Components of Runoff

Runoff means the draining or flowing off of precipitation from a catchment area through a surface channel enters into a stream channel. It represents the output from catchment in a given unit of time. Fig. 18.1 shows components of runoff.

Fig. .Components of runoff.

Consider a catchment area receiving precipitation. For a given precipitation, when the evapotranspiration, initial loss, infiltration and detention storage requirements are satisfied, the excess precipitation moves over the land surfaces to reach smaller channels. This portion of runoff is called overland flow and involves building up of storage over the surface and draining off the same.Flows from several small channels join bigger channels and flows from these in turn combine to form a larger stream, and so on, till the flow reaches the catchment outlet. The flow in this mode, where it travels all the time over the surface as overland flow and through the channels as open-channel flow and reaches the catchment outlet is called surface runoff.

A part of the precipitation that infilters moves laterally through upper crusts of the soil and returns to the surface at some locations away from the point of entry into the soil. This component of runoff is known variously as interflow, through flow, storm seepage, subsurface flow or quick return flow. Depending upon the time delay between the infiltration and the outflow, the interflow is sometimes classified into prompt interflow, i.e. the interflow with the least time lag and delayed interflow.

  Another route for the infiltered water is to undergo deep percolation and reach the groundwater storage. The time lag, i.e. the difference in time between the entry into the soil and outflows from it is very large, being of the order of months and years. This part of runoff is called groundwater runoff or groundwater flow.

 Based on the time delay between the precipitation and the runoff, the runoff is classified into two categories; as

(a) Direct runoff

(b) Base flow.

a)  Direct runoff

It is the part of runoff which enters the stream immediately after the rainfall. It includes surface runoff, prompt interflow and rainfall on the surface of the stream. In the case of snow-melt, the resulting flow entering the stream is also a direct runoff. Direct storm runoff and storm runoff are also used to designate direct runoff.

 b)  Base flow

The delayed flow that reaches a stream essentially as groundwater flow is called base flow.

 Factors Affecting Runoff

The main factors affecting the runoff from a catchment area are:

a)       Precipitation characteristics

b)      Shape and size of catchment

c)       Topography

d)      Geologic characteristics

e)       Meteorological characteristics

f)        Storage characteristics of a catchment

Precipitation Characteristics

Precipitation is the most important factor, which affects runoff.  The important characteristics of precipitation are duration, intensity and areal distribution. 

Duration Total runoff depends on the duration of rainstorm.  For a given rainfall intensity and other conditions, a longer duration rainfall event will result in more runoff. 

Intensity Rainfall intensity influences both rate and volume of runoff.  The runoff volume and also runoff rate will be greater for an intense rainfall event than for less intense event. 

Areal Distribution It also influences both the rate and volume of runoff.  Generally, the maximum rate and volume of runoff occurs when the entire watershed contributes.

Shape and Size of Catchment

The runoff from a catchment depends upon the size, shape and location of the catchment. The following are the general observations:

a)  More intense rainfall events are generally distributed over a relatively smaller area, i.e., larger the area lower will be the intensity of rainfall.

b)  The peak normally decreases as the area of the basin increase. (peak flow per unit area)

c)   Larger basins give a more constant minimum flow than the smaller ones. (effect of local rains and greater capacity of the ground-water reservoir)

d)  Fan shaped catchments give greater runoff because tributaries are nearly of same size and hence time of concentration of runoff is nearly same.  On the contrary, discharges over fern leaf arrangement of tributaries are distributed over long period because of the different lengths of tributaries.

Fig. a.Fan shaped catchment.  Fig. b.Leaf shaped catchment.

Topography

The runoff depends upon surface condition, slope and land features.  Runoff will be more from a smooth surface than from rugged surface.  Also, if the surface slope is steep, water will flow quickly and adsorption and evaporation losses will be less, resulting in greater runoff.  On the other hand if the catchment is mountainous, the rainfall intensity will be high and hence runoff will be more.

 Geologic Characteristics

Geologic characteristics include surface and sub-surface soil type, rocks and their permeability.  Geologic characteristics influence infiltration and percolation rates.  The runoff will be more for low infiltration capacity soil (clay) than for high infiltration capacity soil (sand).

 Meteorological Characteristics

Temperature, wind speed, and humidity are the major meteorological factors, which affect runoff.  Temperature, wind speed and humidity affect evaporation and transpiration rates, thus soil moisture regime and infiltration rate, and finally runoff volume.

 Storage Characteristics of a Catchment

Presence of artificial storage such as dams, weirs etc. and natural storage such as lakes and ponds etc. tend to reduce the peak flow.  These structures also give rise to greater evaporation.