Geography Reference
In-Depth Information
infiltrate into the soil or pond-up and flow over the surface as
overland flow.
Infiltration is influenced by vegetation cover, soil texture, soil
structure, the amount and connectivity of pore spaces and compac-
tion. People often measure the
infiltration rate
of soils, which is
the volume of water passing into the soil per unit area per unit
time. They measure it because it is useful information to under-
stand how fast water can soak into a soil and therefore how much
water might run over the land surface during heavy rain. Water
moving over the land surface tends to get into rivers more quickly
than water that infiltrates into soil and moves within the soil. The
maximum rate of infiltration when there is a plentiful water supply
is the
infiltration capacity
. The infiltration capacity of a soil gen-
erally decreases during rainfall. Therefore, if rainfall is at a constant
rate then the water arriving at the soil surface may at first all infil-
trate into the soil but then as infiltration capacity decreases more of
the water will run over the land's surface. Compaction of the soil,
surface crusts or a frozen surface can restrict infiltration even if
within the soil itself water percolation rates could be quite fast.
These restrictions to infiltration can lead to fast generation of over-
land flow and potentially large river flood peaks. Soils with lots of
humus and a deep litter layer, such as those within tropical rainfor-
ests, tend to have large infiltration capacities.
If surface water supply is greater than the rate of infiltration then
overland flow can begin once small surface depressions begin to
overflow. This process is called
iniltration-excess overland
low
. This type of flow is uncommon in many temperate areas
except in urban locations, along roads and paths or perhaps along
compacted soils in arable fields created by tractor wheels or over-
grazing by animals. Iniltration-excess overland flow is more
common in semi-arid regions where soil surface crusts have
developed and rainfall rates can be rapid. It is also more likely in
areas where the ground surface is often frozen, such as northern
Canada or parts of Siberia.
However, one of the most common misconceptions is to think
that iniltration-excess overland flow is the only way overland flow
can be developed. In fact, there is another important process
known as
saturation-excess overland flow.
. When all the pore
spaces are full of water then the soil is said to be saturated and the
