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

Rainfall

Percolation

Soilsurface

Infiltration

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

Changes in Infiltration Capacity Over Time (Horton, 1933)

Time

Infi

ltra

tio

n C

apac

ity

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

Leaking Bucket

Storage S

Input A

Leakage B/S

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

Partial Area Concept in Drylands

Source areas of runoff produced by combination of: topography soils vegetation

Patchy over a catchmentConnectivity important