RUNOFF - Fundamentals of Hydrology - page 81

Table 5.1 Some typical infiltration rates compared to rainfall intensities

Soil and vegetation Infiltration rate Rainfall type Rainfall intensity

(mm/hr) (mm/hr)

Forested loam 100-200 Thunderstorm 50-100

Loam pasture 10-70 Heavy rain 5-20

Sand 3-15 Moderate rain 0.5-5

Bare clay 0-4 Light rain 0.5

Source : From Burt (1987)

on the observations from the eastern USA: that
during a storm it was common to find all the rainfall
infiltrating a soil. Hewlett and Hibbert (1967)
hypothesised that during a rainfall event all the
water infiltrated the surface. This hypothesis was
confirmed by a comprehensive field study by Dunne
and Black (1970).
Through a mixture of infiltration and through-
flow the water table would rise until in some places
it reached the surface. At this stage overland flow
occurs as a mixture of return flow (i.e. water that
has been beneath the ground but returns to the
surface) and rainfall falling on saturated areas. This
type of overland flow is referred to as saturated
overland flow . Hewlett and Hibbert (1967)
suggested that the water table was closest to the
surface, and therefore likely to rise to the surface
quickest, adjacent to stream channels and at the base
of slopes. Their ideas on stormflow were that the
areas contributing water to the hydrograph peaks
were the saturated zones, and that these vary from
storm to storm. In effect the saturated areas imme-
diately adjacent to the stream act as extended

channel networks. This is referred to as the variable
source areas concept . This goes a step beyond the
ideas of Betson (1964) as the catchment has a partial
areas response but the response area is dynamic;
i.e. variable in space and time.
So who was right: Horton, or Hewlett and
Hibbert? The answer is that both were. Table 5.2
provides a summary of the ideas for storm runoff
generation described here. It is now accepted that
saturated overland flow (Hewlett and Hibbert) is the
dominant overland flow mechanism in humid, mid-
latitude areas. It is also accepted that the variable
source areas concept is the most valid description of
stormflow processes. However, where the infiltration
capacity of a soil is low or the rainfall rates are high,
Hortonian overland flow does occur. In Table 5.1 it
can be seen that there are times when rainfall inten-
sities will exceed infiltration rates under natural
circumstances. In arid and semi-arid zones it is not
uncommon to find extremely high rainfall rates (fed
by convective storms) that can lead to infiltration
excess overland flow and rapid flood events; this is
called flash flooding .

Table 5.2 A summary of the ideas on how stormflow is generated in a catchment

Horton Betson Hewlett and Hibbert

Infiltration Controls overland flow Controls overland flow All rainfall infiltrates

Overland flow Infiltration excess Infiltration excess Saturated overland flow

mechanism
Contributing area Uniform throughout Restricted to certain areas Contributing area is variable

the catchment of the catchment in time and space