Coefficient, KDescribes percentage of runoff resulting from a rainstorm and highly variable depending on above catchment specific factors.K = Runoff (mm)Rainfall (mm)

Design Model for Catchment : Cultivated Area Ratio

C:CA calculation for crop production systemsRule: WATER HARVESTED = EXTRA WATER REQUIRED

Interpolating the above we obtainCWR – Design Rainfall________________ = CDesign Rainfall X Runoff Coeff X Eff Factor CA

N.B Runoff coeff is proportion of rainfall which flows along ground as surface runoff (ranges between 0.1 and 0.5)Efficiency factor takes to account the inefficiency of uneven distribution of water within field as well as evaporation losses and deep percolation (ranges between 0.5 and 0.75)

Example on C:CA calculation for crop productionClimate: Semi-Arid

RWH System: External Catchment (e.g. trapezoidal bunds) Crop: 110 day Sorghum • Crop Water Requirement = 525 mm• Design Rainfall = 375 mm (P = 67%)• Runoff Coefficient = 0.25• Efficiency Factor = 0.5

C = (525-375)CA (375x0.25x0.5)i.e: The catchment area must be 3.2 times larger than the cultivated area. In other words, the catchment: cultivated area ratio is 3.2:1.Comment: A ratio of approximately 3:1 is common and widely appropriate.

C:CA calculation for tree systems• C:CA is difficult to determine for systems where trees

are intended to be grown• Only rough estimates are available for the water

requirements of the indigenous, multi-purpose species commonly planted in WH systems.

• Trees almost exclusively grown in micro-catchment systems where it is difficult to determine which proportion of the total area is actually exploited by the root zone

• Therefore, its considered sufficient to estimate only the total size of the micro-catchment (MC)

Example: Microcatchment system (Negarim microcatchment) for trees MC = RA x WR - DRDR – K - EFF

where: MC = total size of microcatchment (m2)RA = area exploited by root system (m2)WR = water requirement (annual) (mm)DR = design rainfall (annual) (mm)K = runoff coefficient (annual)EFF = efficiency factorAs a rule of thumb, it can be assumed that the area to be exploited by the root system is equal to the area of the canopy of the tree.

Example: Semi-arid area, fruit tree grown in Negarim microcatchment Annual water requirement (WR) = 1000 mmAnnual design rainfall (DR) = 350 mmCanopy of mature tree (RA) = 10 m2

Runoff coefficient (K) = 0.5Efficiency factor (EFF) = 0.5

Total size MC = 10 x {(1000-350)/(350 x 0.5 x 0.5)} = 84m2 As a rule of thumb, for multipurpose trees in ASAL, the size of the microcatchment per tree (C and CA together) should range between 10 and 100 m2, depending on the aridity of the area and the species grown. Flexibility can be introduced by planting more than one tree seedling within the system and removing surplus seedlings at a later stage if necessary.

C:CA calculation for systems for rangeland and fodder• In most cases it is not necessary to calculate the ratio

C:CA for systems implementing fodder production and/or rangeland rehabilitation

• As a general guideline, a ratio of 2:1 to 3:1 for microcatchments (which are normally used) is appropriate.

E. FACTORS TO CONSIDER BEFORE SELECTING A WH1. Social , economic & cultural considerations

Most communities have coping mechanisms Carry out a PRA priority ranking and consider Alternatives sources, use , cost and access to

water Consider quality, quantity and operational cost Timing and initial cost of investment

2. Technical considerations Slope >5% large volume of work or small area Soils- should be suitable for irrigation. Should not

have sandy texture Costs – affects quantity of earth works

Design and Construction of Negarim microcatchments

• Negarims are derived from the Hebrew word “Negev” meaning runoff. It was popularized in Israel negev desert. Negarims are appropriate in arid conditions with as low as 100-150mm rainfall pa.

• Structure- negarims are diamond shaped basins surrounded by small earth bunds with an infiltration/retention ditch at the lowest corner.

