Some basic principles that govern the movement of water into the soil can be used to predict infiltration. The infiltration capacity, f (L), is the maximum rate that a soil in a given condition can absorb water and generally decreases as soil moisture increases. If the rainfall rate is less than the infiltration capacity, then infiltration proceeds at the capacity rate. However, if the rainfall rate exceeds the infiltration capacity, then infiltration proceeds at the capacity rate, and the excess rainfall ponds on the surface or runs off. As the time from the onset of rainfall increases, infiltration rates decrease due to soil moisture increases, raindrop impact, and the clogging of soil pores, until a steady-state infiltration rate is reached (Fig. 8).24 Existing infiltration models use empirical, approximate, or physical approaches to predict infiltration.25
Empirical. Empirical infiltration models generally utilize a mathematical function whose shape as a function of time, t, matches observations and then attempts a physical explanation of the process.
Kostiakov26 proposed the simple infiltration rate,/(L/T) model:
where a and y are constants that have no particular meaning and must be evaluated by fitting the model to experimental data.
where S is a parameter called sorptivity, t is time from ponding, and A is a constant that depends on soil properties. In this model, the infiltration rate approaches a constant equal to the hydraulic conductivity at the surface water content, and the wetting front advances without changing its shape and approaches a constant velocity.
Physical. Recent advances in numerical methods and computing has facilitated the practical application of the Richards equation to realistic flow problems. Such packages can simulate water infiltration and redistribution using the Richards equation and including precipitation, runoff, drainage, evaporation, and transpiration processes.30
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