Discussion
Figures 10 and 11 illustrate the implications of the model when
extrapolated to longer slopes and more extreme rainfalls. Using the same
parameter as in the field data, runoff coefficients are estimated as a
function of storm rainfall for slopes of 5 to 50m in figure 12, where
they are compared with SCS curve number relationships. Over the storm
sizes seen in the field data, the greatest relative divergences between
the two approaches are found for the large number of storms of less than
30 mm and with less than 10% runoff, for which the SCS method
consistently underpredicts the small volumes of runoff.
The importance of the proposed alternative runoff estimate is not,
however, seen to lie in the quality of fit to individual data sets, all
of which show wide variations that can be contained within either form
of expression. What seems much more important and useful is that the
proposed expression explicitly includes scaling for both rainfall
intensity and slope length, providing a model with much greater
possibilities for transference across scales and between sites and
climates. Experimentation within the model environment also shows that
the parameters in equation (4) also responds rationally to changes in
infiltration parameters and their spatial variability, to gradient and
to micro-topography expressed through the potential for locally
divergent flow.
The potential to apply a model at different spatial scales within a
catchment is of value, not only in support of field experiments but also
to distribute runoff and sediment transport within a field area or
within a landscape evolution model.