• Suitability- negarims are with soil that have at depth of 1.5-2m for adequate water storage, uneven topography and a slope of 1%- 5%

• Use- runoff collected and stored in the infiltration/retention ditch are suitable for growing trees or bushes for re-vegetation and are also as land reclamation and soil conservation tools.

• Size- size of each unit is usually based on type and no of trees to be established, however area range between 10-100 sqm

• Bund height- depends on the %slope and area of the negarim unit but range between 25-60cm on a 1-5%slope.

• The spacing and area of the trees/ fruit crop is calculated from the C:CA to meet optimum requirement during seasons of low rainfall

• Compaction- The bund to hold water and reduce risk of break must be compressed with feet or any heavy equipment

• Grass is planted on the bund to stabilize it

Negarims under construction in different parts of Kenya

Taita Taveta district Taita Taveta district

Taita Taveta district Turkana district

• Negarim MC typical ground appearance

• Recommended Bund height for different slopes (field experience)

Micro catchments area Bund height (cm)

2-3% slope 4-5% slope

10-40sqm 25cm 35-45cm

40-60sqm 25-35cm 45-55cm

60—100sqm 35-60cm 60cm

>100m2 40-60 Not recommended

V-Shaped micro-catchments Sometimes, open-faced V shaped MC may be constructed to allow surplus water to overflow

• Setting up the control contour for negarim micro-catchments• contour line using simple survey equipment are used to lay the

control contour. MC are established after this

Construction• Construct a infiltration/retention ditch above the contour

if there a risk of excess flow• Tree seedling are planted at the onset of the rains in the

infiltration ditch one at the bottom and another a step above the ditch floor to reduce risks of water logging.

• Negarims are appropriate for fruit trees, re-vegetation of degraded lands/ASAL rehabilitation. Established area must therefore be protected from livestock until the trees establish and pasture re-seeding occurs

Zai pits micro-catchments• Zai pits are traditional land rehabilitation technologies originally

practiced in Burkina Faso. These are small pits 60cm long by 60cm wide by 60cm deep dug in ASALs and where degraded soils are prevalent.

• Pits are dug during the dry season. Manure placed at the bottom of the pit and fast maturing dry land crops planted at the start of the rains. The pits collect and concentrate water at the root system guarantee conservation and availability.

• Zai pits are effective with small cultivated areas. Useful in ASAL zone with rainfall of 200-700mm and where soil hardpan and poor infiltration is common. Slope can be up to 2%on any uneven topography

• C:CA ratio. Catchment area is the pit and immediate surrounding of the crop. On average, C:CA ratio range between 1:1 to 3:1 depending on the level of aridity and the crop being planted

Zai pits constructed in Kilifi district

Construction of Zai pits• Zai pits may not necessarily follow the

contour• Excavate the pits and place the excavated

earth immediately down the slope of the pit to allow water to flow in. Manure is placed at the bottom of the pit.

• Zai pits can be used for over 4 seasons if properly managed before being re-done

• Mulch is recommended on the floor of the pit to conserve moisture and increase organic content

CONTOUR BUNDS FOR TREES PRODN• Contour bunds/ridges for trees establishment are

simplified MC which can easily be mechanized, more like terraces but target to retain as much runoff as possible

• Suitable in ASAL with rainfall of 200-550 mm pa and soils- 1.5-2m depth for food crops. For fodder an even shallower depth is ok

• Slope is up to 5% with a even topography. For effective infiltration and retention for trees, it recommended that infiltration pits/retention ditches are dug at the base of the contour. Bunds are spaced at 5-10m on slopes of up to 2%, while bund height range between 25-60cm with a base of 50-75cm depending on the slope

• It is imperative to always calculate the C:CA ratio on identified sites in order to maximize on runoff collection and ensure management of the water harvest is properly invested.

• Identity and measure the Catchments area. Where excessive runoff is likely to occur, a retention/diversion ditch at the top of the contour is necessary. However, in case the catchment may not yield sufficient runoff to meet the water balance of the crop/fodder, runoff from neighboring catchments maybe directed/intercepted towards cultivated area to meet the crops/fodder water requirements using diversion channels

• Layout; - contour bunds will consist of a series of parallel or almost parallel earth bunds approximating the contour at 5-10m apart depending on the slope.

