Infiltration : Objectives
• Understand the three processes of infiltration
• Understand the difference between rate (flux) and
volume of infiltration
• Identify physical and biological factors that affect
infiltration rates and volumes
• Learn about methods for measuring infiltration
• Understand why infiltration varies in space and time
• Identify management effects on infiltration rates and
volumes
What is infiltration?
The movement of water through the air-soil
interface
It is one of the things that can happen to precipitation
that reaches the soil surface
Infiltration is the actual rate at which water is entering
the soil at any given time
Infiltration
Rainwater that soaks into the ground and may reach the
groundwater table.
Processes of infiltration
• Entry through the soil surface (infiltration)
• Storage in the soil profile (soil moisture)
• Transmission through the soil profile(percolation)
Infiltration...........
Precipitation reaching the ground may
infiltrate.
This is the process of moving from
the atmosphere into the soil.
Infiltration may be regarded as either
a rate or a total. For example: the soil
can
infiltrate
1.2
inches/hour.
Alternatively, we could say the soil has
a total infiltration capacity of 3 inches.
Note that in both cases the units are
Length or length per time!
Infiltration
Infiltration is nearly impossible to
measure directly - as we would disturb
the sample in doing so.
We can infer infiltration in a variety
of ways
The exact point at which the
atmosphere ends and the soil
beings is very difficult to define and
generally we are not concerned
with this fine detail.
In other words, we mostly want to
know how much of the precipitation
actually enters the soil.
Percolation.....
Once the water infiltrates into the ground, the
downward movement of water through the soil
profile may begin.
Percolation.....
The percolating water may
move as a saturated front under the influence of
gravity…
Or, it may move as
unsaturated flow mostly
due to capillary forces.
Percolation….the point
• The vertical percolation of the water into various
levels or zones allows for storage in the
subsurface
.
• This stored subsurface water is held and
released as either:
• evaporation,
• transpiration, or as
• streamflow eventually reaching the watershed
outlet.
Infiltration nomenclature
• i = intensity of rainfall (rate) (length/time)
• f = infiltration rate- measure of hydraulic
conductivity (length/time)
• F = infiltrated volume(Length3) or depth (L)
If i < f what happens?
If i > f what happens?
Infiltration
• Infiltration is the actual rate at which water is
entering the soil at any given time
What factors affect infiltration?
• Flow influences
– Head (ponding)
– Viscosity (function of temperature)
– Water quality
– Soil chemistry
– Soil and water temperature
– Air entrapment
What factors affect infiltration?
• Soil surface conditions
– Land use
– Vegetation cover
– Roughness and slope
– Cracking and crusting
– Surface sealing, swelling
What factors affect infiltration?
• Hydrophobicity
– Dryness
– Heat
– Plant chemicals
– Aromatic oils
– Other chemicals
– Fire
What factors affect infiltration?
• Subsurface conditions
– Soil
•
•
•
•
•
•
•
•
Hydrologic group (A B C D)
Texture
Porosity
Depth
Shrink and swell
Layering
Spatial variability
Structure
What factors affect infiltration?
• Subsurface conditions
– Root system
– Water table depth
– Subsurface drainage
– Water release relationship
– Hydraulic conductivity
Factors that affect surface and
subsurface conditions that affect
infiltration
•
•
•
•
•
•
•
•
Mechanical processes, plowing,
Frost- freeze-thaw cycles
Litter layer, organic matter
Compaction
Antecedent soil water condition
Chemical activity
Biological activity
Microbial activity
Soil Type Effects on Infiltration
• Sand soils have the highest infiltration rates
• Clay soils have the lowest infiltration rates.
• High organic matter improves infiltration rates.
How do we measure infiltration?
• Rainfall simulators
–
–
–
–
Needle drip systems
Stand pipes
Sprinkler nozzles
Rotating boom
• All measure input of water and output of water (runoff)difference is the amount infiltrated
• Plot scale
• Need lots of water, vehicles, plot boundaries
How do we measure infiltration?
• Average infiltration method
– Small basins or plots
– Use storms with bursts of rain
– Compute the amount of rain in the burst
– Separate the runoff volume due to the burst
– Difference is infiltrated volume
How do we measure infiltration?
• Soil surveys
– Usually report infiltration ranges for various
soil types
– Example rates
•
•
•
•
•
Sand
Sandy loam
Loam
Silt loam
Light clay
124 mm/hr
50 mm/hr
13.2 mm/hr
1.05 mm/hr
0.44 mm/hr
How do we measure infiltration?
• An infiltrometer is a device used to measure
the rate of water infiltration into soil or other
porous media.
• Commonly used infiltrometers are single-ring
and double-ring infiltrometers, and also disc
permeameters
How do we measure infiltration?
• Single ring infiltrometer
– Constant head (ponded depth)
– Results tend to be higher than that due to
rainfall
– Point scale
Infiltrometer ring
http://ocw.mit.edu/NR/rdonlyres/Civil-and-Environmental-Engineering/1-72
Fall-2004/9227A886-A1FE-408F-BFED-1665F5E5B6B8/0/1_72_lecture_13.pdf
Management effects on infiltration
• Compaction or alteration of soil surface
and vegetative cover
– Grazing, skidding logs, recreational use,
