Infiltration
INFILTRATION - is the process by which water from the surface gets
into the ground by means of gravitational and capillary forces. The rate of
infiltration defines how much water will be left on the surface to
contribute to the rapid runoff and how much will go into the ground and further
slowly travel through the porous media.
The factors that influence infiltration may be grouped into three categories.
- Related to climatic conditions. The amount of water infiltrated depends
upon the duration and intensity of rainfall. A 24 hour drizzle can be entirely
accommodated by the soils, whereas the same amount of water received during a
20 minute downpour will most probably end up in the surface water runoff.
Temperature also matters. When ground is frozen the intensity of infiltration
drops.
- Related to surface characteristics. Land use and landcover translates
into the imperviousness of the surface. A parking lot will leave little
water to infiltrate, whereas a forest may capture the entire amount of
water coming. On the other hand, forests can intercept the incoming rainfall
with leaves and trees in such a way that a certain portion of the incoming
water never reaches the ground. This moisture is only exposed to
evaporation. Slope also matters. In a flat area there is more time for
water to enter the ground, while on a hill it starts traveling downwards
along the surface as soon as it hits the ground.
- Related to soil characteristics. Sand is an excellent media for
infiltration. On the contrary, clay can block almost all infiltration. Besides,
if the soil is already saturated with water (soil moisture content is high),
there will be little space left in the pores for additional water to infiltrate.
A typical infiltration event evolves both in space and in time. As the rainfall
starts some water begins to seep into the ground, gradually increasing the
soil water content (curves 1-3) at the top of the soil layer. As more water
comes with the rain it keeps entering the soil pores. The gravitation removes
some water from the top layers and makes it travel further deeper into the
ground. If this vertical movement is fast enough to free up the space on
the top for the additional incoming water, then all the rain is absorbed.
If the soil characteristics do not allow water to travel fast enough
through the soil, then
the pores on the top get all filled up (curve 4-6) and the additional water
will be left on the surface to travel with overland flows. This is when
ponding may occur. The wave of
saturated water propagates downwards through the soil. Once the rain
stops the pores at the top start to dry out getting ready to accommodate a
new rainfall event.
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