Soil water

Recent advances in developing methods to solve the equations governing combined saturated‐unsaturated flow in two and three dimensions [ Freeze , 1971; Stephenson and Freeze , 1974] have focused attention on the need to determine effective field values of the hydraulic properties of the soil. Because of natural variation from point to point, hydraulic properties for field‐size units are difficult to characterize. A detailed field study was conducted by Nielsen et al. [1973] to determine the field variability of the hydraulic properties of the soil and to test various field methods of measuring these properties. They concluded that even seemingly uniform land areas manifest large variations in hydraulic conductivity. For a given point, methods for measuring the soil hydraulic properties will give values that are more accurate than those required to characterize an entire field because of the heterogeneity of the soil. Thus the ability to make predictions over a large area from soil properties determined at one location can range from good to unsatisfactory, depending on the prediction parameter of interest. Because of field variation in the hydraulic properties of the soil, simplified methods for calculating soil water flux and water contents during redistribution were found to be satisfactory when they were compared with more detailed numerical methods and with field measurements. Furthermore, when field variability is considered, simplified methods for measuring hydraulic conductivity or soil water diffusivity are sufficiently accurate for characterizing field conditions. The results of this study will have continued application in evaluating the worth of both simple and complex models for predicting water movement in the field.