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4.
Evaporation
Part of rainfall water received on the surface or intercepted by plants and leaves
or in depressions or from the water surfaces of streams is lost due to
evaporation and transpiration. It has already been discussed about the role played
by evapotranspiration in the hydrologic cycle.
Evaporation losses will depend upon several factors, like area of water surface,
depth of water in the water body, humidity, wind velocity, temperature,
atmospheric pressure and quality of water.
Factors Affecting Run Off
When rainfall occurs on the earth's surface its disposal in run off
depends upon several factors such as :
1) Intensity of rainfall and characteristics of precipitation,
2) Topography,
3) Size and shape of the catchment area,
4) Geology of the area,
5) Meterological characteristics,
6) Character of the catchment surface,
7) Storage characteristics.
Rainfall can be of very short duration or it can be continuous over a period of
time. More is the rainfall, more will be run-off. For short duration rain or drizzles,
most of rain water may be evaporated or infiltrates into the soil.
Nearness of nalas or streams or drainage channels also influences run off. In
case of fan shaped catchment or watershed, most of drainage channels join
the main stream at a time, thus inducing greater runoff. Topography greatly
influences run-off. In case of hills, rainwater reaches the main stream with
greater velocity than those occurring in flat areas. This is again influenced by
the geological terrain. In hard rock areas with steep gradient of hillocks, run-off
is greater. Even in flat lands covered by hard rocks run-off is higher. In flat
alluvial terrain water percolates through the soil, thus reducing the surface
run-off. If the surface area is cultivated, or away from any drainage channel run
