Infiltration.ppt

INFILTRATION • • • • • •

Definition of Infiltration Factors Affecting Infiltration and Infiltration measurements Horton Model and Philip’s Equation Green-Ampt Model Ponding Time Fitting Infiltration models to infiltration data using excel

Definition of Infiltration Infiltration is the process by which water on

the ground surface enters the soil.

Infiltration is governed by two forces:

• •

Gravity Capillary Action

Reference: https://www.sciencedaily.com/terms/infiltration_(hydrology).htm

Definition of Infiltration

Reference: https://pubs.usgs.gov>wsp>report

Definition of Infiltration Infiltration is also defined as a downward entry of water into the soil. The velocity at which water enters the soil is called infiltration rate.

Infiltration Rate • •

In soil science is a measure of the rate at which a particular soil is able to absorb rainfall or irrigation. It is the rate at which the water actually infiltrates through the soil during a storm and it must be equal the infiltration capacities or rainfall rate, which ever is lesser.

Infiltration Capacity •

The maximum rate at which a soil in any given condition is capable of absorbing water. Reference: https://www.sciencedaily.com/terms/infiltration_(hydrology).htm; soilquality.org/indicators/infiltration.html ; mgebrekiros.github.io>infiltration

Definition of Infiltration Difference between Infiltration and Percolation Infiltration- The entry of water into the soil through the soil surface Percolation- The process of downward movement of water into soil, once water enters into the soil. NOTE: When Percolation Stop, Infiltration also stop. When Infiltration Stop, Percolation is continuing.

Reference: https://www.slideshare.net/mobile/AnantPatel18/infiltration-80

Factors Affecting Infiltration

Infiltration rates vary widely. It is dependent on the condition of the land surface (cracked, crusted, compacted, etc.), land vegetation cover, surface soil characteristics (grain size & gradation), storm characteristics (intensity, duration &magnitudes), surface soil and water temperature, chemical properties of water and soil.

Reference: mgebrekiros.github.io>infiltration

Factors Affecting Infiltration

Precipitation. The greatest factor controlling infiltration is the amount and characteristics (intensity, duration, etc.) of precipitation that falls as rain or snow.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration

Soil Characteristics. Some soils, such as clays, absorb less water at a slower rate than sandy soils. Soils absorbing less water result in more runoff overland into streams.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration

Conditions at Soil Surface. Even if the subsoil has excellent under drainage but at the surface soil pores are sealed due to turbid water or by in wash of fine soil particles it may prevent entry of water into the soil and infiltration rate will be low.

3/28/2019

Factors Affecting Infiltration

Soil Saturation. Like a wet sponge, soils already saturated from previous rainfall can’t absorb much more, thus more rainfall will become surfacerunoff.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration

Type of Vegetative/Land Cover. Some land covers have a great impact on infiltration and rainfall runoff. Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration

Soil Temperature. If saturated soil mass gets frozen due to severe low temperature it becomes nearly impermeable. It affects the infiltration.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration

Human Activities on Soil Surface. If the soil surface gets compacted due to construction of roads, operation of tractors and other farm implements and machinery the porosity of the soil is decreased.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration

Slope of the Land. Water falling on a sleepy-sloped land runs off more quickly and infiltrates less than water falling on flat land.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Factors Affecting Infiltration Evapotranspiration. Plants need the shallow groundwater to grow, and by the process of evapotranspiration, water is moved back into the atmosphere.

Reference: https://www.usgs.gov/edu/watercycleinfiltration.html

Infiltration Measurements

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 are described below.

Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Areal Measurement

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Infiltration Measurements Hyetograph

Hydrograph

Infiltration Measurements Point Infiltration Measurement Point infiltration measurement are normally made by applying water at a specific site to a finite area and measuring the intake of the soil. Infiltrometer is a device used to measure the rate of water infiltration into soil. There are types of infiltrometer: 1. Rainfall Infiltrometer 2. Flooding Infiltrometer A. Basin Infiltrometer B. Ring– Type Infiltrometer b. Single & Double Ring

Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Rainfall Infiltrometer Basically a rainfall infiltrometer simulates rainfall with the use of special spray nozzles set a certain distance (usually 2 to 3 m) above the soil surface. The soil surface tested is usually enclosed so that once runoff commences it can be collected at an opening and the volume measured with time. The difference between the application rate and the runoff rate is taken to be the infiltration. Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Rainfall Infiltrometer With the help of rain simulator, water is sprinkled at a uniform rate in excess of the infiltration capacity, over a certain experimental area. The resultant runoff R is observed, and from that the infiltration f using f= (P-R)/t. where: P= Rain Sprinkled R= Runoff Collected t= Duration of rainfall

Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Flooding Infiltrometer Flooding infiltrometers enclose an area and pond water to a specified depth. The infiltration rate is calculated from the drop in water level per unit time or the amount of water required to maintain the specified depth or head of water per unit time. Flooding infiltrometers measure the maximum rate of entry of water into the soil. They do not simulate raindrop activity; they measure water penetration rather than rainfall infiltration. Basically there are two types of flooding infiltrometers;

•Basin infiltrometer which uses earth retaining walls; and • Ring infiltrometer which uses metal rings inserted into the ground to retain the water.

Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Types of Ring Infiltrometer: •Single Ring Infiltrometer

•Double Ring Infiltrometer

Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Example 1. A USGS rain-simulator infiltrometer experiment was conducted on a sandy loam soil. Rainfall was simulated at the rate of 20 cm/hr. The rainfall and runoff data are given in the table. a.) Find and Plot the mass- infiltration curve from the experimental data b.) Plot an infiltration rate curve. Table EX.1. Rain- simulator Infiltrometer Data and infiltration capacity calculation

Reference: mgebrekiros.github.io>infiltration

Infiltration Measurements Solution. The measured data are given in column 1, 3, 4. Cumulative infiltration F is calculated by subtracting the cumulative runoff from the cumulative rainfall. Infiltration rate is determined by driving the F by the total duration of infiltration. The result is placed in figure below.

Reference: mgebrekiros.github.io>infiltration

Horton Model

In general, for a given constant storm, infiltration rates tend to decrease with time. The initial infiltration rate is the rate prevailing at the beginning of the storm and is maximum. Infiltration rates gradually decrease in time and reach a constant value.

Reference: mgebrekiros.github.io>infiltration

Horton Infiltration Horton observed the above facts and concluded that infiltration begins at some rate fO and exponentially decreases until it reaches a constant fc. He proposed the following infiltration equation where rainfall intensity i greater than fp at all time.

Reference: mgebrekiros.github.io>infiltration

Horton Model Note: That infiltration takes place at capacity rates only when the intensity of rainfall i equals or exceeds fp; that is f= fp when i > fp, but when for i < fp , f < fp and f= i.

Reference: mgebrekiros.github.io>infiltration

Horton Model Table 1. Estimated Values of Horton Parameters

Reference: mgebrekiros.github.io>infiltration

Horton Model Example 2. Given an initial infiltration capacity fo of 2.9 in/hr and a time constant of 0.28 1/hr , derive an infiltration capacity versus time curve if the ultimate infiltration capacity is 0.50 in/hr.

Reference: mgebrekiros.github.io>infiltration

Horton Model

For the times shown in the table below, values of fp are computed and entered into the table. Using excel the curve of Fig. 1 is derived. Time (hr)

Infiltration Capacity (in/hr)

Time (hr)

Infiltration Capacity (in/hr)

4

1.283072

5

1.091833

6

0.947298

7

0.83806

0

2.9

0.1

2.833732

0.2

2.769294

0.3

2.706635

8

0.7555

0.4

2.645706

9

0.693103

1

2.313881

10

0.645944

2

1.870902

15

0.535989

3

1.536105

20

0.508875

Reference: https://www.egr.msu.edu/classes/ce421/lishug/text%20book.pdf

Horton Model Series1 3.5

Infiltration Capacity (in/hr)

3

2.5 2 1.5 1 0.5 0 0

5

10

15

Time (hr)

20

25

Horton Model Example 3. The Horton’s infiltration equation for a basin is given by fp = 6 + 16e-2t where fp is in mm/hr and t in hour. If a storm occurs on this basin with an intensity of more than 22 mm/hr, determine the depth of infiltration for the first 45 minutes and average infiltration rate for first 75 minutes.

Horton Model

Horton Model

Philip Equation (1957, 1969) Philips Solution for Horizontal and Vertical Infiltration

John R. Philips (1957,1969) presented the first analytical solution to Richards’ equation. It is based on a time expansion method considering infiltration as a sorption process with a perturbation generated by the presence of gravity.

