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 1997. 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 believed to be the first time that a hydrological model has been evaluated with in situ soil moisture measurements over such an extensive area spanning several climate zones. Comparisons of time series between the observed and modeled domain-averaged volumetric water content at 5 cm revealed similar phase and amplitude changes, a coefficient of determination (R²) of 0.64, and small mean bias and root-mean-square errors (MBE and rmse) of 0.03 and 0.09, respectively. At 25, 60, and 75 cm, the model performance was slightly worse, with R² values between 0.27 and 0.40, MBE between -0.01 and 0.02, and rmse between 0.11 and 0.13. The model response to changes in soil water at these levels was sluggish, 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 initialize 5-cm soil moisture values in numerical prediction models. At deeper soil levels, however, the relatively small R² values and negligible MBE suggest that the model may be better suited for initializing a regionally averaged soil moisture value rather than unique gridbox values. These results illustrate the difficulty in using point measurements to validate a hydrological model, especially deeper in the soil where moisture values are more dependent on soil properties (which can vary sharply over small distances) and are less dependent on recent rainfall.