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Apart from rainfall characteristics such as intensity, duration and distribution, there
are a number of site (or catchment) specific factors which have a direct bearing on the
occurrence and volume of runoff.
- Soil type
The infiltration capacity is among others dependent on the porosity of a soil which
determines the water storage capacity and affects the resistance of water to flow into
deeper layers.
- Vegetation
The amount of rain lost to interception storage on the foliage depends on the kind of
vegetation and its growth stage. Values of interception are between 1 and 4 mm. A
cereal crop, for example, has a smaller storage capacity than a dense grass cover.
More significant is the effect the vegetation has on the infiltration capacity of the soil.
A dense vegetation cover shields the soil from the raindrop impact and reduces the
crusting effect as described earlier.
In addition, the root system as well as organic matter in the soil increase the soil
porosity thus allowing more water to infiltrate. Vegetation also retards the surface
flow particularly on gentle slopes, giving the water more time to infiltrate and to
evaporate.
In conclusion, an area densely covered with vegetation, yields less runoff than bare
ground.
- Slope and catchment size
Steep slope plots yield more runoff than those with gentle slopes.
In addition, it was observed that the quantity of runoff decreased with increasing slope
length.
This is mainly due to lower flow velocities and subsequently a longer time of
concentration (defined as the time needed for a drop of water to reach the outlet of a
catchment from the most remote location in the catchment). This means that the water
is exposed for a longer duration to infiltration and evaporation before it reaches the
measuring point. The same applies when catchment areas of different sizes are
compared.
The runoff efficiency (volume of runoff per unit of area) increases with the decreasing
size of the catchment i.e. the larger the size of the catchment the larger the time of
concentration and the smaller the runoff efficiency.
Frederick paul answered the question on February 25, 2019 at 13:02