# Module 3

```LEARNING MODULE
SURIGAO STATE COLLEGE OF TECHNOLOGY
Module No. 3
INFILTRATION
I.
Topic:
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Definition of infiltration
Factors affecting infiltration, and infiltration measurements
Horton Model and Phillip’s equation,
Green-Ampt model
Ponding time
Fitting infiltration models to infiltration data using Excel
II.
Time Frame: 8 hours
III.
Introduction:
Infiltration and percolation play an important role in surface runoff, groundwater
recharge, evapotranspiration, soil erosion, and transport of chemicals and subsurface
waters. Infiltration is a very complex process, which can vary temporally and spatially.
Selection of measurement techniques and data analysis techniques should consider these
effects, and their spatial dimensions can categorize infiltration measurement techniques.
A brief introduction of infiltration measurement techniques are described in this lesson.
IV.
Objectives:
At the end of the topic, you are expected to:
1. Assess factors affecting infiltration;
2. Solve problems using different models; and
3. Apply infiltration models to infiltration using excel.
V.
Learning Activities:
DEFINITION OF INFILTRATION
Infiltration is the process by which water enter into a soil from rainfall or irrigation.
Infiltration is governed by two forces which is gravity and capillary action. Infiltration is
measured in inches per hour or millimeters per hour.
Soil water movement known as percolation is the process of water flow from one
point to another point within the soil. When percolation stop, infiltration also stop. When
infiltration stop, percolation continues.
Infiltration rate is the rate at which the water actually infiltrates through the soil
during a storm and it must be equal the infiltration capacities or the rainfall rate, whichever
is lesser. The volume of water that the ground or soil can hold is known as field capacity.
FACTORS AFFECTING INFILTRATION AND INFILTRATION MEASUREMENTS
The maximum rate at which a soil in a given condition is capable of absorbing water
is the infiltration capacity (f)
Actual rate of infiltration (fa) = infiltration capacity (f), when intensity of rainfall (i) &gt; = f
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Factors affecting infiltration:
o Vegetation cover – dense vegetation increase the infiltration
o Moisture content – infiltration rate depends on initial moisture condition of
soil. When soil moisture is high, infiltration rate is slow. When soil moisture
is low, infiltration rate is high.
o Temperature – viscosity of water changes with temperature. Increase in
temperature cause reduction in viscosity. Infiltration is higher when
temperature is high.
o Intensity of rainfall – high intensity rainfall cause mechanical compaction of
soil. Heavy intensity rainfall cause less infiltration. Lesser intensity rainfall
cause higher infiltration.
o Human activity – cultivation on bare land will increase infiltration while
construction of roads and buildings will decrease in infiltration capacity.
o Quality of water – silt and other impurities in water results in reduction of
infiltration
o Movement of man and animals – heavy movements cause compaction of
soil, results in less infiltration.
o Presence of ground water table – if ground water table is near to the earth
surface, it reduce infiltration. For infiltration to continue, ground water table
should not very close.
o Size and characteristics of soil particles – infiltration is directly proportional
to the grain size/diameter for granular soils. However, if the soil has swelling
minerals the infiltration rate will reduce drastically.
Areal infiltration estimation is accomplished by analysis of rainfall-runoff data from
a watershed. For a storm with a single runoff peak, the procedure resembles that
of the calculation of a Ψ index. The rainfall hyetograph is integrated to calculate
the total rainfall volume. Likewise, the runoff hydrograph is integrated to calculate
the runoff volume. The infiltration volume is obtained by subtracting runoff volume
from rainfall volume. The average infiltration rate is obtained by dividing infiltration
volume by rainfall duration.
Point infiltration measurements are normally made by applying water at a specific
site to a finite area and measuring the intake of the soil. There are four types of
infiltrometers: the ponded-water ring or cylinder type, the sprinkler type, the
tension type, and the furrow type. An infiltrometer should be chosen that replicates
the system being investigated. For example, ring infiltrometers should be used to
determine infiltration rates for inundated soils such as flood irrigation or pond
seepage. Sprinkler infiltrometers should be used where the effect of rainfall on
surface conditions influences the infiltration rate. Tension infiltrometers are used
to determine the infiltration rates of soil matrix in the presence of macropores.
Furrow infiltrometers are used when the effect of flowing water is important, as in
furrow irrigation.
Field measurement of the infiltration is done by the instruments known as
infiltrometers. There are two types of infiltrometers used: single tube/ring type and
double tube/ring type.
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HORTON MODEL AND PHILIP’S EQUATION
Horton proposed the following infiltration equation where rainfall intensity I is
greater than fp at all times.
The cumulative infiltration equation F(t) for the Horton Method is found from the
relationship d(F(t)/dt = f(t) = fp and is given by
The table below shows the estimated values of Horton parameters
Soil/cover complex
Standard agricultural (bare)
Standard agricultural (vegetated)
Peat
Fine sandy clay (bare)
Fine sandy clay (vegetated
CE 370 – Hydrology
fo (mm/hr)
280
900
325
210
670
fc (mm/hr)
6 - 220
20 - 290
2 - 29
2 - 25
10 - 30
K
1.6
0.8
1.8
2.0
1.4
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Example 1: The infiltration capacities of a given soil at different intervals of time are
measured and values are given in table. Find an equation for the infiltration capacity.
