module 4 infiltration
TRANSCRIPT
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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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Factors which affect infiltration:
Biological factors-
Vegetation
Type (forb, grass, shrub, trees, etc.) Distribution
Shrublands
Grasslands
type of grass
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Factors which affect infiltration:
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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Some Important Soil Characteristics
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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)