• Correct layout is necessary for stability of the system. Bunds have to be compacted with optimal moisture to ensure stability. Accurate leveling of the tip of the bund to the same elevation will reduce risk of breakage. Stone pitching at the tip of the bund to reduce risk of erosion is also recommended

• Grass established on the bund guarantee stability for a longer time and it is important that the area be cropped is protected from livestock until it is fully established

Contour bunds shortly after rains in Garissa district

Illustration of contour bunds

Contour Ridges• Contour ridges, sometimes called contour furrows or micro-

watersheds, are used for crop production. • This is again a micro-catchment technique. Ridges follow the contour

at a spacing of usually 1 to 2 metres. • Runoff is collected from the uncultivated strip between ridges and

stored in a furrow just above the ridges. Crops are planted on both sides of the furrow.

• The system is simple to construct - by hand or by machine - and can be even less labour intensive than the conventional tilling of a plot.

• The yield of runoff from the very short catchment lengths is extremely efficient and when designed and constructed correctly there should be no loss of runoff out of the system.

• Another advantage is an even crop growth due to the fact that each plant has approximately the same contributing catchment area.

• Contour ridges for crops are not yet a widespread technique. They are being tested for crop production on various projects in Africa.

Contour ridges field layout

Contour ridges in Marigat, Baringo

Semi circular bunds• Semi-circular bunds are earth embankments in the shape of a

semi-circle with tips of the bund on the contour. Mainly applicable to rangeland rehabilitation, fodder production or tree growing.

• Catchment characteristics; medium slope, permeable soils, low vegetation. Water harvest maybe insitu or exsitu. Compared to other WH structures, Semi circular bunds are efficient in rangeland fodder production and rehabilitation.

• Suitability;- effective with rainfall 200-750mm, soil not necessarily deep, slope up to 2%, (at higher elevation of up to 5% shorten the slope), even topography.

• Configuration – establish bunds with approximate radii of up to 20m constituted in a staggered line.

• Recommended C:CA ratio range between 3:1 -5:1 for fodder production upto 2% slope. At higher slopes bunds size and distance reduced.

Construction: Stake out using survey equipment the tip of control contour and using string stake the continuous contour

• The bund is protected at the lower tip by pitching a layer of stones

Management• Enclose the area from livestock to allow

re-generation and seed dispersal to increase productivity

• Repair bunds where breakage occur and establish a tree crop to increase water retention efficiency

TRAPEZOIDAL BUNDS-Design and construction• TBs are used to enclose large area of up to 2ha to

impound water. TBs is derived from the layout- trapezoidal. 3 sides of a plot are enclosed while the upslope is left open to allow runoff. Crops are planted within the enclosed area with excess being discharged at the tips of wings. TBs are efficient water harvesting structure for crop production in the ASALs.

• Suitability- crops, fodder, trees can be grown in arid area with a annual rainfall a low as 200-500mm pa. Recommended soil type - clay loam that have retention capacity. Slope up to 2%. At higher elevation earth work increases.

• Each TB consist of base bund connected to two wing walls extending upslope at an angle of 135 degrees. The size of enclosed area depend on the slope and can vary from 0.1-1ha.

• TBs may be built as single unit or a set of not more 2 TBs lines in a staggered manner with distance between I trapezoidal tip to the next 20-30m. More than 2 rows of TBs mean less water to the 3rd and subsequent rows

• TBs are efficient on low slope upt 1.5%. At higher slopes quantity of earth work is huge and efficiency of water use declines.

Construction• Establish the slope and catchment size• Decide on the maximum length of the bund. This is

dependent on the catchments characteristics (runoff), physical conditions of the soil, socio-economic factors of the beneficiaries

• Using simple survey equipment, peg the wings and base of the embankment

• Calculate the height and base of the bund and peg appropriately

• Mark tips of the bunds and ensure they are shaped with an extension lip to prevent erosion.

