Accuracy of soil water budgets based on a range of relationships for the influence of soil water availability on actual water use

The accuracy of six published functions for deriving dryland water use from evaporative demand and soil water status was assessed by incorporating them in water budgets which were used to estimate dryland soil water status from actual climatic records. Budget-derived estimates were compared with values actually measured under improved pastures in the field over an 842 day period. The root mean square (RMS) of the differences between computed and observed soil water values was used to evaluate the various functions. RMS values were found to vary from 8.1 to 29.5 mm for the various functions tested. Soil water estimations made by using a simple ratio function were generally as good as or better than those made by using more complex functions. The sensitivity of the various functions to changes in their input assumptions was tested. The results of these tests will facilitate the selection of the optimum functions for conditions other than those encountered in this study. Reduced accuracy of soil water prediction resulted from the use of functions to set water use equal to the potential rate, regardless of the overall dryness of the soil profile, whenever recent rainfall was calculated to have made water available in the surface zone.