1.5.4 Runoff

Factors affecting runoff

Precipitation characteristics – Most important factor on which rainfall depends. The more the rainfall, more will be run off. Run-off depends upon the types of storm causing precipitation, also upon its duration. Runoff increases with the intensity of rainfall, with light showers most of water will be lost in infiltration and evaporation etc. If rainfall is in the form of snow runoff will be less.

Shape & size of catchment – Runoff depends on size, shape and location of catchment. A stream collecting water from a small catchment area is likely to give greater runoff intensity per unit area. For very big catchment uniform rain seldom falls over entire area, with the result that only few tributaries will feed at a time, resulting in less runoff.

In fan or sector shaped catchment all tributaries are approximately of same size, such catchment gives greater runoff since the peak flood from the tributaries is likely to reach the main stream approximately at the same time. In case of fern shaped catchment the tributaries are of different lengths and meet the main stream at regular intervals. In such a narrow catchment, peak flood intensity is reduces since discharges are likely to be distributed over a long period of time.

Topography - If surface slope is steep, water will flow quickly and absorption and evaporation losses will be less resulting in greater runoff.

Geological characteristics - are the important characteristics affecting runoff. These include the type of surface soil and subsoil, type of rock, permeability characteristics. If soil and subsoil is pervious seepage will be more, reducing the peak flood. If the surface is solid rocky the absorption will be practically nil and runoff will be more. As against if cracks are fissures are present in the rock water will be lost in seepage.

Meteorological characteristics - Temperature, wind and humidity also affect runoff. High temperature and greater wind velocity give rise to greater evaporation loss and reduce runoff. If the ground is saturated it gives more runoff. If the direction of storm is in direction of stream it increases runoff.

Character of catchment surface - Runoff also depends upon surface conditions-whether surface is drained or un drained, natural or cultivated and whether it is bare or covered with vegetation etc. If the surface has no natural drainage, absorption lass will be more. If more area of catchment is cultivated surface runoff will be less.

Storage characteristics – The artificial storage such as dams, weirs etc and natural storage such as lakes, pond etc tend to reduce peak flow. The storage into the pervious surface soil and subsoil tends to reduce the peak flow and tend to regularize the flow of a stream.

Computation of runoff

Runoff from a catchment can be computed daily, monthly or yearly. Following are the methods

By runoff formulae and tables –

Runoff coefficient – Runoff and rainfall can be interrelated by runoff coefficient by the expression

R = kP

R = runoff in cm, P = rainfall in cm, k = runoff coefficient

Runoff coefficient depends upon all the factors which affect the runoff. This method is used only for small water control project, should be avoided for analysis of major storms. In general runoff coefficients are

Barlow’s table – Barlow carried out studies of catchment under 140 sq.km in U.P and deduced values of k (in percentage) for various classified catchments.

Strange’s tables and curves - Strange gave tables and curves from rainfall in the plains of south India. Strange’s tables and curves give runoff for daily rainfall and take into account three types of catchments i.e. good, average and bad and three surface conditions ( dry, damp & wet)prior to rain.

Inglis’s formula – Inglis derived formula by data collected from 37 catchments in Maharashtra

For Ghat areas R = 0.85 P – 30.5 cm

For non ghat areas R = cm