vehicles, plowing
– Even low ground pressure skidders can
increase bulk density by up to 45% at a depth
of 15 cm – frequent travel over wet soils
Management effects on infiltration
• Compaction or alteration of soil surface
and vegetative cover
– Grazing, cropping and logging
• Changes interception, organic matter layers,
rooting depths, ground cover
Infiltration rate i(t), cm/hr
Infiltration rate over time
Infinite at time t = 0?
i(t) can’t exceed precipitation rate
Zero at time t = ∞?
Infiltration rate i(t), cm/hr
Infiltration rate over time
Infiltration rate can be either
soil-limited or rain-limited
i(t) can’t exceed precipitation rate
Not really.
Soil behind (above) the
wetting front isn’t 100% saturated. Some
people write ic instead.
As t → ∞, i(t) → Ks
Field Tests
24 inch double ring infiltrometer with Mariotte Tubes http://www.hilbec.com/STORMWATER.htm
Infiltration is measured in the field with bottomless rings. Mariotte
Tubes allow for measurement of liquid flow during the infiltration test by providing
a constant water level in the 24 inch Infiltration Rings
Measuring infiltration: ring infiltrometer
Falling head method: Pour in water, wait for steady
flow, then measure water depth over time.
Constant head method: Maintain a constant water
level, and measure how much water that requires
over time.
Single-ring
Double-ring
Measuring infiltration
Water is applied
to the soil
surface at a
positive pressure
There is less effect of the ring size
on the results when using the
double-ring:
Maintain equal depths, but
only
measure flow into inner ring.
Outer ring will supply most of
the horizontal flow, so inner ring
gives mainly vertical
Double Ring Infiltrometer
Measure rate of fall in inner ring
Infilration
http://www.alwi.com/wastewater.php
Double-ring Infiltrometer
• Two rings eliminates overestimating the hydraulic conductivity
• Outer ring contributes to lateral flow , so
• Inner ring is contributing mostly to downward flow.
•Water from Mariotte bottles to rings via tap at base of bottles. Ring
water height equals that of the base of the bubble tube.
•When water moves into the soil, reducing the height of ring water to
below that of the bubble tube, more water is fed into the ring.
Estimating infiltration at the scale of a catchment
(watershed):
 Measure baseflow
before rainfall
 Measure rainfall
 Measure streamflow
 Estimate runoff by
baseflow separation
 Estimate: Infiltration
= rainfall - runoff
Infiltration models
 Green & Ampt (1911)
 Horton (1930)
 Kostiakov (1932)
 Philip (1957)
There are many others, but we won’t study them.
These models have 2 main purposes:
Explain the observed infiltration
patterns Predict future infiltration
Infiltration by Horton’s method
f  f c   f 0  f c e
kt
Estimates 1: Horton’s Equation
Horton: The infiltration capacity decreases
exponentially with time and ultimately reaches a
constant rate
Infiltration capacity
Where ft is the infiltration rate at time t;
f0 is the initial infiltration rate or maximum
infiltration rate;
fc is the constant or equilibrium infiltration rate after
the soil has been saturated or minimum infiltration
rate; NOTE e is a number, ~ 2.718
k is the decay constant specific to the soil.
the f’s have units in/hr and k is a time constant hr -1
Horton’s Infiltration Model for soil capacity
Infiltration starts at a constant rate, f0, and is decreasing exponentially
with time, t. After some time when the soil saturation level reaches a
critical value, the rate of infiltration will level off to the rate fc.
In a few minutes we will do an example using Horton’s
Equation using an average rectangle estimate to the area under
the curve.
Estimates 2: F index
Infiltration Volume = total rainfall volume – runoff volume as
measured in the rain gages and at the outlet gage, respectively.
F assumes infiltration volume resulted from a constant infiltration
rate. It assumes a high initial infiltration is balanced by a low later
infiltration.
Example: Guessing
F
We will find F in this problem by guessing a value for F , calculating
the total runoff that would result, and comparing our answer to the
known runoff.
Kostiakov’s model
i(t)
t
i t   Bt
n
with i : infiltration rate, L/T
t : time, T
B, n : fitting parameters
usually n ≈ 1/2
No theory: this is purely empirical
No physical interpretation of B and n.
Note that i(0) = ∞, and i(∞) = 0.
Frequently this model fits the data better
than more physically-based models.
Green & Ampt’s model
b
i(t)
i
c
t
I t    i t dt
with i : infiltration rate, L/T
ic : final i : i(∞), L/T
t : time, T
b : fitting parameter
I : cumulative infiltration, L
No physical interpretation of b.
Note that i(0) = ∞, and i(∞) = i . c
Assumes all flow is saturated flow
Philip’s model
c
i(t)
with
i
2
t
i : infiltration rate, L/T
c
c
t
t : time, T
s : sorptivity, L/T0.5
Exact solution of Richards’ equation, with
additional assumptions
Infinite series, but only 1st 2 terms used
Doesn’t work well at short times
Sorptivity isn’t used much outside of
Australia (J. R. Philip was
Infiltration into a soil water column
1
2
 h1  h2 
f  K

 z1  z 2 
F  L
Green-Ampt method

 r
Se 
i   r
    i
Relative saturation
Incremental change
in soil water content
as wetting front pass
Green-Ampt Parameter Definitions

 r
Se 
i   r
    i
Relative saturation
Incremental change
in soil water content
as wetting front pass
Math for Green Ampt
GreenAmpt
Parameters