WATER AND ICE - Physical Geography - page 78

water table (highest point below which the soil or rock is satu-
rated) is at the surface. Water can therefore leave the soil and run
out over the surface. This is common in shallow soils or at the
bottom of hillslopes where water running through the soil will
collect, keeping the pore spaces full of water for long periods
allowing water to return to the surface at this point. This means
that saturation-excess overland flow can occur long after it has
stopped raining. If it is raining then this additional rainwater will
find it difficult to enter the soil if it is saturated and so saturation-
excess overland flow can be a mix of fresh rainwater and water that
has been within the soil for some time.
The landscape area that produces saturation-excess overland
flow varies through time. During wet seasons a larger area of soil
will be saturated and able to generate saturation-excess overland
flow than during dry seasons. If the catchment starts off relatively
dry then during a rainfall event not much of the area will generate
saturation-excess overland flow, but as rainfall continues then more
of the catchment becomes saturated, especially in the valley
bottoms, and therefore a larger area of the catchment will produce
saturation-excess overland flow. This means that the source areas
for overland flow are variable whereas the source areas for
iniltration-excess overland flow tend to be the same.
Water moving through soils or rocks is called throughflow. .
Most rivers around the world are mainly fed by throughflow. This
process can maintain river flow during dry periods (the baseflow
of the river). There are different ways water can move through soil
or rock and this affects how quickly water can get to a river
channel and therefore the typical nature of the response of a river
to rainfall. Matrix flow is where water moves through the small
pore spaces within a rock or soil whereas in macropore flow
water moves through larger cracks, fissures or continuous channels
within the rock or soil thereby bypassing contact with most of the
soil mass itself. It is possible to estimate the rate of water flowing
through the matrix of a saturated soil or rock using techniques such
as dye tracing, pumping water out of a well and timing the length
of time it takes to rise up the well again, or taking small samples
back to the laboratory and running water through them. Most
water in soils tends to move through the matrix and therefore there
are opportunities for cation exchange (see Chapter 3 on soil