WATER-RETENTION IN AUSTRALIAN SOILS .1. DESCRIPTION AND PREDICTION USING PARAMETRIC FUNCTIONS

The soil water characteristic (SWC) is a fundamental property controll ing soil water storage and movement. The SWC is often described by ana lytical functions because this is convenient in the solution of numeri cal flow equations as well as in implementation of closed-form methods of predicting unsaturated hydraulic conductivity. The analytical func tions must adequately describe the measured SWC data for these applica tions to be successful. We used two large sets of SWC data from Austra lian soils to evaluate five widely used equations on the basis of how well they describe the measured data. In general, sigmoidal equations described the measured SWCs best, but the simpler power-law equations also gave a very good description. Most of the error with the power-la w functions occurred at water contents near saturation from physically unrealistic equation discontinuity around the air entry potential. Th e good performance of the power-law equations enabled development of a method to predict SWC data from only two measured SWC points and a kn owledge of soil bulk density. When a power-law equation was used toget her with parabolic smoothing and parameter values were obtained using just two measured SWC points, then a very good SWC prediction was obta ined within the 0 to -150 m matric potential range. This method does i ncrease the reliance on the accuracy of measurement of the two points that are used for interpolation or extrapolation. The 'two-point' pred iction method significantly reduces the cost of obtaining SWC data, th us easing an important constraint to the widespread application of soi l water simulation models based on Richards' equation.