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Infiltration and Surface Water
Definition of InfiltrationDifference between drylands and
humid regionsFactors Affecting InfiltrationModelling InfiltrationRunoff Production
Infiltration
Infiltration Process of water entry into the soil
through the soil surfaceInfiltration Capacity
Maximum rate water is absorbed by the soil
Infiltration Rate Infiltration occurring at less than capacity
Infiltration Capacity Controlled By:
Soil Surface ProcessesSoil Profile Processes
i.e. distinguish between infiltration and percolation
Factors Affecting Infiltration Capacity
RainfallSoil CompactionDepth of surface
detentionSlopeCracksCultivationVegetation
LitterTramplingSoil MoistureTemperatureSoil PorosityCrustingSoil TypeUrban Areas
Soil Surface
Can impose upper limit to infiltrationInfiltration capacity reduced by:
surface compaction fines blocking pores frost action
Infiltration capacity increased by: cracks and fissures slope
Vegetation
Complex effect on infiltrationReduces raindrop impactImproves soil structureRetards surface water movementGround litter
Soil Water Movement
Water Content
Dep
th
WWetting Fromt
Wetting Zone
Transmission Zone
Transition Zone
Saturated Zone
During Wetting
Wetting front advances due to passage of water through transmission zone
Transmission zone becomes longerMoisture only changes significantly
in the wetting zone and wetting front
During a storm:
Infiltration capacity decreases with time
Due to: swelling of clays splash cause fines to block pores reduction of flow processes in the soil
Green and Ampt (1911)
L
HLHKf c )( 0
Where f = infiltration capacity
L = depth of wetting front
K = effective hydraulic conductivity
Ho = depth of ponded water
Hf = capillary suction at wetting front
Modified Green and Ampt
S
BAf
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
Runoff
Overland flow Hortonian Saturation Excess
Variable source areas Temperate model Dryland model
Runoff
Overland Flow Water that fails to infiltrate and travels
over the ground surface towards a channelHortonian Overland Flow
Infiltration excess overland flow i.e. rainfall intensity exceeds infiltration capacity
Saturation Excess Runoff caused by rain falling into saturated
areas and therefore cannot infiltrate
Hortonian Infiltration Common with:
Thin vegetation coverThin soilsFrozen soilsTracksSemi-arid and arid areas
Horton’s assumptions
Infiltration capacity can be measured to calculate overland flow
Soil surface acts as plane of separation
Sheet of water can accumulate on and flow over this hypothetical surface
Hewlett and Hibbert (1967)
Couldn’t see Hortonian overland flowAll rainfall infiltratedSaw overland flow as an rapid
expansion of the channel network
Variable Source Areas
Developed for humid areasSaturated areas produce the storm runoffWater table rises over an expanding areaSpreads up low order tributaries, then
unchannel swales and gentle footslopesRelated to geology, topography, soils,
rainfall, vegetation
Application to drylands
Difficult to apply variable source model to drylands because: runoff produced by Hortonian overland
flow long dry periods between rainfall transmission losses high