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Infiltration

Infiltration is the term applied to the process of water entry into the soil. The rate of infiltration determines The time at which superficial water appears on the soil surface The amount of runoff that will form over the soil surface during rainfall or irrigation If the rate of infiltration is limiting, the entire water balance in the root zone will be affected. The infiltration process When rain falls or sprinkling occurs, the water supply rate may either be less than or greater than the saturated hydraulic conductivity of the surface soil. If the rate is less than the hydraulic conductivity, all the water enters the soil. In this case the infiltration rate, that is, the rate at which water enters the soil, is equal to the water supply rate. In this case the only way that runoff can occur is if the soil becomes saturated above an impermeable layer, or if the soil has a layer in which the hydraulic conductivity is less than the hydraulic conductivity of the layers above it. If the water supply rate is greater than the hydraulic conductivity and the soil is dry, then for a while all the water enters the soil. In this case the rate at which water enters the soil is greater than the saturated hydraulic conductivity. This occurs because water not only flows in response to gravity, but also in response to the soil suction. As the water content of the soil decreases, the soil suction also decreases. Sooner or later, the supply rate begins to exceed the capability of the soil to absorb the water. At this point, water begins to build up on the soil surface and runoff begins. The time between the start of the rainfall and the initiation of runoff is known as the time to ponding. The infiltration rate continues to decrease and asymptotically approaches the saturated hydraulic conductivity. The steady state infiltration rate is termed

the steady state infiltrability. It is approximately the same as the field saturated hydraulic conductivity of the surface soil.

INFILTRATION RATE (f)

Application Rate (R)

Ks

Tp

TIME (t)

Schematic representation of the infiltration process

Factors affecting the infiltration process 1. Soil properties. The steady state infiltration rate is approximately equal to the hydraulic conductivity. Therefore soils with higher hydraulic conductivities tend to have more infiltration and less runoff. In addition, the pore size distribution influences the rate of change of infiltrability. Generally speaking, the wider the range of pore sizes the more gradual the change in the infiltration rate. The pore size distribution is a mirror image of the particle size distribution. 2. Initial moisture content. For a given rainfall rate, the longer is the time to ponding and the more gradual is the change in infiltration rate. For saturated soils, the infiltration falls to the saturated hydraulic conductivity almost instantaneously. 3. Rainfall Rates. For rates less than Ks in a deep homogeneous soil, infiltration may continue indefinitely for rainfall rates less than Ks. Extremely high

rainfall rates may cause destruction of the peds at the soil surface leading to surface sealing or the formation of soil crusts 4. Surface sealing and crusting. Changes in the hydraulic conductivity of the surface soil has a stronger influence on the infiltration rate than most other factors. Fifty years ago, the decrease in infiltration rate was thought to be the results of progressive formation of a surface seal. The formation of a seal 5 mm thick lead to a 75% decrease in infiltration rate. 5. Layered soils. When the wetting front in the soil reaches a layer with either a coarser or a finer texture, there is a decrease in the infiltration rate for some period of time. If the layer has a coarser texture, the infiltration rate will recover when the large pores in that layer become filled. For soil in which the layer is finer textured, the infiltration rate will stay low. 6. Movement and entrapment of soil air. If air is trapped in the soil, the hydraulic conductivity is reduced. Estimating the Volume of Water Infiltrated The infiltration rate curve serves to partition the water supply between infiltration and runoff. The volume of water infiltrated is given by the area under the infiltration rate curve. The volume of runoff is the area between the supply line and the infiltration rate curve.

INFILTRATION RATE (f)

R

Runoff Volume Infiltration Volume

Ks

Tp

TIME (t)

In order to estimate the volume of infiltration, the curve may be treated as a series of trapezia. The volume of water infiltrated is the sum of the area of the individual trapezia.

Area = (L 1 + L 2) * W 2

INFILTRATION RATE (f)

R L1 L2

W Ks

T T T T T

Tp

If equal time increments are used then the area can be estimated by the trapezium rule. The smaller the time increments, the more accurate the resulting estimate of infiltration volume. In the limit, the area under the curve can be evaluated exactly using the principles of calculus. This is beyond the scope of this course. In order to determine the infiltration rate at any time approximate infiltration rate equations have been developed. Most of these equations are empirical, that is they are based on observed behavior. In some equations the parameters have no physical meaning. Kastiakov Equation f t = K k t - + Ks where ft is the infiltration rate at time t; and Kk and are constants. These constants have to be evaluated from experimental data. One of the benefits of this equation is that it can be used to determine when ponding will occur.

Horton Equation f t = K s + (R - K s )e -(t-t p ) for t tp

where R is the rainfall (sprinkler) rate; tp is the time to ponding; and is a soil parameter which depends on initial conditions and application rate. In most irrigation designs, the point at which water is applied is fixed, so can be treated as a constant. (e is a number commonly used in mathematical formulations. It has a value approximately equal to 2.7183).

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