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off will be less. Meteorological conditions also controls run-off. A frozen surface
or stream gives rise to more run off. Rise in temperature induces more
evaporation and thus run off is reduced. Wind direction of the storm in the
direction of streams give higher run off. If there are artificial storage like dam
and weirs, run-off will be reduced.
Measurement of Run-off :
Measurement of run-off is very important for the Irrigation Engineers for
designing the dams, weirs and other water holding structures. There are
several methods which are adopted by the Engineers, but details of the
methodologies are beyond the scope of this book. In brief, methods adopted
are:
a) Run-off formulae and tables.
b) By infiltration method.
c) By Unit Hydrograph.
d) Rational Method.
Run Off Formulae and Tables :
One of the most common method used by using the formulae for small areas
R = K x P
Where R = run-off in cm
P = rainfall in cm.
K = run-off co-efficient.
Run-off co-efficient depends on various factors like the urban area,
vegetative area, farm lands etc. Various values of K, which are commonly used,
are shown in the Table 1.3. (After S.K. Garg, 1988)
