Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour.
     The rate at which water infiltrates into a ground is called the infiltration capacity. When a soil is dry, the infiltration rate is usually high compared to when the soil is moist. For an initially dry soil subjected to rain, the infiltration capacity curve shows an exponentially decaying trend as shown in Figure 10. The observed trend is due to the fact that when the soil is initially dry, the rate of infiltration is high but soon decreases, as most of the soil gets moist. The rate of infiltration reaches a uniform rate after some time.
Infiltration Curve
Interestingly, if the supply of continuous water from the surface is cutoff, then the infiltration capacity starts rising from the point of discontinuity as shown in below.
For consistency in hydrological calculations, a constant value of infiltration rate for the entire storm duration is adopted. The average infiltration rate is called the Infiltration Index and the two types of indices commonly used are explained in the next section.
Infiltration indices
The two commonly used infiltration indices are the following:
φ – index
W – index
The φ - index
This is defined as the rate of infiltration above which the rainfall volume equals runoff volume, as shown in Figure
φ – index
The method to determine the - index would usually involve some trial. Since the infiltration capacity decreases with a prolonged storm, the use of an average loss rate in the form of - index is best suited for design storms occurring on wet soils in which case the loss rate reaches a final constant rate prior to or early in the storm. Although the - index is sometimes criticized as being too simple a measure for infiltration, the concept is quite meaningful in the study of storm runoff from large watersheds. The evaluation of the infiltration process is less precise for large watersheds. The data is never sufficient to derive an infiltration curve. Under the circumstances, the - index is the only feasible alternative to predict the infiltration from the storm.
The W – index
This is the average infiltration rate during the time when the rainfall intensity exceeds the infiltration rate.
Thus, W may be mathematically calculated by dividing the total infiltration (expressed as a depth of water) divided by the time during which the rainfall intensity exceeds the infiltration rate. Total infiltration may be fund out as under:
Total infiltration = Total precipitation – Surface runoff – Effective storm retention The W – index can be derived from the observed rainfall and runoff data. It differs from the - index in that it excludes surface storage and retention. The index does not have any real physical significance when computed for a multiple complex watershed. Like the phi-index the - index, too is usually used for large watersheds.

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