Infiltration rate and infiltration test

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Infiltration rate and infiltration testINFILTRATION RATE

Infiltration is the process by which water on the ground surface enters the soil. Infiltration

is governed by two forces, gravity, and capillary action. While smaller pores offer greater

resistance to gravity, very small pores pull water through capillary action in addition to and

even against the force of gravity.

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 measured in inches per hour or millimeters per hour. The

rate decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltrationrate, runoff will usually occur unless there is some physical barrier. It is related to the

saturated hydraulic conductivity of the near-surface soil.

The rate of infiltration is affected by soil characteristics including ease of entry, storage

capacity, and transmission rate through the soil. The soil texture and structure, vegetation

types and cover, water content of the soil, soil temperature, and rainfall intensity all play a

role in dictating infiltration rate and capacity. For example, coarse-grained sandy soils have

large spaces between each grain and allow water to infiltrate quickly. Vegetation creates

more porous soils by both protecting the soil from pounding rainfall, which can close

natural gaps between soil particles, and loosening soil through root action. This is why

forested areas have the highest infiltration rates of any vegetative types.

The top layer of leaf litter that is not decomposed protects the soil from the pounding action

of rain, without this the soil can become far less permeable. In chapparal vegetated areas,

the hydrophobic soils in the succulent leaves can be spread over the soil surface with fire,

creating large areas of hydrophobic soils. Other conditions that can lower infiltion rates or

block them include dry litter that resists re-wetting, or frosts can lower infiltration. If soil is

saturated at the time of an intense freezing period, the soil can become a concrete frost on

which infiltration rates would be around zero. But any of these infiltration reducing

conditions would not be over an entire watershed, there are most likely gaps in the concrete

frost or hydrophobic soil where water can infiltrate.

Horton (1933) suggested that infiltration capacity rapidly declines during the early part of a

storm and then tends towards an approximately constant value after a couple of hours for

the remainder of the event. Previously infiltrated water fills the available storage spaces and

reduces the capillary forces drawing water into the pores. Clay particles in the soil may

swell as they become wet and thereby reduce the size of the pores. In areas where the

ground is not protected by a layer of forest litter, raindrops can detach soil particles from



the surface and wash fine particles into surface pores where they can impede the infiltration

process.

Once water has infiltrated the soil it remains in the soil, percolates down to the ground

water table, or becomes part of the subsurface runoff process.

The process of infiltration can continue only if there is room available for additional water

at the soil surface. The available volume for additional water in the soil depends on the

porosity of the soil and the rate at which previously infiltrated water can move away from

the surface through the soil. The maximum rate that water can enter a soil in a given

condition is the infiltration capacity. If the arrival of the water at the soil surface is less than

the infiltration capacity, all of the water will infiltrate. If rainfall intensity at the soil surface

occurs at a rate that exceeds the infiltration capacity, ponding begins and is followed by

runoff over the ground surface, once depression storage is filled. This runoff is called

Horton overland flow. The entire hydrologic system of a watershed is sometimes analyed

using hydrology transport models, mathematical models that consider infiltration, runoff

and channel flow to predict river flow rates and stream water quality.

The infiltration rate is the velocity or speed at which water enters into the soil. It is usuallymeasured by the depth (in mm) of the water layer that can enter the soil in one hour. Aninfiltration rate of 15 mm/hour means that a water layer of 15 mm on the soil surface, willtake one hour to infiltrate.

In dry soil, water infiltrates rapidly. This is called the initial infiltration rate. As more waterreplaces the air in the pores, the water from the soil surface infiltrates more slowly andeventually reaches a steady rate. This is called the basic infiltration rate (Table 7).

The infiltration rate depends on soil texture (the size of the soil particles) and soil structure

(the arrangement of the soil particles: see Volume 1) and is a useful way of categorizingsoils from an irrigation point of view (see Table 8).

The most common method to measure the infiltration rate is by a field test using a cylinderor ring infiltrometer.