Time (hr)
fp (cm/hr)
0
10.4
0.25
5.6
0.50
3.2
0.75
2.1
1.00
1.5
1.25
1.2
1.50
1.1
1.75
1.0
2.00
1.0
Solution:
The infiltration capacity reaches a constant value equals to fc = 1.0 cm/hr. Plotting
the log 10 (fp – fc) with t on linear scale and estimating slope of the line m = - 1/1.31
The φ-index method is the simplest method and is calculated by finding infiltration as a
difference between gross rainfall and observed surface runoff. The φ-index method
assumes that the loss is uniformly distributed across the rainfall pattern.
Example 2: Estimate the φ-index of the catchment having an area 2.26 km2. The
observed runoff caused by the rainfall given in the table is 282.097 m3
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Philip proposed an equation to estimate cumulative infiltration F(t) by
Example 3: A small tube with a cross-sectional area of 40 cm2 is filled with soil and laid
horizontally. The open end of the tube is saturated, and after 15 minutes, 100 cm3 of water
have infiltrated into the tube. If the saturated hydraulic conductivity of the soil is 0.4 cm/hr,
determine how much infiltration would have taken place in 30 minutes if the soil column
had initially been placed upright with its surface saturated.
GREEN-AMPT MODEL
The Green-Ampt model is an approximate model utilizing Darcy’s Law. The model
is developed with the assumption that water is ponded on the ground surface. By
definition the cumulative depth of water infiltrated into the soil F is given by:
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From Darcy’s Law
And in this case q = -f because q is positive upward while f is positive downward
Here the head at h1 is h0 and the head at the dry soil below the wetting front h2 = - Ψ-L.
h0 is negligible if it is assumed that ponded water becomes surface runoff
Table below shows the USDA Soil Texture Green-Ampt Infiltration Parameters
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Mean steady-state matric potential drop Ψi across seals by soil texture
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Example 4: Compute the infiltration rate f and cumulative infiltration F after one hour of
infiltration into a silt loam soil that initially had an effective saturation of 30%. Assume
water is ponded to a small but negligible depth on the surface.
Solution:
The cumulative infiltration at t = 1 hour is calculated employing the equation
taking a trial value of F(t) = Kt =0.65 cm
Substituting F = 1.27 cm in equation we get 1.79 cm and after number of iteration F
converges to a constant value of 3.17 cm. Infiltration rate after one hour is estimated by
VI.
Self-Evaluation Test:
1. The infiltration rate s a function of time for silt loam are given below. Determine the best
values for the parameters fo, fc and k for Horton’s equation to describe the infiltration of
the silt loam soil at the locality.
Time in hours 0
0.07 0.16 0.27 0.43 0.67
1.10 2.53
fp (mm/hr)
6.6
5.3
4.3
3.3
2.2
1.3
0.7
0.25
2. For clay soil at a given location parameters of Philip’s equation were found as S = 45
cm/hr0.5 and K = 10 cm/hr. Determine the cumulative infiltration and the infiltration rate
of 0.5 hr increments for a 3-hour period. Plot both as functions of time. Assume
continuously ponded conditions.
3. Use the Green-Ampt method to evaluate the infiltration rate and cumulative infiltration
depth of a silt clay soil at at 0.1 hour increments up to 6 hours from the beginning of
infiltration. Assume initial effective saturation 20% and continuous ponding.
CE 370 – Hydrology
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VII.
SURIGAO STATE COLLEGE OF TECHNOLOGY
Review of Concepts:
Infiltration is the process of water entry into a soil from rainfall, or irrigation. Soil
water movement (percolation) is the process of water flow from one point to another point
within the soil. Infiltration rate is the rate at which the water actually infiltrates through the
soil during a storm and it must be equal the infiltration capacities or the rainfall rate,
whichever is lesser. Infiltration capacity the maximum rate at which a soil in any given
condition is capable of absorbing water. The rate of infiltration is primarily controlled by the
rate of soil water movement below the surface and the soil water movement continues
after an infiltration event, as the infiltrated water is redistributed. Infiltration and percolation
play a key role in surface runoff, groundwater recharge, evapotranspiration, soil erosion,
and transport of chemicals in surface and subsurface waters.
VIII.
References:
David Chin, Water Resources Engineering, 3rd Ed., Pearson , 2013
Ram S. Gupta Hydrology and Hydraulics System 4th Edition 2017
Bedient Philip B.,Huber Wyne C., Vieux Baxter E. Hydrology and Floodplain Analysis 5th
Edition, Pearson, 2013
McCuen, R.H., Hydrologic Analysis and Design, Prentice Hall, 1989
Linsley, R.K., M.A. Kohler and J.L.H. Paulhus, Hydrology for Engineers by; McGraw-Hill,
1988
Applied Hydrology by VenTe Chow, David Maidment and Larry Mays;McGRAW-HILL
International Editions; 1988
Handbook of Hydrology David Maidment, 1993
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