• If catchment is too big its advisable to construct a diversion ditch. If catchment too small interception ditches constructed to lead water into the bunds

Source; FAO Water Harvesting Manual 2

Layout of trapezoidal bunds and a

completed structure in Makueni district

Trapezoidal bunds Field arrangement of a large catchment area

Recommended dimensions of one TB unit%slope Length of

base bund (m)

Length of wing wall (m)

Distance between tips (m)

Earth work per bund

Cultivated area per bund (sqm)

0.5% 40 114 200 355 9600

1.0% 40 57 120 220 3200

1.5% 40 38 94 175 1800

Consequences of poor compaction of TBs

Flood water spreading bunds• Applied in situations where TBs are not suitable • Depends on seasonal water from a stream or water

course• Very effective in areas where slope is max. 1%. • Water is directed from the seasonal river/flood area and

impounded by huge soil bunds • The bund are build at intervals and allow for overflowing

from one to the other and an exit for excessConstruction• Bunds must be heavily compacted. • Water intake from seasonal river is designed and

constructed• With land slope approx. 1%, large areas of up to 10 ha

can be established on one bund.

Intake from stream

Field layout of water spreading bunds

Permeable rock dams• Permeable rock dams are flood water farming

techniques where runoff is spread at the valley bottoms for improved crop production. Developing gullies are healed at the same time

• The structures are built with rocks across developing gullies to form long, low dam walls . Permeable rocks are a form of terrace used to heal previously productive land. It is highly labour intensive, moving large quantities of stone; require group participation and where possible is mechanized

• Suitability- area with rainfall 200-750mm, all types of soil- poor soil easily treated with sediment, slope; below 2%, topography wide and shallow valley beds

• The central part of the dam is perpendicular to the water course, while the extension of the wall to either side of the curve back following the contour.

• The runoff is initially concentrate at the gully, then spread out. The gully fills with fertile deposits

• For an effective system a series of permeable dams are built to progressively increasing in size downstream along the same water course.

Land slope Spacing between dams

Volume of stone /ha cultivated

0.5% 140m 70

1.0% 70m 140

1.5% 47 208

2.0% 35 280

Permeable rock check dams for gully control and catchment conservation in a water project site, Makueni district

• C:CA are not easily determined for permeable rock catchments, however it is advisable to estimate the volume of water likely; to determine the size of the dams

• Design- the main part of the dam can be upto 0.7m. With the gully being filled before dam construction may reach 2m above the gully floor. The dam wall or spreader will depends on the slope but can be as long as 1000m across wide. But most commonly 50-200m

• Dam wall is built with loose stones carefully arranged with large boulders forming the framework. The slope sides 3:1 on downstream, with 1:1 -2:1 for the upstream. Need to protect the valley floor from undermining.

• Design variation- vary depending on the % slopes of the land. Where it is flat, dams may be nearing straight. Gabion maybe used on the spillway and part of the gully to stabilize. Downstream must be protected with stone pitching from scorching.

• For a series of dams, an appropriate vertical interval (VI) is selected. In principle, the height of one dam approximates the base of the next dam. Thus for a dam of say 70cm,VI is 70cm.always use a line level to measure.

• Horizontal spacing (HI) between dams can be determined by

HI= (VI*100)/%slope.Example 1% slope, HI= 0.7*100/1% =70m2% slope, HI =0.7*100/2% =35M

F. ACCOMPANYING TECHNOLOGIES TO WH FOR CROP MGT

Soil & Water Conservation Structures• Soil conservation structures are commonly used to

manage erosion problems. Extensive investigations show that erosion is increased as water runs along steeper slopes over long distances

• The principal behind all soil conservation structures is to:

a) reduce length of slope

b) reduce steepness of slope

Terraces and other structures are spaced according to a vertical and horizontal interval. • The vertical interval (VI) is the vertical distances

between them• The horizontal interval (HI) is their horizontal spacing

between them• The vertical interval is given by

VI=0.3x(S+2) m4where S = percent slope• The horizontal interval is a function of the vertical

interval and is given byHI=V I mS

Angles may be measured in degrees or percent.

