Arid Zone Hydrology

1. Arid Zone Hydrology

2. PhD Work 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 Cyclical variations Inter. Model Complicity Complex

10. RAINFALL CANOPY EVAPORATION TRANSPIRATION THROUGHFALL SOIL EVAPORATION NET RAINFALL ROOT EXTRACTION 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

11. NET RAINFALL FREE SURFACE EVAPORATION OVERLAND FLOW CHANNEL FLOW INFILTRATION STREAM / AQUIFER INTERACTION 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)

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 !