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.