Texas; Rainwater Harvesting Soil Storage and Infiltration Systems - Part 2
INFILTRATION “The surface entry of water into the soil”
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Transcript of 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.