Infiltration is the movement of water into the soil under the driving forces of gravity and capillarity, and limited by viscous forces involved in the flow into soil pores as quantified in terms of permeability or hydraulic conductivity. The infiltration rate, f, is the rate at which this process occurs. The infiltration rate actually experienced in a given soil depends on the amount and distribution of soil moisture and on the availability of water at the surface. There is a maximum rate at which the soil in a given condition can absorb water. This upper limit is called the infiltration capacity, fc. Note that this is a rate, not a depth quantity. It is a limitation on the rate at which water can move into the ground. If surface water input is less than infiltration capacity, the infiltration rate will be equal to the surface water input rate, w. If rainfall intensity exceeds the ability of the soil to absorb moisture, infiltration occurs at the infiltration capacity rate. Therefore to calculate the actual infiltration rate, f, is the lesser of fc or w. Water that does not infiltrate collects on the ground surface and contributes to surface detention or runoff (Figure
35). The surface overland flow runoff rate, R, is the excess surface water input that does not infiltrate.
R = w - f (28)
This is also often referred to as precipitation excess.
The infiltration capacity declines rapidly during the early part of a storm and reaches an approximately constant steady state value after a few hours (Figure 7). The focus of this section on at a point infiltration models for calculating runoff is on how to calculate runoff accounting for the reduction of infiltration capacity. We use accumulated infiltration depth, F, as an independent variable and write infiltration capacity as a decreasing function fc(F), then as F increases with time fc is reduced. fc may be a gradually decreasing function, or a threshold function, as in the case of saturation excess runoff where there is a finite soil moisture deficit that can