ERRORS IN THE ESTIMATION OF SOIL-WATER PROPERTIES AND THEIR PROPAGATION THROUGH A HYDROLOGICAL MODEL

The Agricultural Catchments Research Unit model (ACRU) includes a deci sion support system (DSS) for estimating the water content of soil at field capacity (theta(FC)) and wilting point (theta(WP)) when these ch aracteristics are not directly measurable. Three methods of estimation are proposed: (a) based on silt and clay content and bulk density, (b ) based on clay content only, and (c) based on soil series. These thre e pedotransfer functions are compared with respect to both the estimat ion of theta(FC) and theta(WP) and the propagation of errors when the actual evapotranspiration of a wheat crop (E) is predicted over the gr owing season by the ACRU model. The standard error of estimation was b etween 0.066 and 0.082 m(3)/m(3) for theta(FC), between 0.056 and 0.06 9 m(3)/m(3) for theta(WP) and between 29.9 and 34.8 mm of water for E. The method based on silt and clay contents and bulk density predicted theta(FC) and theta(WP) for non-swelling soils most precisely. The me thod based on soil series was better than other methods for swelling s oils. It also performed better for estimating available water capacity and consequently for predicting E from a conceptual soil water model. The propagated error of estimating theta(FC) and theta(WP) using the DSS reached 15-18% of the simulated E. The error in the prediction of E can reach 26-30% when spatial variation in soil properties is also e stimated.