INFILTRATION

“The surface entry of water into the soil”

P = Total PrecipitationF = Total InfiltrationQ = Total Surface RunoffD = Detention StorageS = Depression Storage

I = O +/- SP = {F + Q} +/- {S + D}F = P - Q +/- D +/- S

WATER BALANCE FOR INFILTRATION

AIR

SOIL

P

F

QDDS

Despite strict definition, infiltration is really a 3-phase process:

1. Surface Entry

2. Transmission

3. Soil Storage Depletion

Moisture Content

0.0 0.1 0.2 0.3 0.4 0.5

Dep

th (

m)

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

18 min

39 min

78 min

159 min

Surface

PENETRATION OF WETTING FRONT

All times refer to minutes since since infiltration began.

After, Carson and Kirkby, 1972.

Volume of voids occupied by water-------------------------------------------Total volume of soil

18 Minutes

Moisture Content

0.0 0.1 0.2 0.3 0.4 0.5

Dep

th (

m)

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

18 min

39 min

78 min

159 min

Surface

PENETRATION OF WETTING FRONT

All times refer to minutes since since infiltration began.

After, Carson and Kirkby, 1972.

Volume of voids occupied by water-------------------------------------------Total volume of soil

159 Minutes

LAND USE AND CUMULATIVE INFILTRATION

Time (min)

0 20 40 60

Cum

ulat

ive

Sur

face

Inf

iltra

tion

(mm

)

0

20

40

60

Inch

es

0.0

0.5

1.0

1.5

2.0

2.5

Crusted bareground

Clean, tilledbare ground

Weeds or grains

Strip crops

Newly grazedpasture

Hay

4-8 Yr-oldpasture

Old permanentpasture

HORTON'S INFILTRATIONCAPACITY CURVE.

Time, t (hrs)

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e, f

t (m

m h

r-1)

0

2

4

6

8

10fo = 10 (mm hr-1)

k = 1.0 hr-1

ft = fc + (fo - fc). e-k.t

fo

fc

fo - fc

fc fc fcfc

(fo - fc). e-k.t fc = 2 (mm hr-1)

ft = Infiltration rate (L T-1) at time t

fo = Initial infiltration rate (L T-1) at time 0

fc = Stabalized infiltration rate (L T-1) if

saturated (Field capacity)k = Empirical constant (T-1) ~ "Conductivity"

Changing Field Capacity, fc.

Time, t (hrs)

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e, f

t (m

m h

r-1)

0

2

4

6

8

10fo = 10 (mm hr-1)

k = 1.0 hr-1

fc = 5.0 mm hr-1

fc = 3.5 mm hr-1

fc = 2.0 mm hr-1

fc = 1.0 mm hr-1

fc = 0.5 mm hr-1

ft = fc + (fo - fc). e-k.t

Changing Initial InfiltrationRate, f0.

Time, t (hrs)

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e, f

t (m

m h

r-1)

0

5

10

15

20

fc = 2 (mm hr-1)

k = 1.0 hr-1

20 mm

hr -115 mm

hr -1

10 mm

hr -17.5 mm hr -15 mm hr -1

ft = fc + (fo - fc). e-k.t

CHANGING k

Time, t (hrs)

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e, f t (

mm

hr-1

)

0

2

4

6

8

10

k = 0.5k = 0.75k = 1.0

k = 1.5k = 1.75

fo = 10 (mm hr-1)

fc = 2 (mm hr-1)

Units of k, hr-1

ft = fc + (fo - fc). e-k.t

EXAMPLES OF ESTIMATED INFILTRATION RATES

Source: Bedient, Huber and Vieux, 2008. after Rawls, 1976

fc fo k

EXAMPLE mm/hr mm/hr (/hr)

1 35.56 482.6 38.29

2 44.958 375.158 19.64

3 66.802 88.138 1.4

4 61.468 158.496 4.7

5 43.942 288.036 7.7

6 45.72 584.454 32.71

Time (mins)

0 20 40 60 80

Cum

ulat

ive

Dep

th (

mm

)

0

10

20

30

40

50 P = 50.80

F = 20.47

Q = 28.23

End ofStorm

INTERACTION OF TOTALS OF VARIABLES IN THE INFILTRATION PROCESS.

PARTITIONING OF PRECIPITATION ASFUNCTION OF STORM DURATION

Duration of Storm (mins)

0 10 20 30 40 50 60

Per

cent

age

of P

reci

pita

tion

Tot

al

0

20

40

60

80

100

Constant Precipitation Intensity50.8 mm/hr or 0.847 mm/min

INFILTRATION CAPACITY CURVEWETTING

Time (hr)

0 1 2 3 4 5 6

Infil

tra

tion

Ra

te (

mm

/hr)

0

2

4

6

8

10

fo = 10 mm/hr

fc = 2 mm/hr

k = 2 hr-1

HORTON'S INFILTRATION CAPACITYFOR DRYING SOIL.

(INFILTRATION RECOVERY)

Time (hr)

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e (m

m/h

r)

0

2

4

6

8

10

fo = 10 mm/hr

fc = 2 mm/hr

kd = (0.5*k) = 0.5*2 = 1 hr-1

ft = fo - (fo - fc). e-kd.t

fc

fo

After Viessman and Lewis, 2003.

INFILTRATION CAPACITIES UNDERINTERMITTENT WETTING AND DRYING

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e (m

m/h

r)

0

2

4

6

8

100 1 2 3 4 5 6

Infiltration ratesDrying rates

Infiltration trajectoriesDrying trajectories

Change point:Wetting-dryingChange point:Drying-wetting

Rainfall

fo = 10, fc = 2, k = 2, kd = 1.

INFILTRATION CAPACITIES UNDERINTERMITTENT WETTING AND DRYING

0 1 2 3 4 5 6

Infil

trat

ion

Rat

e (m

m/h

r)

0

2

4

6

8

100 1 2 3 4 5 6

Rainfall

P = 6mm/hr

Time (hr)

0 1 2 3 4 5 6

Run

off

Rat

e(m

m/h

r)

0

2

4