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

Infiltration rate and run-off.

  • 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:

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


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-index & run-off

  • 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.