Dynamic hydrologic models of areas that are potentially larger than characteristic storm sizes must give explicit consideration to the effect of storm size on the rainfall‐runoff process. The local response of an element of the modeled area can be parametrized in terms of proportion of saturated and impermeable area and infiltration parameters of the unsaturated permeable areas. When a very simple spatial description of the storm depth and duration is provided, it is possible to integrate the local response over the entire area to obtain the average, or lumped, input‐output behavior of the large area on a volume basis. Where infiltration‐excess runoff is significant, the rainfall‐runoff response of a large area is extremely sensitive to the storm size, with a fixed volume of precipitation producing more runoff when it is concentrated over smaller areas. Where saturated or impermeable source areas provide most of the surface runoff, this scale effect on runoff volume is absent. Whenever the effect is significant, it becomes important for rainfall‐runoff models to include not only the rainfall volume but also some description of its areal variability, as forcing
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