Scaling Soil-Water Retention Curves to Field-Measured Maximum and Minimum Moisture Contents to Improve Model Predictions of Infiltration in a Semi-Arid Watershed.

We investigated the effects of scaling soil hydraulic property measurements and estimates on 1-D model predictions of soil moisture content in the Dry Creek Experimental Watershed (DCEW). The DCEW is a non-agricultural, mountainous site near Boise ID, USA. The soil hydraulic properties were obtained from in-situ measurements, laboratory multistep outflow tests, several pedotransfer functions, and finally, by inversion of field pressure head and moisture content data using HYDRUS-1D (Simunek et al., 2005). Two different scaling techniques were also applied: the Basile et al. (2003) approach of combining field and laboratory measured parameters, and secondly, replacement of the predicted or measured saturated and residual moisture contents by the field-measured minimum and maximum values. The resulting soil-moisture retention curves were compared to the in-situ curves, and evaluated for goodness of fit. The various parameter estimates were then used as inputs for HYDRUS-1D to determine the effects of different inputs on predictions of soil moisture for an extended period. Not surprisingly, the inversely estimated inputs yielded the best predictions of moisture content with RMSE values of 0.008 to 0.01 cm³/cm³. The scaled inputs yielded RMSE error values that ranged from 0.02 to 0.03 cm³/cm³, which represented a significant improvement over results when unscaled inputs were used. Implications of these results for experimental design of future field sampling and watershed monitoring studies will be discussed.