ABSTRACT
In this chapter, infiltration rate will imply capacity or potential rate that is the maximum rate at which water enters the soil under no restriction on the supply of water. Clearly, the actual rate of infiltration is less than or equal to the potential rate, depending on the availability of water. It is known that infiltration is a function of antecedent soil moisture, soil characteristics, vegetation, land use, climatic characteristics, land slope, and supply of water or rainfall. Some of these factors vary in space, some vary in time, and some vary in both space and time. Soil characteristics vary significantly from one place to another, and antecedent soil moisture, which defines the initial infiltration, also significantly varies spatially. The infiltration parameters determined using point measurements are point values, or at best reflect average values. Although large spatial variability in infiltration is recognized, little effort has been made to account for its probabilistic characteristics, except for a few watershed models, as for example the BASINS (formerly Stanford Watershed Model) (Crawford and Linsley, 1966; Donigian and Imhoff, 2006). Crawford and Linsley (1966) were probably the first to consider spatial variations in infiltration capacity; from empirical data reported in the literature (Burgy and Luthin, 1956), they found large variations in infiltration capacity even in relatively homogeneous soils (uniform Yolo silt loam) and over small areas (40 ft × 20 ft). Considering infiltration capacity as a random variable, they expressed the cumulative probability distribution function of infiltration capacity as a function of area.
