The factors affecting the runoff are as follows: 1. Precipitation 2. Size and Shape of the Catchment Area 3. Geographical Characteristics 4. Meteorological Characteristics 5. Drainage Net 6. Other Factors.
Factor # 1. Precipitation:
The runoff is clearly a function of precipitation, its intensity, its duration and its coverage. More the intensity, more will be the runoff. The infiltration rate reduces after some time; hence more the duration, proportionately more will be the runoff. Similarly, more the area covered by the storm, more will be the runoff.
Direction of movement of a storm over the catchment area has a definite effect on the runoff. If the storm moves in the direction of the flow, the base period of hydrograph will be less and more peak flow may be expected. On the other hand, if the storm over against the flow direction, then the base period will be comparatively more and less peak flow may be expected (Fig. 2.7).
Factor # 2. Size and Shape of the Catchment Area:
The size of catchment has a definite effect on the runoff. More the area, more will be the runoff. So also, the shape will have a definite effect on the runoff.
In case of a fan-shaped catchment area, the base period of the resulting hydrograph will be less and thus more peak flow may be expected. In case of an elongated catchment, the base period of the resulting hydrograph will be comparatively more and thus more will be the infiltration losses and less will be the runoff (Fig. 2.8).
The average value of:
i. Manning’s coefficient,
ii. The shape factor,
iii. Stream order,
iv. Drainage density,
v. Stream density,
vi. Circularity ration,
vii. Elongation ratio,
viii. Compactness coefficient, and
ix. The slope of the channel and so on of the catchment thus definitely influences.
Factor # 3. Geographical Conditions:
The nature of the soil, its permeability, has an effect on the infiltration rate and has indirect effect on the runoff. Impervious tock outcrops will increase the runoff. Also, impervious sub-surface layers at higher levels than groundwater table level increase the runoff.
Faults, fissures and cracks in the surface formations will allow more infiltration of the overland flow and the water thus infiltrated may find its opening in the adjoining catchment area or the same catchment somewhere else and may indirectly affect the runoff.
Factor # 4. Meteorological Conditions:
The temperature has an effect on the evaporation and infiltration and may indirectly affect the runoff. The barometric pressure, altitude and wind will not only affect the storm and its movement, but may also affect the runoff.
Factor # 5. Drainage Net:
The rainwater after meeting all abstractions, first flows through small rivulets and then flows to bigger ones. The pattern of the various tributaries normally known as drainage net or drainage pattern will affect drainage of the surface low. In each case, the draining time will differ and will affect the infiltration and thus indirectly the runoff.
Factor # 6. Other Factors:
Other factors affecting run off are:
a. Cultivation in an area,
b. Contour bunding,
d. Deforestation, and
e. Urbanization and so on have a direct effect on the runoff.