MACROSCOPIC CAPILLARY LENGTH, SORPTIVITY, AND SHAPE FACTOR IN MODELING THE INFILTRATION-RATE

Infiltration tests in the field involve measurements of sorptivity and macroscopic capillary length. These two parameters are strongly relat ed through the shape factor, which is a measure of the nonlinearity of the soil hydraulic diffusivity. In this study, relationships were dev eloped between the macroscopic capillary length, the sorptivity, and t he shape factor and the parameters of the Brooks and Corey and van Gen uchten expressions of hydraulic conductivity and diffusivity. These re lationships are important for users of numerical models who need to es timate the parameters of these expressions to predict water dow and co ntaminant transport in soils. Numerical simulations with a dimensionle ss form of Richards' equation show that the predicted infiltration rat e will not be very sensitive to small variations in the shape factor, provided the macroscopic capillary length is the same. This result is encouraging because the shape factor Is difficult to determine accurat ely in the field. The similarity of the dimensional infiltration solut ions implies that the macroscopic capillary length is a scale factor, because (i) it renders predictions of infiltration rates fairly insens itive to the expressions of hydraulic conductivity and diffusivity use d, and (ii) it reduces the number of parameters needed to characterize infiltration. Therefore, the infiltration curve into a particular soi l can be deduced by choosing units of length and time (i.e., scaling) of a generalized infiltration solution.