The process, whereby water enters the surface strata of the soil and thus moves downward towards the water-table is know infiltration.
In fact when water falls on the soil, a small part of it is first of all absorb by the top thin layer of soil so as to replenish the soil moisture deficiency.
After this any excess water moves downward where it is trapped in the voids and becomes ground water.
The amount of stored ground water mainly depends upon the number of voids present in the soil.
The number of voids further depend upon the size, shape, arrangement, and degree of compaction of the soil.
Hence different soils will have different number of voids and hence different capacities to absorb water.
The maximum rate at which a soil in any given condition is capable of absorb water.
It is evident that rain water will enter the soil at full capacity rate only during the periods when rainfall.
When the rainfall intensity is less than the infiltration capacity, the prevailing infiltration rate is approximately equal to the rainfall rate.
Hence the actual prevailing infiltration rate may be equal to or less than the infiltration capacity.
This actual prevailing rate at which the water is entering the given soil at any given time is know infiltrationrate.
If the rainfall intensity exceeds the infiltration capacity the difference is call the rainfall excess rate.
This excess water is first of all accumulated on the ground as surface detention and then flows over land into,streams.
The water below the water-table is know as the ground water and the water abovetable is know as soil moisture.
When a graph is drawn between capacity and duration of rainfall in hours we get a curve know.
This method ofcomputing run-off is use for small catchments.