Table 7 BASIC INFILTRATION RATES FOR VARIOUS SOIL TYPES

Soil type Basic infiltration rate (mm/hour)

sand less than 30

sandy loam 20 - 30

loam 10 - 20

clay loam 5 - 10

clay 1 - 5

FIELD INFILTRATION TEST

Equipment required



Shovel/hoeHammer (2 kg)Watch or clock5 litre bucketTimber (75 x 75 x 400)Hessian (300 x 300) or jute cloth

At least 100 litres of water

Ring infiltrometer of 30 cm diameter and 60 cm diameter. Instead of the outer cylinder abund could be made to prevent lateral water flow.

Measuring rod graduated in mm (e.g. 300 mm ruler)

Figure 74 Set-up of field test

Method

Step1:

Hammer the 30 cm diameter ring at least 15 cm into the soil. Use the timber to protect thering from damage during hammering. Keep the side of the ring vertical and drive themeasuring rod into the soil so that approximately 12 cm is left above the ground.

Step2:

Hammer the 60 cm ring into the soil or construct an earth bund around the 30 cm ring to thesame height as the ring and place the hessian inside the infiltrometer to protect the soilsurface when pouring in the water (Figure 75).

Step3:

Start the test by pouring water into the ring until the depth is approximately 70-100 mm. At thesame time, add water to the space between the two rings or the ring and the bund to thesame depth. Do this quickly.

The water in the bund or within the two rings is to prevent a lateral spread of water from theinfiltrometer.



Step4:

Record the clock time when the test begins and note the water level on the measuring rod.

Step5:

After 1-2 minutes, record the drop in water level in the inner ring on the measuring rod andadd water to bring the level back to approximately the original level at the start of the test.Record the water level. Maintain the water level outside the ring similar to that inside.

Step6:

Continue the test until the drop in water level is the same over the same time interval. Takereadings frequently (e.g. every 1-2 minutes) at the beginning of the test, but extend theinterval between readings as the time goes on (e.g. every 20-30 minutes).

Note that at least two infiltration tests should be carried out at a site to make sure that thecorrect results are obtained.

Figure 75 Cylinder infiltrometers with second ring or bund

Table 8 and Figure 76 show how to record these measured data.

Table 8:

- Column 1 indicates the readings on the clock in hours, minutes and seconds.

- Column 2 indicates the difference in time (in minutes) between two readings.

- Column 3 indicates the cumulative time (in minutes); this is the time (in minutes) since the teststarted.



- Column 4 indicates the water level readings (in mm) on the measuring rod: before and after filling(see step 5).

- Column 5 indicates the infiltration (in mm) between two readings; this is the difference in themeasured water levels between two readings. How the infiltration is calculated is indicated in

brackets.

- Column 6 indicates the infiltration rate (in mm/minute); this is the infiltration (in mm; column 5)divided by the difference in time (in minutes, column 2).

- Column 7 indicates the infiltration rate (in mm/hour); this is the infiltration rate (in mm/minute,column 6) multiplied by 60 (60 minutes in 1 hour).

- Column 8 indicates the cumulative infiltration (in mm); this is the infiltration (in mm) since the teststarted. How the cumulative infiltration is calculated is indicated in brackets.

DATA SHEET: INFILTRATION RATE

Figure 76:



In Figure 76, the cumulative time (in minutes, column 2) is set out against the cumulativeinfiltration (in mm, column 8) and a curve is formed. From Figure 73 it can, for example, beobserved that for the soil type used in the example it takes 70 minutes to infiltrateapproximately 74 mm of irrigation water.

The basic infiltration rate can be determined from Table 8, column 7: the infiltration rate in

am/hour. Once the values of the Infiltration rate are constant, the basic infiltration rate hasbeen reached. In this example the basic infiltration rate is 27 mm/hour and was reachedafter 60 minutes. After 60 minutes the cumulative infiltration was 69 mm. After the first 60minutes the infiltration rate is constant: 27 mm/hour. So after 120 minutes (2 hours) thecumulative infiltration will be 69 + 27 = 96 mm (indicated on the graph with a dotted line).After 3 hours the cumulative Infiltration will be (96 + 27 =) 123 mm, etc. Once the curvehas been established it is possible to determine how long it will take to infiltrate a certainamount of water. This is of course important to know when determining the irrigation time.

Figure 76 Example of an infiltration curve

Note: The infiltration curve should be determined for normal soil moisture conditionsbefore irrigation takes place, i.e. usually when the top soil is dry.



DATA SHEET: INFILTRATION RATE

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Infiltration rate and infiltration test