By: Muamaraldin Mhanna

Promoter: Prof. W. Bauwens

Arid Zone Hydrology

PhD Work

Rainfall Generator Model WH Techniques

Rainfall - Runoff Model

Title

Rainwater Harvesting in Arid and Semi-Arid Areas

Content

1. Distinctive Features of Arid Zone Hydrology

2. Hydrological Processes in Arid Zones

3. Problems of Data Availability

Distinctive Features

Features underline the need for modeling approach

Arid zones are often in a delicate hydrological balance

Rainfall tends to be more variable in both space and time

Phreatophytes:

- Groundwater recharge

- Channel transmission losses

Shallow aquiferDeep roots

Distinctive Features

Features underline the need for modeling approach

Plant Cover:

- Generally Sparse

o Xerophytes

o Ephemeral Grasses

o Small leafy plants

- Xerophytes are able to survive with little available water

- There is a wide variation in the soil water balance

Distinctive Features

Features underline the need for modeling approach

There is a relative absence of organic matter:

* Interception * Infiltration

* Evapotranspiration * Runoff

The surface soil largely is the first point of contact by rainfall

The water table is typically below stream beds and disconnected from

the surface drainage system

(Flood Events)

Hydrological Processes

Precipitation

High spatial and temporal variability

Significantly different in various regions:

Tropical Areas (e.g. USA) Summer

Elsewhere (e.g. China) Winter

Unique rainfall characteristics:

- Rain storms are random events (small frequency of occurrence)

- More drought conditions lower magnitude of frequent storms

- Rain storms result from convective type rainfall events

(short duration and high intensities)

Hydrological Processes

Precipitation

The significance of spatial and temporal variability of rainfall on the

hydrological models :

- Large uncertainties in runoff estimation

- Produces more surface runoff

(Infiltration-excess runoff )

- Explain the decreasing of runoff coefficients within increasing slope

- Induces variability in the phenology of vegetation

(Leaf area index and Evapotranspiration)

Hydrological Processes

Interception

Generally:

- Site scale Often highly significant

- Catchment scale: - Runoff is due to low intensity rainfall

- Gross variations in the density of plant cover

Simple

Inter.

Complex

Cyclical variationsTime-invariant

Zero (Desert)

Hydrological Processes

Evapotranspiration

Evaporation from bar soil is important than transpiration:

- Greater area of bare soil

- Frequency of small rainfall events

At least 95% of the precipitation:

- Periods between rainfall events are longer

- Soil water stores are empty at the beginning of the storm

- Decrease the impact of errors in estimating soil water losses

THROUGHFALL

NET RAINFALL

ROOT

EXTRACTION

SOIL EVAPORATION

RAINFALL

CANOPY

EVAPORATIONTRANSPIRATION

Hydrological Processes

Runoff Processes

The lack of vegetation cover :

- Large reduction in infiltration capacity

- Generation of extensive overland flow

Overland flows converge on channel network, producing:

Flood Hydrographs

- Extremely rapid rise times (e.g. 15–30 minutes)

CHANNEL

FLOW

OVERLAND FLOW

INFILTRATION

FREE SURFACE

EVAPORATION

STREAM / AQUIFER

INTERACTION

NET RAINFALL

Hydrological Processes

Runoff Processes

Transmission Loss:

- Losses from the flood hydrograph is reducing the flood volume

(The flood wave moves downstream)

- The transmission loss is varied:

o From point to point along a channel

o The degree of saturation of the alluvium

- A major source of potential groundwater recharge

- A major cause of the differences in runoff depths

Hydrological Processes

Runoff Processes

Partial-area Runoff:

- These processes are attributed to:

o The presence of saturation in valley bottoms

o Potential rates of infiltration prevent any runoff

- The predominant runoff mechanism is Hortonian overland flow

- Occurs in several regions under Specific conditions

- It needs to be considered in arid zone runoff modelling

- The characteristics of each region need to be carefully assessed

Hydrological Processes

Wadi Flows

Typical for the semi- to hyper-arid climatic zones

Particular rainfall-runoff features of wadi flows:

- Rain storm floods characterized by :

* Sudden occurrence * Rapid rise and fall

* Great yearly variation * High sediment loads

- Flash floods are significant

- Very small fraction of rainfall becomes runoff

- It is intermittent and tends to be lost before reaching the sea

Hydrological Processes

Rapid Time to Ponding and Onset of Runoff

Short times to ponding (< 10 minutes):

- The rapid onset of runoff after the start of rain

- The production of runoff with small depths of rain

Western (Arid) Runoff occurs:

>16 mm of rain

(storm event)> 5 mm of rain

(hour)

Eastern (Humid) Runoff occurs:

35 mm of rain

125 mm of rain

(Dry antecedent

conditions)

New South Wales

Hydrological Processes

Rapid Time to Ponding and Onset of Runoff

These significant differences point to:

- The need for specific care in modeling some arid zone rainfall-runoff

processes

- Avoidance of a blind acceptance of characteristics of the runoff

process in the humid zone

- Simple extrapolation from the humid zone models is likely to be

unsuccessful

Problems of Data Availability

The Availability of Data

The scarcity of data is the greatest problem in arid zone modeling

Routine stream gauging networks are typically sparse:

- The high cost of obtaining such data

- The low economic potential of most arid regions

(This situation is unlikely to change)

Data from very few regions have been used in published studies

(Few years to be available !!!!)

Problems Of Data Availability

The Accuracy of Data

The accuracy of arid zone stream flow data is generally low:

- Isolation of most stations, and difficulty of access in rainy periods

- The high variability and irregular occurrence of flow

- Lack of suitable natural control sections

- Difficulty of current metering with high sediment and debris loads

Local experience will continue to play a part in the designing and

modeling in the water management section