SOIL PROPERTIES

Hydrological Pathways involved in different runoff
generation processes. Infiltration excess pathways are shown in
red. Saturation excess and subsurface stormflow pathways are
shown in blue. Groundwater and baseflow pathways in black and
Evapotranspiration is green. (Courtesy of Mike Kirkby)

The general climatic regime controls the total volume of runoff in
any region through its effect on the water balance. In a broad sense,
over a time scale long enough that storage changes average out (are
negligible), and over a region large enough or with boundary defined
so that inflows (surface and subsurface) are negligible, the water
balance may be stated as
P = Q + E (1)
where P is the precipitation rate, Q the runoff rate, and E the
evapotranspiration rate. This equation indicates that the precipitation
input is disposed of either into runoff or evapotranspiration. In
general the climatic regime controls the overall proportioning. Here
groundwater recharge supplying baseflow is included in Q. Because
the quantities in equation (1) must be positive, this equation places
limits on the values of Q and E given any specific P. Both Q and E
are constrained to be less than P. This may be visualized in a
space
The domain of valid solutions is below the 1:1 line E=P. There is in
general an upper limit on the possible evapotranspiration, due to the
energy inputs required to evaporate water. This limit is related to the
solar radiation inputs as well as the capacity of the atmosphere to
transport evaporated water away from the surface (related to wind
and humidity). This limit has been denoted as Ep (potential
evapotranspiration) in Figure 14. The line E=Ep provides another
upper limit to the domain of valid solutions

precipitation is large (P >> Ep) water is not going to be limited at the
earth surface so E will approach Ep asymptotically for P tending to
infinity. Also, if precipitation is small (P << Ep) water is very limited
at the earth surface and may all evaporate, so E will approach the line
E=P asymptotically as P tends to 0 approaching the origin. These
constraints suggest a solution for the E versus P function of the form
indicated in Figure 14. In Figure 14 the axes have been scaled by Ep
to make them dimensionless. A nonlinear increase in Q with P as
P/Ep increases is suggested, and the index P/Ep serves as an index
of regional humidity or aridity, with P/Ep large (>1) in humid
regions and small (<1) in arid regions

Generalised dependence of Runoff Coefficient and
Style of Overland Flow on Arid-Humid scale and on Storm Rainfall
Intensities (Courtesy of Mike Kirkby)
These regional water balance considerations based on climatic regime
serve as first order controls on the generation of runoff. However
data plotted in the form of Figure 14, shows scatter due to other
effects. Precipitation intensity is also important. Figure 15 shows the
interplay between humidity/aridity and precipitation intensity on
runoff processes and the runoff coefficient. In this figure, similar to
P/Ep the ratio E/Ep serves as a measure of aridity, with E/Ep