1. How do we compare angles between the different forms?

By developing a relationship between degrees and percent slope using trigonometric relationships.

2) How do VI and HI vary with slope? VI increases with increasing slope as HI decreases.Note: • that going by the definition of slope, a 45o slope is

equivalent to 100% slope.• Values of VI do not change much for land slopes up to

20%. Beyond this slope, it is necessary to consider the effect of slope on terrace spacing.

• The VI and hence HI are not rigid but guidelines. When permanent row crops are grown, the VI can be adjusted to fit the crop recommended spacing.

Fanya Juu Design and constructionDescription

• This is the most common soil conservation method in both high and low rainfall areas of Kenya.

• It is mostly made manually.

• Made by digging a trench along the contour and heaping the soil uphill to form an embankment.

• Appropriate on slopes between 15-30%.

• Useful in semi-arid areas to harvest and conserve water.

General, slope-dependent dimensions of Fanya Juu terraces:Slope,

%VI,m

HI, m

Width,m

Depth, m

Channel area,

m

5 1.00 20 0.50 0.50 0.25

10 1.35 14 0.50 0.55 0.28

15 1.73 12 0.60 0.55 0.33

20 1.80 9 0.60 0.60 0.36

Layout and construction of fanya juu terraces

• Start by measuring the slope of the farm• Calculate the VI and HI as shown before• Lay and mark all positions of the trench with or without

gradient as required.• Mark out a berm position along the upper side of the

trench at 15-30cm distance• Mark ridge positions with pegs that guide development

of required minimum depth

Measuring slopes and marking contours using line level

NOTE: The gully in the left…. What do we do when the fanya juu has to cross such an area?

DEMONSTRATIONS: Field assembly of an A – Frame and field calibration as well as usage of the A – Frame to mark contours

A – Frames very precise but not recommended for layout of terraces in large areas….. WHY???

Can A- Frame be used to measure/determine slopes??

What are these people doing?

Discuss the above photos

StabilizationEnsure that constructed embankments are stable• The stability and effectiveness of soil structures

depends on the stability of the embankment.• During construction, ensure the soil forming the

embankment is compacted to reduce risks of breakage.

Soil embankment stabilized with napier grass. A grass adds value so that no part of the land is perceived as wasted.

• Establish vegetation from the first rainy season and ensure it is protected from livestock.

• Plant either single or a grass mixture in rows about 20 cm apart.

• Plants should be regularly trimmed and used either for animal feed or soil fertility enhancing through composting.

• All embanks should be planted with perennial grasses to stabilize the soil against erosion. Suitable stabilization material includes Napier grass, Signal grass , Donkey grass, Makarikari or Guinea grass.

• Root crops such as sweet potato and cassava SHOULD NEVER be planted on the embankment. Why?

Which soil & water conservation structure is this??

Establish regular maintenance of the structures at least once every year to:a) repair broken embankments.b) expand by adding more embankments to expand cultivated areac) replace dead grasses on embankment.

PLENARY DISCUSSION….1.Fanya Chini terraces• Trash lines• Stone lines• Any other soil & water conservation

structures not mentioned

Management of RWH structures• Protect from destruction by enclosing a RWH area until

it is fully established• Repair any damages as soon it occurs, lack of this will

create a weakness for water to break the RWH structures.

• Most bunds stabilize when planted with grass• Where quantity of target water for harvest cannot be

predicted or design rainfall likely to be exceeded or is too low include diversion or collection ditches as appropriate

• Where communities participate in the development and realize the benefits of RWH; sustainability, replication and up scaling will be guaranteed.

• Establish vegetation /trees around or within the RWH structures to increase soil stability, organic matter and reduce evapotranspiration giving lengthy life.

• Put an economic value together with social good to RWH

• THIS IS NOT “THE END”……….………………

• BUT THE “BEGINNING” OF IMPROVING FOOD SECURITY THROUGH EFFECTIVE RAINWATER HARVESTING AND MANAGEMENT TECHNOLOGIES!!

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