Runoff

Runoff

It is the amount of rain water that flows on the surface of the earth. It is helpful in evaluating amount of water available in the river over long periods and thus it is helpful in estimating the storage capacity of the reservoir, and the fixed height of the dam to be constructed for the storage of the water. Therefore these determinations are useful when storage reservoir is to be used as a source of water. However, when a river or a stream is to be directly used as a source of water, day to day quantities of flow in that river or stream is determined. Here are some formula:

Estimating Run-off and Yield of a Basin:

a) English Formula: It is used in South India, 

                          For hilly catchment: Qy= 0.85Py- 304

                          For plane catchment:  Qy = Py[ (Py-17.8)/254]

  where, Qy = yearly run-off in cm depth over the basin

               Py=  yearly precipitation or annual rainfall in cm.

b)Khosla's Formula:  Qy = Py - 0.4813Tm (Tm>4.5.C)

c) Qy = KPy

where K is impervious factor.