HOW MUCH WATER ENTERS THE
SOIL?
Infiltration
Precipitation
Evaporation
Infiltration
Runoff
What Factors Control Infiltration?
What Factors Control Infiltration?
A) Soil Characteristics
What Factors Control Infiltration?
A) Soil Characteristics
1) Grain Size (size of pores in soil)
Large Grains
Large Grains
Medium Grains
Large Grains
Medium Grains
Fine Grains
Gravel
Large Grains
Sand
Silt
Medium Grains
Clay
Fine Grains
What Factors Control Infiltration?
A) Soil Characteristics
1) Grain Size (size of pores in soil)
2) Antecedent moisture
HYGROSCOPIC
WATER
CAPILLARY
WATER
FREE or GRAVITATIONAL
WATER
FREE or GRAVITATIONAL
WATER
10
-1
Infiltration Rate, ft (mm hr )
MAXIMUM RATES OF INFILTRATION
8
6
4
2
0
0
1
2
3
Time, t (hrs)
4
5
6
MAXIMUM RATES OF INFILTRATION
Dry Soil.
Hygroscopic
Water only.
Lots of soil
air to fill
with water.
10
-1
Infiltration Rate, ft (mm hr )
Initial
Infiltration
Rate is
high
8
6
4
2
0
0
1
2
3
Time, t (hrs)
4
5
6
MAXIMUM RATES OF INFILTRATION
Moist soil.
Capillary
Water.
Progressively
fewer voids
to fill with
water.
10
-1
Infiltration Rate, ft (mm hr )
Initial
Infiltration
Rate is
high
Exponential
decline in
rate
8
6
4
2
0
0
1
2
3
Time, t (hrs)
4
5
6
MAXIMUM RATES OF INFILTRATION
Saturated
soil.
Free or
Gravitational
Water.
Voids to fill
with water
determined
by rate of
loss.
10
-1
Infiltration Rate, ft (mm hr )
Initial
Infiltration
Rate is
high
Exponential
decline in
rate
Minimum
rate, or
“field
capacity”
set by
drainage
8
6
4
2
0
0
1
2
3
Time, t (hrs)
4
5
6
What Factors Control Infiltration?
A) Soil Characteristics
1) Grain Size (size of pores in soil)
2) Antecedent moisture
B) Rainfall intensity (how fast rain is arriving!)
Precipitation
Evaporation
Infiltration
Overland Flow
Runoff
CONSTANT HEAVY RAIN FOR FOUR HOURS
Precipitation and Infiltration
Rates (mm/hr)
0
1
2
3
4
35
35
30
30
25
25
20
20
15
15
10
10
5
5
0
0
0
1
Precipitation Intensity
2
Hours
3
4
DECREASING INFILTRATION RATE FOR FOUR HOURS
Precipitation and Infiltration
Rates (mm/hr)
0
1
2
3
4
35
35
30
30
25
25
Rainfall to
Overland Flow
20
15
20
15
Rainfall that
could be
Infiltrated
10
5
10
5
0
0
0
1
Precipitation Intensity
Infiltration Rate
2
Hours
3
4
INCREASING OVERLAND FLOW FOR FOUR HOURS
0
1
2
3
4
20
15
25
20
15
10
5
10
5
0
0
0
1
Precipitation Intensity
Infiltration Rate
Overland Flow
2
Hours
3
4
Overland Flow
Generation (mm/hr)
30
No Overland
Flow
Precipitation and Infiltration
Rates (mm/hr)
35