Module 4 Infiltration

8/11/2019 Module 4 Infiltration

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Infiltration & Soil Water


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Definitions

Infiltration: process by which water enters the soilsurface

Infiltration capacity: maximum rate at which

water can enter the soil Percolation (Soil Hydraulic Conductivity):

movement of water through soil (saturated andunsaturated flow)

Soil Water: water held in soil pores

Plant available water


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Factors which affect infiltration:

Soil type

Texture

Depth

Initial soil moisture content

Rainfall

IntensityAmount/duration/distribution

Quality of water


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Biological factors-

Vegetation

Type (forb, grass, shrub, trees, etc.) Distribution

Shrublands

Grasslands

type of grass


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How does management influence infiltrationprocess and soil moisture??


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Management Impacts


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Infiltration Capacity

Diminishes over time during an infiltration event

Pore size generally decreases with depth

Decrease in matric potential

Air entrapment at depth

Influence of pore size:

Where:

R = pore radius

p = pressure drop

= viscosity

L

pRQ

8

4


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Some Important Soil Characteristics


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Saturated vs. Unsaturated Conditions

Potential Gradient

Saturated soil: moving force is the gradient ofa positivepressure potential

Unsaturated: moving force is the direction of anegativematric potential


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Saturated vs. Unsaturated

Hydraulic Conductivity

Saturated:

conductivity is maximal and constant.

Most conductive are soils with large, continuouspores. Least: those with micropores

Unsaturated:

conductive portion of the soil decreases (A)

increased tortuosity

Ks decreases more steeply for sandier soils than forclayey soils


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Darcys Law (1856)

Experimentalfindings using a soilcolumn

Discharge rate (Q) isproportional to thecross-sectional areaand to the hydraulicdrop (H)

Q is inverselyproportional to thelength of the column

L

HKAQ


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Hydraulic Conductivity in Soils

Representative Texture Ks (mm/h)

Sand 210.0

Loamy Sand 61.0

Fine Sandy Loam 26.0

Loam 13.0

Sandy Clay Loam 4.3

Clay Loam 2.3

Clay 0.6


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Graph of infiltration rate


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Cumulative Infiltration


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Thinking About Infiltration Rates

When doesinfiltration occur?

When does

infiltration rate =infiltration capacity?

When does pondingor runoff occur?


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Hortons Infiltration Capacity f

Horton (1933 - 1940)studied the response ofdifferent soils toapplication of water at

varying rates

Rate of rainfall mustexceed the rate ofinfiltration andantecedent condition is animportant parameter

Sand > Silt > Clay


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Hortons Infiltration Concept

f(t) = Rate of water loss into soil

f = fc + (fo - fc) exp (-kt)

fc = final rate value

fo = initial rate value

K = decay rate

Can integrate to get

F(t) = Vol of infiltration


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Hortons Eqn

VolArea fdt A [fc (f0 fc )ekt]dt

F


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Determination of infiltration

Although infiltration changes with timeduring a storm, it is accepted as constantfor hydrograph analysis problems.

The W index method

(Total precipitation surface runoff)/duration ofrainfall


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Determination of infiltration

The index method gives the averagevalue above which the rainfall equals thesurface runoff.


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Runoff of 75 mm

0

5

10

15

20

25

30

35

40

1 2 3 4 5

Rainfall mm/hr

Rainfall mm/hr


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How to Measure Infiltration?

Double-ring infiltrometer

Inner = 30cm

Outer = 50cm

Measure the inner; maintaina head

Issues??

Disc Permeameter and

Tension Infiltrometer Water held under tension

Passes through disc at soilsurface; measure rate

Wilkipedia.com

http://www.cig.ensmp.f
http://en.wikipedia.org/wiki/Image:Double_ring.JPGhttp://upload.wikimedia.org/wikipedia/en/3/36/Ponded_disc.jpghttp://upload.wikimedia.org/wikipedia/en/3/36/Ponded_disc.jpghttp://en.wikipedia.org/wiki/Image:Double_ring.JPG


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Double Ring Infiltrometer

Measure rate of fall in inner ring

Infilration

http://www.alwi.com/wastewater.php


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How to Measure Infiltration?

Rainfall Simulators:

measures infiltration under rainfall

conditions Drop formers

Rotating boom

Variable Intensity


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Computer Controlled

Intensities:

13 178 mm/hr 2m by 6m plot

Oscillating boom

4 VeeJet nozzles

Rainfall energy closeto natural rainfall

Variable Intensity Rainfall Simulator

Walnu t Gulch Rainfall Simu lator


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Multiple Intensities

0

50

100

150

200

0 10 20 30 40 50 60

time (min)

rate(mm/hr)

RainfallRunoffInfiltration = rainfall - runoff

When multiple

rainfall rates are

applied,

the steady state

infiltration rate

frequently increaseswith increasing

rainfall rate


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0

20

40

60

80

100

120

0 50 100 150 200

intensity (mm/hr)

infiltration(mm/h

r)

Intensity - Infiltration

Sandy Loam Upland Deep

Loamy sand

Good condition

High variability

Ecological

Site

f(mm/hr)

ave (range)

SLUD 80 (55-125)


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Intensity - Infiltration

Clay Loam Upland

Cobbley clay loam

Fair condition

Much Less Variability

Ecological

Site

f(mm/hr)

ave (range)

SLUD 80 (55-125)

LiS 64 (40-85)

CLU 24 (18-36)SLUD (Sandy Loam Upland Deep)LiS (Limey Slopes)CLU (Clay Loam Upland)

0

20

40

60

80

100

120

0 50 100 150 200

intensity (mm/hr)

infiltration(mm/hr)

Module 4 Infiltration

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