Implications and Application of the Raats Superclass of Soils Equations

According to the Richards equation, the capacity of a soil to hold and conduct water is determined by the water retention and hydraulic conductivity characteristics. Many mathematical relationships have been proposed in the literature to describe these characteristics. Raats introduced a general functional relationship with only four parameters that included as special cases four pre-1990 models found in the literature, including the well-known relationships by Mualem-van Genuchten and Brooks and Corey. The aims of this study were (i) to discuss this general functional relationship and its four special cases, (ii) to present expressions for the differential moisture capacity, water diffusivity, and matric flux potential corresponding to the general functional relationship and its special cases, (iii) to discuss methods for determining the parameter elasticity and sensitivity, and (iv) to apply the Raats model to experimental data. Soil water retention and hydraulic conductivity data for 11 soils were used to optimize the values of the four major parameters in the Raats model. In none of the cases did the optimized coefficients indicate that the Raats model approached one of the four submodels that it includes.