Geoscience Reference
In-Depth Information
Fig. 11.3 Schematic illustration of
the overland flow (OF)
mechanism as saturation
excess: (a) the position of
the water table (WT) prior
to the onset of precipitation
and (b) during the
precipitation event.
The precipitation rate
P
is smaller than the
infiltration capacity over
the unsaturated portion of
the land surface; overland
flow takes place where the
water table has risen to the
ground surface.
L S
( a )
W T
P
I n f i l t r a t i o n
( b )
W T
OF
in 8% of the precipitation events infiltration excess overland flow was the predominant
runoff process; rainfall intensity, rather than rainfall amount, was the decisive factor for
its occurrence, although soil surface crusting also played a role. Occurrence and yield of
overland flow varied spatially on account of the variability of the infiltration capacity.
In general, infiltration excess overland flow appears to be rare in natural basins covered
with a thriving vegetation in more humidclimates.
Saturation excess overland flow
This type of surface runoff occurs over land surfaces that are saturated by emerging
subsurface outflow from below and perched water tables, regardless of the intensity
of the rainfall (or snowmelt) (see Figure 11.3). It is a rapid and almost immediate
transport mechanism to the stream channel, for the seepage outflow water and for the
rainwater falling (or snow melting) on such areas. It usually takes place in conjunction
with subsurface flow to the channel, but the relative magnitudes of surface and subsurface
flows into the channel depend largely on the nature of the catchment and the precipitation.
It is most often observed over limited areas in the immediate vicinity of the river channel
where downslope subsurface flows emerge, and in wetlands, where the water table can
rise rapidly to the surface; but it can also occur higher up in slope hollows, where elevation
contours display strong curvature, thus forcing convergence of the flow paths. Outside
of these saturated areas all the precipitation and other input can generally enter the soil
surface.
