watershed physics: Soil variability criteria

Variability criteria for input parameters to the moisture characteristic and hydraulic conductivity models are developed. The parameters include the water contents at air entry and at 15 bars as well as the hydraulic conductivity used in matching the predicted values with the experimental values at saturation. The available data suggest that the water contents at 15 bars and at air entry have a normal distribution whereas the hydraulic conductivity has a log normal distribution. The distributions of water contents and conductivities for different soil series with depth and over an area are illustrated, and variability is expressed as the coefficient of variation. It is suggested that in watershed studies a soil be considered uniform with depth or over an area if the coefficient of variation with respect to water content does not exceed 15% at 15 bars and 10% at air entry and the logarithmic standard deviation for hydraulic conductivity used in matching is less than log 2. If the preceding conditions are met, the water contents at 15 bars and at air entry as well as the saturated conductivities may be averaged.