Infiltration

• Infiltration is the process by which water penetrates from ground surface into the soil.

• Infiltration rate is governed by:– rainfall rate– hydraulic conductivity of soil surface– vegetative cover at top of soil profile– ability of deeper soil profile to store and transmit incoming water.

• Infiltration occurs first by capillary action, then by gravity

• Our goal - predict infiltration rate f (L/T) - rate at which water enters

soil surface or cumulative infiltration F (L). The remainder of the non-infiltrating water becomes runoff which ultimately contributes to streamflow

Infiltration

• In general there are three conditions that should be distinguished:

– No ponding. Infiltration equals rainfall rate and is less than or equal to the soils ability to infiltrate water (i.e. infiltration capacity)

– Saturation from above. Ponding occurs because rainfall rate exceeds that infiltration rate. In this case infiltration rate equals the infiltration capacity

– Saturation from below. Ponding occurs because the water table has risen to or above the land surface and the entire soil is saturated. In this case infiltration rate is zero.

Measuring Infiltration

• Infiltration is measured with a ring infiltrometer.

• A ponded condition within the ring infiltrometer is created by flooding the surface or by applying a high rate of simulated rainfall.

• The rate of infiltration is obtained by – 1) measuring the rate at which the level of ponded

water decreases, or

– 2) measuring the rate at which water has to be added to maintain a constant level of ponding.

General Infiltration Behavior

time to ponding

rainfall rate

runoffinfiltration rate (typically

exponential decay after ponding)

Ksat

t

finfiltration rate (darcy flux at ground surface)q(z=0)

if rainfall rate < Ksat all infiltrates - no ponding, soil never becomes sat’d

General Infiltration Behavior

• For rainfall rates less than saturated conductivity of soils all rainfall will infiltrate, no runoff will occur soil never becomes saturated.

• For rainfall rates > Ksat but less than the soils maximum infiltration capacity, initially all water will infiltrate. Since rate > Ksat all water cannot be transmitted down, water storage in soil will increase until soil is saturated.

• When soil becomes saturated rate of infiltration will decrease because only will take in water which can be transmitted down. No more storage to fill called time to ponding.

General Infiltration Behavior

• After ponding infiltration rate decreases approximately exponentially initially driven by both capillary gradients and gravitational gradients when moisture approximately uniformly distributed through profile capillary gradients 0 infiltration driven by gravity gradients asymptotic value Ksat.

• For rainfall rates greater than maximum infiltration capacity get immediate ponding and

• exponential decay from maximum infiltration capacity toward minimum infiltration capacity.

Horton Infiltration Model

• one of earliest infiltration equations developed (1933) and the most common empirical equation used to predict infiltration if ponding occurs from above:

• Instantaneous infiltration

• Cumulative infiltration

• fc, minimum infiltration capacity (approximately saturated hydraulic conductivity)

• fo, maximum infiltration capacity (function of saturated conductivity and soil tension)

• k constant representing exponential rate of decrease of infiltration

ktcc ffftf exp)()( 0

tKtco

cK

fftfdftF

0)exp1()()(

Horton Infiltration Model

• All are empirical parameters which must be fit to each soil type using data from a ring infiltrometer experiment

• Horton’s equations are only valid after ponding. Therefore all water the soil has potential to infiltrate is available at soil surface. Ponding will only occur if i > f(t). Should only be used during very high intensity precipitation events over small areas

fc

fo rate of decay governed by k,increase k, increase rate of decay

(analogous to Ksat)

t

F(t)

f(t)

(time after ponding)

Example• Suppose that the parameters for Horton's equation are fc= 1.0

cm/hr, fo=5.0 cm/hr and k=2 hr-1 .

– Determine the infiltration rate and cumulative infiltration after 0.,0.5,1.0, 1.5, 2.0 hours if the rainfall rate is 6 cm/hr.

– Plot as a function of time. – What would be the infiltration rate if the rainfall rate were

0.6 in/hr?• NOTE: There are many other physically based models to

predict infiltration from particular storm events when upper soil ponding is the limiting factor, Horton's equation is just one example.

SCS Method

• Generally applied to total rainfall event not time distribution of rainfall.

• Predicts total volume of infiltration and total volume of runoff, not rate over time.

• Most often used to predict effects of land development on runoff from "design storms" for permitting purposes.

SCS Method

• Based on mass conservation principles, but derived based on empirical observations:

• Total storage in basin depends on antecedent moisture conditions, soil type and land use.

• By studying many experimental watersheds SCS developed a relationship between basin storage and curve numbers which depend on land use, antecedent conditions and soil type:

SP

SPPe 8.0

2.0 2

101000 CN

S

SCS Method

• impervious surface and water surface: CN = 100 S = 0

• natural surfaces: CN < 10 CN = 30 meadow

• CN = 70 residential

• Curve numbers for various land uses and soil types are tabulated for normal antecedent moisture conditions (AMC II). Depend on soil type.

GroupA: deep sand B: sandy loam C: clay loam D: heavy clay

all Florida soils classified according to hydrologic group in county soil survey

SCS Method

• For extremely dry conditions modify CN: (AMC I)

• For extremely wet conditions (AMC III):

• For mixed land uses compute a weighted curve number based on percent area in that land use

n

nd CN

CNCN

58.010

2.4

n

nw CN

CNCN

13.010

23

i

iiA

CNACN

Dunne Runoff

• Also called Dunne Overland Flow or Saturation Overland Flow) - All rainfall infiltrates and results in a raising of the watertable.

• If rains long enough saturation of soil occurs from below. Get no more infiltration Overland flow. Occurs in shallow water table flatwoods regions of Florida. First in low-lying areas near streams and wetlands.

increased outflow to stream

Dunne Runoff

• How do we determine when Dunne runoff will occur?

• Total available soil moisture storage is

• Time to onset of Dunne runoff is:

• Before onset of Dunne runoff: runoff rate=0 (all rainfall infiltrates to fill storage)

• After onset of Dunne runoff: runoff rate=rainfall rate

LavailableStorage is

i

Lis

rate rainfall

available Storagerunoff toTime