ABSTRACT
Precipitation, in the form of rain or snow, is the primary
source of runoff or surface drainage water. When rain falls
on the surface of the soil, part or all of it is absorbed by the
soil itself. If the rainfall intensity rate is less than the
absorption rate of the soil, all of the rain will be absorbed
and there will not be any surface runoff. The rate at which
soil can absorb water depends on the soil type. Coarse sand
and gravel soils can absorb water at a very high rate, so
there is usually no surface drainage or runoff from such
soils. Clay soils absorb water much more slowly than sand.
The ability of soil to absorb water also changes over time.
When the soil is dry, it can absorb water rapidly. As a soil
becomes more wet, the rate of absorption decreases. The
way a given soil absorbs water is called the infiltration
characteristic of the soil. Fig. 1 shows how the infiltration
rate of a Columbia Silt Loam soil changes over time. The
infiltration rate of a soil is measured in mm hr-1, which is
the same as the way rainfall rates are measured. As time
increases, the rate at which the soil can absorb water
decreases. Whenever the rainfall rate is less than the infil-
tration rate of the soil, the rain will all be absorbed and
there will be no runoff or surface drainage. In Fig. 1, if the
rainstorm continued at a rate of 18 mm hr-1 for a period of
more than 30 minutes, surface runoff would begin to
occur. Also, if the rainfall intensity was greater than
60 mm hr-1, surface runoff would begin immediately,
inasmuch as the rainfall rate is higher than the infiltration
of the dry soil. Floods or surface drainage that occurs due to
very high intensity, short duration rainfalls, are called flash
floods. Flash floods are of short duration and high volume
and are very dangerous because there is little fore-warning
of their occurrence.
