Factors Affecting Runoff
The runoff rate and its volume from an area, mainly in influenced by following tow factors:
a) Climatic factors and
b) Physiographic factors
Factors Affecting Runoff – Climatic Factors:
The climatic factors of the watershed affecting the runoff are mainly associated with the characteristics of precipitation, which include.
1. Type of precipitation
2. Rainfall intensity
3. Forms of precipitation
4. Duration of rainfall
5. Rainfall distribution
6. Direction of prevailing wind and
7. Other climatic factors.
1. Type of precipitation: Types of precipitation have a great effect on the runoff. For example a precipitation which occurs in form of rainfall, starts immediately in from of surface flow over the land surface, depending upon its intensity as well as magnitude, while a precipitation which takes place in form of snow or hails the flow of water on ground surface will not take place immediately, but after melting of the same. During the time interval of their melting the melted water infiltrates into the soil and results a very little surface runoff generation.
2. Rainfall intensity: The intensity of rainfall has a dominating effect on runoff yield. If rainfall intensity is greater than infiltration rate of the soil the surface runoff takes place very shortly while in case of low intensity rainfall, where is found a reverse trend of the same. Thus high intensities rainfall yield higher runoff and vice-versa.
3. Duration of rainfall : Rainfall duration is directly related to the volume of runoff due to the fact, that infiltration rate of the soil goeson decreasing with the duration of rainfall till it attains constant rate. As a result of this even a mild intensity rainfall lasting for longer duration may yield a coverderable amount of runoff.
4. Rainfall distribution: Runoff them a water sheed depends very much on the distribution of rainfall. The rainfall distribution for this purpose can but expressed by a team "distribution coefficient which may be defined as the ratio of maximum rainfall at a point to the mean rainfall of the watershed. For a given total rainfall, if all other conditions are the same, the greater the value of distribution coefficient, greater will be the peak runoff and vice – versa. However, for the same distribution coefficient, the peak runoff would be resulted from the storm, falling on the lower part of the basin i.e. near the outlet.
5. Direction of prevailing wind: The direction of prevailing wind, affected greatly the runoff flow. If the direction of prevailing wind is same as the drainage system then it has great influence on the resulting peak flow and also on the duration of surface flow, to reach at the outlet. A storm moving in the direction of stream slope produces a higher peak in shorter period of time than a storm moving in opposite direction.
6. Other climatic factors: The other climatic factors, such as temperature wind velocity, relative humidity, annual rainfall etc. affect the water losses from the watershed area to a great extent and thus the runoff is also affecter accordingly. If the losses are more the runoff will be less and vice -versa.