FACTORS AFFECTING THE RUN-OFF

## FACTORS AFFECTING THE RUN-OFF

• The characteristics of the rainfall play an important part in determining the amount of consequent run-off’.
• The various factors that affect the run-off can be summarise under two heads.
1. Characteristics of precipitation,
2. Characteristics of drainage basin.

1. Characteristics of Precipitation

(a) Type of precipitation. Precipitation may be in the form of rain or drizzle.

• Run-off pattern or the hydrograph of run off is considerably governed by this factor.
• If precipitation occurs in the form of heavy rain,it will immediately produce bulk of run off (Peak flow of short duration).
• If precipitation is in the form of a drizzle it will produce run off at a slow and steady rate.

(b) Rain intensity. Rain intensity has a lot of effect on the run off.

• If the intensity of rain increases, the run off increases rapidly.
• For example, if the intensity is increase four times, the run off may increase nine times or so.
• For example let there be a rain in progress sufficient to make the infiltration capacity constant,
• say 0.5 cm/hr. Now if the intensity of rain is 0.8 cm/hr the run off (strictly speaking excess rain) will occur at the rate of 0.3 cm/hr.
• Now let the intensity of rain be increased to a value say 12 cm/hr (four times), the resulting run off rate will be equal to 2.7 cm/hr (nine times).
• Thus an intense rain of the type shown in Fig. 6.8 (a) will definitely produce much more run off than a uniform rain of
• the type shown in Fig. 6.8 (b) provid the infiltration capacity remains the same throughout the storm period.

• Although the total amount of rain in Fig. 6.8 (a) and (b) is same, still rain
• (a) will produce higher amounts of run off while rain
• (b) is likely to produce much less run off.

(c) Rainfall duration. It is important because infiltration capacity goes on reducing with longer duration of rainfall till it becomes constant.

• If the infiltration is less, the surface run-off will be more.
• The water table may reach the ground level and in such case there will be no filtration.

(d) Rainfall distribution. Run-off from a basin is very much dependent upon the rainfall distribution.

• The rain may fall either on the whole basin or on a small part of it.
• For small drainage basins the peak flows are generally the result of intense rains falling over small areas.
• On the other hand for large drainage basins, the peak flows are the result of storms of lesser intensity but covering large areas.
• The rainfall distribution is generally express by the distribution coefficient which for a given storm can be obtaine by dividing.
• the maximum rainfall at the point by the mean rainfall on the basin.

(e) Soil moisture deficiency. The run off also depends upon the soil moisture present at the time of the rainfall.

• If a rainfall occurs after a long dry spell of time, the soil is dry and it can absorb large amounts of water.
• In such a condition even intense rain may fail to produce any appreciable run off.
• On the other hand if there are persistent rains, the soil will be already wet and infiltration will be very small.
• In such conditions even small rains may cause considerable floods.

(f) Direction of the storm. If the direction of draining water and that of storm are same, more floods are likely to occur because storms will be increasing velocity of surface flow, giving very little time for infiltration.

(g) Climatic conditions. Various other climatic factors such as temperature, wind, humidity, etc.

• affect the run off. More the losses lesser will be the available runoff.

2. Drainage Basin Characteristics

(a) Size of the basin. If spread or the area of the basin is large the total flood flow will require more time to pass through an outlet.

• This fact will widen the base of the flood hydrograph and thus reduce the peak flow.
• It is so because total volume of water passing the outlet is same, but time is more.

(b) Shape of the basin. The shape of the drainage basin appreciably affects the rate at which water enters the stream.

• The shape of the basin may be (1) Fan shaped and (2) Fern leaf shaped.

• Fan shaped basins give greater run off than Fern-leaf basins.
• This is because the tributaries in the case of Fan shaped basin are nearly of same size and length and lead their discharges at the outlet point
• practically at the same time.
• In the case of Fern-leaf catchments flood discharge is less as water from the tributaries lying near the outlet point passes out of outlet point, before water from upper areas reaches the outlet point

(c) Slope of the catchment. If catchment area has more slope, discharge available in form of run-off will be more.

(d) Nature of surface of basin. If surface of the catchment is very permeable to large depth, run-off available will be small.

• Such soil conditions absorb more of rain water.

(e) Topography of the basin. The catchment having more rugged surface, more of vegetation gives less amount of run off.

• If artificial storages are available in the catchment, available run-off is again decreased.

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