INFILTRATION

Infiltration and Surface Water

• Definition of Infiltration• Difference between drylands and humid

regions• Factors Affecting Infiltration• Modelling Infiltration• Runoff Production

Infiltration

• Infiltration – Process of water entry into the soil through the

soil surface• Infiltration Capacity

– Maximum rate at which water is absorbed by the soil

• Infiltration Rate– Infiltration occurring at less than capacity

Infiltration Capacity Controlled By:

• Soil Surface Processes• Soil Profile Processes

i.e. distinguish between infiltration and percolation

Rainfall

Percolation

Soilsurface

Infiltration

Factors Affecting Infiltration Capacity

• Rainfall• Soil Compaction• Depth of surface

detention• Slope• Cracks• Cultivation• Vegetation

• Litter• Trampling• Soil Moisture• Temperature• Soil Porosity• Crusting• Soil Type• Urban Areas

Soil Surface

• Can impose upper limit to infiltration• Infiltration capacity reduced by:

– surface compaction– fines blocking pores– frost action

• Infiltration capacity increased by:– cracks and fissures– slope

Vegetation

• Complex effect on infiltration• Reduces raindrop impact• Improves soil structure• Retards surface water movement• Ground litter

Infiltration• General

– Process of water penetrating from ground into soil

– Factors affecting• Condition of soil surface,

vegetative cover, soil properties, hydraulic conductivity, antecedent soil moisture

– Four zones• Saturated, transmission,

wetting, and wetting front

depth

Wetting Zone

TransmissionZone

Transition ZoneSaturation Zone

Wetting Front

During Wetting

• Wetting front advances due to passage of water through transmission zone

• Transmission zone becomes longer• Moisture only changes significantly in the

wetting zone and wetting front

Changes in Infiltration Capacity Over Time (Horton, 1933)

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)

LHLHKf 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

SBAf

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 channel• Hortonian 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 cover• Thin soils• Frozen soils• Tracks• Semi-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 flow• All rainfall infiltrated• Saw overland flow as a rapid expansion of the

channel network

Variable Source Areas

• Developed for humid areas• Saturated areas produce the storm runoff• Water table rises over an expanding area• Spreads up low order tributaries, then

unchannel swales and gentle footslopes• Related 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 catchment• Connectivity important

Infiltration

• Infiltration rate– Rate at which water enters the soil at the surface (in/hr

or cm/hr)• Cumulative infiltration

– Accumulated depth of water infiltrating during given time period

t

dftF0

)()(

)(tf

dttdFtf )()(

Quote of the today

"There is only one good, that is knowledge” “There is only one evil, that is ignorance.“ Socrates (470-399 BC)

InfiltrometersSingle Ring Double Ring

http://en.wikipedia.org/wiki/Infiltrometer

Infiltration Methods

• Horton and Phillips – Infiltration models developed as approximate

solutions of an exact theory (Richard’s Equation)• Green – Ampt

– Infiltration model developed from an approximate theory to an exact solution

Hortonian Infiltration

ktcc effftf )()( 0

f0 initial infiltration rate, fc is constant rate k is decay constant

Hortonian Infiltration

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2

Time

Infil

trat

ion

rate

, f

k1

k3

k2

k1 < k2 < k3

fc

f0

Green – Ampt Infiltration

Wetted Zone

Wetting Front

Ponded WaterGround Surface

Dry Soil

0h

L

n

i

z

LLtF i )()(

dtdL

dtdFf

zh

Kz

Kf

fzhKqz

MoistureSoilInitialFront WettingtoDepth

i

L

Green–Ampt Infiltration

• Apply finite difference to the derivative, between – Ground surface– Wetting front

Kz

Kf

Wetted Zone

Wetting Front

Ground Surface

Dry Soil

L

i

z

0,0 z

fLz ,

KL

KKz

KKz

Kf f

00

FL

LtF )(

1

FKf f

Kz

Kf

1

LK

dtdL f

1

FKf f

dtdLf

Green–Ampt Infiltration

LtF )(

Wetted Zone

Wetting Front

Ground Surface

Dry Soil

L

i

z

LdL

dLdtK

f

f

CLLtKff

)ln(

Integrate

Evaluate the constant of integration

)ln( ffC

0@0 tL

)ln(L

LKtf

ff

Green–Ampt Infiltration

)ln(L

LKtf

ff

)1ln(f

fFKtF

1

FKf f

Wetted Zone

Wetting Front

Ground Surface

Dry Soil

L

i

z

Nonlinear equation, requiring iterative solution.

Soil Parameters

• Green-Ampt model requires – Hydraulic conductivity (K), Porosity (n), Wetting Front

Suction Head ()Soil Class Porosity Effective

Porosity Wetting Front

Suction Head

Hydraulic Conductivity

n e K (cm) (cm/h) Sand 0.437 0.417 4.95 11.78 Loam 0.463 0.434 9.89 0.34 Clay 0.475 0.385 31.63 0.03

re n

ees )1(

e

res

Effective saturation

Effective porosity

Effective porosity is that portion of the total void space of a porous material that is capable of transmitting a fluid. Total porosity is the ratio of the total void volume to the total bulk volume.

Ponding time

• Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)

Ponding Time

• Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate)

if ptiF *

1

FKf f

1

* p

f

tiKi

)( KiiKt f

p

Potential Infiltration

Actual Infiltration

Rainfall

Accumulated Rainfall

Infiltration

Time

Time

Infil

trat

ion

rate

, fC

umul

ativ

e In

filtr

atio

n, F

i

pt

pp tiF *

Example

• Silty-Loam soil, 30% effective saturation, rainfall intensity of 5 cm/hr

30.0/65.0

7.16486.0

e

e

shrcmK

cm

340.0)486.0)(3.01()1( ees

340.0*7.16

min 10.18hr17.0)65.00.5(0.5

68.565.0)(

KiiKt f

p

Assume that the time evolution of the infiltration capacity for a given soil is governed by Horton's equation (Note that this equation assumes an infinite water supply at the surface, that is, it assumes saturation conditions at the soil surface).

(1)

For this soil, the asymptotic or final equilibrium infiltration capacity is fc = 1.25 cm/h; and the initial infiltration capacity is fo = 8 cm/h. The rate of decay of infiltration capacity parameter is k = 3 h-1. For the precipitation hyetograph tabulated below, carry out a complete infiltration analysis, including evaluation of cumulative infiltration and rate of production of precipitation excess, + v.

Assignment

Time (min) Rainfall Intensity (cm/h)10 1.520 330 840 550 460 370 0.8