Functional sensitivity analysis of four methods to generate soil hydraulic functions

Rapid advances in model building have led to the understanding that applicability of future simulations depends, to a great extent, on the availability of accurate soil hydraulic functions obtained with efficient methods rather than on new models. In this study, four different methods were used to generate hydraulic functions: Method A, direct on-site measurement; Method B, use of measured hydraulic functions averaged on a regional scale; Method C, use of measured hydraulic functions averaged on a national scale; and Method D, use of van Genuchten parameters correlated with soil texture and organic-matter content. Accuracy of these methods was tested by comparing the simulated water storage with the measured water storage of the upper 0.5 m of three soil profiles over a period of 7 yr. Differences in performance of the four methods were not significant. Agreement between measured and simulated water storage was best, however, when directly measured hydraulic functions (Method A) were used. Next best agreement was obtained when continuous (Method D) and two types of class pedotransfer functions (Methods B and C) were used, which relate textures and soil horizons to physical characteristics. Costs involved in obtaining directly measured soil hydraulic functions are prohibitively high, compared with costs for the other methods. With regard to both accuracy and costs, the development of a data base of measured soil hydraulic functions and use of this information to derive continuous and class pedotransfer functions assures, in many cases, optimal spending of limited available resources.