Using a soil hydrology model to obtain regionally averaged soil moisture values

The Soil Hydrology Model (SHM) was modified, and daily simulations of soil volumetric water content were made at 38 Oklahoma Mesonet sites for July 19 97. These model results were compared with soil moisture observations made at the mesonet sites at depths of 5. 25, 60, and 75 cm. This work is believ ed to be the first time that a hydrological model has been evaluated with i n situ soil moisture measurements over such an extensive area spanning seve ral climate zones. Comparisons of time series between the observed and modeled domain-averaged volumetric water content at 5 cm revealed similar phase and amplitude chan ges, a coefficient of determination (R-2) Of 0.64, and small mean bias and root-mean-square errors (MBE and rmse) of 0.03 and 0.09, respectively. At 2 5, 60, and 75 cm, the model performance was slightly worse, with R-2 values between 0.27 and 0.40, MBE between -0.01 and 0.02, and rmse between 0.11 a nd 0.13. The model response to changes in soil water at these levels was sl uggish, possibly because of, among other things, a lack of ability to model preferential downward water flow through cracks in the soil. The results of this study suggest that SHM can be used effectively to initi alize 5-cm soil moisture values in numerical prediction models. At deeper s oil levels, however, the relatively small R-2 values and negligible MBE sug gest that the model may be better suited for initializing a regionally aver aged soil moisture value rather than unique gridbox values. These results i llustrate the difficulty in using point measurements to validate a hydrolog ical model, especially deeper in the soil where moisture values are more de pendent on soil properties (which can vary sharply over small distances) an d are less dependent on recent rainfall.