# USE OF INFILTRATION INDICES

The infiltration capacity curve as shown in Fig. 6.11 cannot be used for computing

- run-off. run-off from large basins.
- It is, because, in large basins the infiltration capacity as well as rainfall rate vary from point to point.
- Moreover sub-surface flow (interflow)
- will also be substantial. Since this water-flow is a part of infiltration, it will not normally be included in the run-off compute by using infiltration capacity curve determined on a small test plot.
- Run-off volumes for large areas are computed using infiltration indices.
- W and φ are the two commonly used indices.
- W-index in the average infiltration rate or the infiltration capacity averaged over the whole storm period and is given as follows:

\[W-index= \frac{P-R}{Tr}\]

where

P = Total precipitation or rainfall.

R = Total run-off.

Tr = Duration of rainfall in hours.

- φ-index may be defined as the average rate of loss of precipitation such that.
- the volume of rainfall in excess of that rate will be equal to the volume of direct run-off φ-index can also be stated.
- As the rate of rainfall above which the rainfall volume equals the run-off volume.
- φ-index can be represent show

- φ and W-indices will be equal for a uniform rainfall but may not be equal for non-uniform rainfalls.
- However for rains which are reasonably uniform or for heavy rains, these two indices are found to be nearly equal.
- The run-off coefficient k can be determined as follows if W-index is known.

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