Testing soil water retention estimation with the MUUF pedotransfer model using data from the southern United States

Soil hydraulic properties have often been estimated rather than measured. Such estimates are made using pedotransfer functions (PTFs), i.e. regression equations relating soil hydraulic properties to soil basic properties. PTFs were developed from about 18,000 samples from the national soil characterization database that used the clay activity ratio along with texture to characterize the effect of soil minerals on soil hydraulic properties. With those PTFs, the Map Unit User File (MUUF) software was developed to use soil series name and textural class of the uppermost soil horizon to estimate soil water retention. Objectives of this study were: (a) to test the MUUF with a database from Southern Plains, and (b) to see whether the MUUF PTFs will be more accurate with actual soil texture data as compared with the soil texture data for the representative pedon of the soil series. The root-mean-square error (RMSE) of the MUUF estimates of water content at −33 kPa in the uppermost horizon was about 0.04 m3 m−3, when only textural class and soil names were used. The accuracy did not change when the actual soil texture was entered in PTFs. In deeper horizons, the RMSE was about 0.065 m3 m−3 when only soil name and textural classes were used. The accuracy improved significantly as the actual texture was used instead of the texture derived from the representative pedon. Using actual texture halved the RMSE in estimates of water contents at −1500 kPa. The worst estimates were obtained for silt texture. As compared with PTFs found in literature, the MUUF algorithms provide an average or better accuracy of water retention estimates in the uppermost soil horizons from the soil series name and textural class. For deep soil horizons, the MUUF pedotransfer algorithms are capable of the same degree of accuracy provided the information on texture of the deeper soil horizons is available.