1. Arid Zone Hydrology

2. PhD Work Title Rainwater Harvesting in Arid and Semi-Arid Areas
Rainfall Generator Model
WH Techniques
Title
Rainfall - Runoff Model

3. Content Distinctive Features of Arid Zone Hydrology
Hydrological Processes in Arid Zones
Problems of Data Availability

4. 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 aquifer
Deep roots

5. Distinctive Features Features underline the need for modeling approach
Plant Cover:
Generally Sparse
Xerophytes
Ephemeral Grasses
Small leafy plants
Xerophytes are able to survive with little available water
There is a wide variation in the soil water balance
Cacti

6. 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)

7. 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)

8. 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)

9. 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
Zero (Desert)
Time-invariant
Inter.
Cyclical variations
Complex
Model Complicity

10. 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
EVAPORATION
TRANSPIRATION

11. 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

12. 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:
From point to point along a channel
The degree of saturation of the alluvium
A major source of potential groundwater recharge
A major cause of the differences in runoff depths

13. Hydrological Processes
Runoff Processes
Partial-area Runoff:
These processes are attributed to:
The presence of saturation in valley bottoms
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

14. 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

15. 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
New South
Wales
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)

16. 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

17. 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 !!!!)

18. 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

19. Thank You !