Drinking Water Supplies - The Drinking Water - page 63

• Soil composition —The composition of the surface soil directly affects
the amount of runoff; for example, it is obvious that hard rock sur-
faces result in 100% runoff. Clay soils have very small void spaces
that swell when wet; when these void spaces fill and close, they do
not allow infiltration. Coarse sand possesses large void spaces that
allow water to flow easily through it, thus producing the opposite
effect of clay soil, even in a torrential downpour.
• Vegetation cover —Groundcover limits runoff. Roots of vegetation and
pine needles, pine cones, leaves, and branches create a porous layer
(a sheet of decaying natural organic substances) above the soil. This
porous, organic layer readily allows water into the soil. Vegetation
and organic waste also act as cover to protect the soil from hard,
driving rains, which can compact bare soils, close off void spaces,
and increase runoff. Vegetation and groundcover work to maintain
the infiltration and water-holding capacity of the soil and also work
to reduce soil moisture evaporation.
• Ground slope —When rain falls on steeply sloping ground, 80% or
more may become surface runoff. Gravity moves the water down the
surface more quickly than it can infiltrate the surface. Water flow off
flat land is usually slow enough to provide opportunity for a higher
percentage of the rainwater to infiltrate the ground.
• Human inluences —Various human activities have a definite impact
on surface water runoff. Most human activities tend to increase the
rate of water flow; for example, canals and ditches are usually con-
structed to provide steady flow, and agricultural activities generally
remove groundcover that would work to retard the runoff rate. At
the opposite extreme, manmade dams are generally built to retard
the flow of runoff.
DID YoU KNoW?
Paved streets, tarmac, paved parking lots, and buildings are imper-
vious to water infiltration, greatly increasing the amount of storm-
water runoff from precipitation events. These manmade surfaces
hasten the flow of surface water and often cause flooding, some-
times with devastating consequences. In badly planned areas, even
relatively light precipitation can cause local flooding. Impervious
surfaces not only present flooding problems but also do not allow
water to percolate into the soil to recharge groundwater supplies—
a potentially devastating blow to a location's water supply.