Recall:

Darcy’s Law

where: q= the flux which at the soil surface equals the infiltration rate H= total hydraulic head which is the sum of the pressure head (Hp) and the gravity head (Hg) K= hydraulic conductivity

Richards’ Equation

Reference: Trieste, Miramare (May 1994). Soil Physical Properties Influencing Parameter in Philip and Kostiakov Infiltration Models. Retrieved from: www.google.com Tuller, M. & Or, D. (2002-2004). Infiltration to Soils. Retrieve from: https://slideplayer.com/slide/5240097

Philip Equation (1957, 1969) Philips Solution for Horizontal and Vertical Infiltration

Horizontal Infiltration For Horizontal infiltration Philip showed that the cumulative and instantaneous infiltration rates are given by:

Cumulative:

Rate:

Reference: Trieste, Miramare (May 1994). Soil Physical Properties Influencing Parameter in Philip and Kostiakov Infiltration Models. Retrieved from: www.google.com Tuller, M. & Or, D. (2002-2004). Infiltration to Soils. Retrieve from: https://slideplayer.com/slide/5240097

Philip Equation (1957, 1969) Philips Solution for Horizontal and Vertical Infiltration

Horizontal Infiltration When a sharp approximated by:

wetting

front

exists,

the

sorptivity

may

be

Reference: Trieste, Miramare (May 1994). Soil Physical Properties Influencing Parameter in Philip and Kostiakov Infiltration Models. Retrieved from: www.google.com Tuller, M. & Or, D. (2002-2004). Infiltration to Soils. Retrieve from: https://slideplayer.com/slide/5240097

Philip Equation (1957, 1969) Philips Solution for Horizontal and Vertical Infiltration

Vertical Infiltration

Reference: Trieste, Miramare (May 1994). Soil Physical Properties Influencing Parameter in Philip and Kostiakov Infiltration Models. Retrieved from: www.google.com Tuller, M. & Or, D. (2002-2004). Infiltration to Soils. Retrieve from: https://slideplayer.com/slide/5240097

Philip Equation (1957, 1969) Philips Solution for Horizontal and Vertical Infiltration

Vertical Infiltration For practical purpose, the series in the previous equation is commonly truncated and only the first two terms are retained: Cumulative:

Rate:

Reference: Trieste, Miramare (May 1994). Soil Physical Properties Influencing Parameter in Philip and Kostiakov Infiltration Models. Retrieved from: www.google.com Tuller, M. & Or, D. (2002-2004). Infiltration to Soils. Retrieve from: https://slideplayer.com/slide/5240097

Philip Equation (1957, 1969) Example 4. 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 0.40 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.

Reference: mgebrekiros.github.io>infiltration

Green- Ampt Method 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. Consider a vertical column of soil of unit horizontal cross- section area and let a control volume be defined around the wet soil between the surface and depth I.

Reference: mgebrekiros.github.io>infiltration

Green- Ampt Method

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Green- Ampt Method

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Green- Ampt Method

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Green- Ampt Method Parameters in the Green- Ampt model

Table 2. USDA Soil Texture Green- Ampt Infiltration Parameters (Maidment, 1993)

Reference: mgebrekiros.github.io>infiltration

Green- Ampt Method Note: that for bare ground cover conditions K = Ks/2, for the area which is bare under canopy the effective hydraulic conductivity can be assumed to be equal to the saturated hydraulic conductivity Ks of the soil. The area which has ground cover is assumed to contain macroporosity, and the effective hydraulic conductivity is equal to the saturated hydraulic conductivity Ks times a macroporosity factor A. For areas which don not undergo mechanical disturbance like range land macroporosity factor A is determined from:

A = exp(2.82 - 0.099S +1.94BD)

[Eq. 9]

And for undisturbed agricultural areas A can be determined from

A = exp(0.96 - 0.032S + 0.04C - 0.032BD) where;

[Eq. 10]

S = Percent sand C = percent clay BD = bulk density of the soil (< 2 mm), g/cc, and A > 1.0. Reference: mgebrekiros.github.io>infiltration

Green- Ampt Method

Reference: mgebrekiros.github.io>infiltration

Green- Ampt Method

Reference: mgebrekiros.github.io>infiltration

Green- Ampt Method

Reference: mgebrekiros.github.io>infiltration

example:

Ponding Time 

Ponding Time RAINFALL STARTS

SOIL SUCKS WATER

PONDING STARTS

SUCTION FORCE WEAKENS

SOIL INCREASE IN MOISTURE

SOIL BECOMES SATURATED

Ponding Time

Ponding Time

Ponding Time

Ponding Time Example 6 Solution:

Ponding Time Example 6

Solution: