Infiltration

1
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

2
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.

3
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.

4
General Infiltration Behavior
time to ponding
rainfall rate
f infiltration rate (darcy flux at ground
surface) q(z0)
infiltration rate (typically ? exponential decay
after ponding)
runoff
Ksat
t
if rainfall rate lt Ksat all infiltrates - no
ponding, soil never becomes satd
5
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 gt Ksat but less than the soils
  maximum infiltration capacity, initially all
  water will infiltrate. Since rate gt 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.

6
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.

7
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

8
Horton Infiltration Model

• All are empirical parameters which must be fit to
  each soil type using data from a ring
  infiltrometer experiment
• Hortons 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 gt f(t). Should only be used
  during very high intensity precipitation events
  over small areas

9
Example

• Suppose that the parameters for Horton's equation
  are fc 1.0 cm/hr, fo5.0 cm/hr and k2 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.

10
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.

11
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

12
SCS Method

• impervious surface and water surface CN 100 S
  0
• natural surfaces CN lt 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
13
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

14
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
?
15
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 rate0
  (all rainfall infiltrates to fill storage)
• After onset of Dunne runoff runoff
  raterainfall rate