Question

Please explain in detail the effects of different moisture conditions (saturated/dry) in soil to rainfall-runoff relationships? Expound on the concepts of rainfall volume, runoff volume, runoff coefficient, and interflow.

Answer 1

After a particular duration of rainfall, Runoff volume mainly depends on the surface conditions v.i.z.,. Characteristics of soil such as Soil texture, Porosity, Structure and hydraulic conductivity.

Soils having wet / saturated moisture conditions will show a high runoff capacity at initial stages as capillary infringement in the soils prevent water to drain off / percolate at initial stages into the sub surface. This is due to pore water pressure exerted by the saturated soil profile. However, Dry sandy soil will have low runoff than rainfall due to ground water movement.

Higher the infiltration capacity(i.e., directly proportional to porosity), Runoff volume observed will be low with respect to perception taken place(i.e., rainfall amount).

The relation between infiltration and time for different loamy soils are as given below:

Rainfall volume: The total amount of precipitation observed over a particular area recorded for a particular duration of a total rainfall time is known as Rainfall Volume.

Runoff volume is the observed stream flow after the discharge of run off over the stream after a duration of rainfall. Any runoff stream accompanied by natural abstractions / valleys / streams will have a discharge to duration pattern as drawn below:

Ground water table with infiltrated water gives inflow to the stream, which is considered as Inter flow. Inter flow reduces the initial losses such as loss due to Evaporation, Evapotranspiration etc.,.

Runoff coefficient gives amount of rainfall water passes through a particular area over certain duration. It is denoted by "C".

C = Q/(iA)

where, Q = discharge or runoff

i = hydraulic gradient

A = area over which storm occured / catchment area.