2. Infiltration and Surface Water zDefinition of Infiltration zDifference between drylands and humid regions zFactors Affecting Infiltration zModelling Infiltration zRunoff Production

3. Infiltration zInfiltration yProcess of water entry into the soil through the soil surface zInfiltration Capacity yMaximum rate water is absorbed by the soil zInfiltration Rate yInfiltration occurring at less than capacity

4. Infiltration Capacity Controlled By: zSoil Surface Processes zSoil Profile Processes i.e. distinguish between infiltration and percolation

6. Factors Affecting Infiltration Capacity zRainfall zSoil Compaction zDepth of surface detention zSlope zCracks zCultivation zVegetation z Litter z Trampling z Soil Moisture z Temperature z Soil Porosity z Crusting z Soil Type z Urban Areas

7. Soil Surface zCan impose upper limit to infiltration zInfiltration capacity reduced by: ysurface compaction yfines blocking pores yfrost action zInfiltration capacity increased by: ycracks and fissures yslope

8. Vegetation zComplex effect on infiltration zReduces raindrop impact zImproves soil structure zRetards surface water movement zGround litter

9. Soil Water Movement

10. During Wetting zWetting front advances due to passage of water through transmission zone zTransmission zone becomes longer zMoisture only changes significantly in the wetting zone and wetting front

11. Changes in Infiltration Capacity Over Time (Horton, 1933)

12. During a storm: zInfiltration capacity decreases with time zDue to: yswelling of clays ysplash cause fines to block pores yreduction of flow processes in the soil

13. Green and Ampt (1911) Where f = infiltration capacity L = depth of wetting front K = effective hydraulic conductivity H o = depth of ponded water H f = capillary suction at wetting front

14. Leaking Bucket

15. Modified Green and Ampt Where f = infiltration rate (mm/s) A = steady infiltration rate due to gravity S = total infiltrated so far into suction store B = constant so B/S is suction component

16. Runoff zOverland flow yHortonian ySaturation Excess zVariable source areas yTemperate model yDryland model

17. Runoff zOverland Flow yWater that fails to infiltrate and travels over the ground surface towards a channel zHortonian Overland Flow yInfiltration excess overland flow i.e. rainfall intensity exceeds infiltration capacity zSaturation Excess yRunoff caused by rain falling into saturated areas and therefore cannot infiltrate

18. Hortonian Infiltration Common with: zThin vegetation cover zThin soils zFrozen soils zTracks zSemi-arid and arid areas

19. Horton’s assumptions zInfiltration capacity can be measured to calculate overland flow zSoil surface acts as plane of separation zSheet of water can accumulate on and flow over this hypothetical surface

20. Hewlett and Hibbert (1967) zCouldn’t see Hortonian overland flow zAll rainfall infiltrated zSaw overland flow as an rapid expansion of the channel network

21. Variable Source Areas zDeveloped for humid areas zSaturated areas produce the storm runoff zWater table rises over an expanding area zSpreads up low order tributaries, then unchannel swales and gentle footslopes zRelated to geology, topography, soils, rainfall, vegetation

22. Application to drylands Difficult to apply variable source model to drylands because: yrunoff produced by Hortonian overland flow ylong dry periods between rainfall ytransmission losses high

23. Partial Area Concept in Drylands zSource areas of runoff produced by combination of: ytopography ysoils yvegetation zPatchy over a catchment zConnectivity important