Using the Soil Hydrology Model to initialize soil moisture profiles for numerical prediction models

A Soil Hydrology Model (SHM) was modified and daily predictions of soil volumetric water content were made at 38 Oklahoma Mesonet sites during July 1997. These predictions were then compared to soil moisture observations made at the Mesonet sites at 5, 25, 60, and 75 cm.

Comparisons of time series between the observed and model-predicted 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 (MBEs and RMSEs) of 0.03 and 0.09, respectively. At 25, 60, and 75 cm, the model performance was slightly worse, with R² values between 0.27-0.40, MBEs between -0.01-0.02, and RMSEs between 0.11-0.13. The model response to changes in soil water at these levels was sluggish, possibly due to a lack of ability to model preferential downward water flow through cracks in the soil.

Sensitivity tests revealed the importance of specifying the proper infiltration depth, while refuting a previous assertion that the model output is independent of initial soil water content. Tests also showed that the modeling of the conductive processes was relatively insensitive to the specification of the soil hydraulic parameters; and that the processes of precipitation infiltration and evapotranspiration overpower the changes in soil water content associated with vertical water transfer.

Finally, the viability of using a much simpler model to initialize soil moisture in the topsoil was demonstrated. However, since much of the water that is extracted from the ground comes from beneath the topsoil, the use of SHM is recommended to resolve the complete vertical profile of soil water. The ability to model the phase and amplitude changes of soil moisture is unique and provides an opportunity to initialize both weather and storm-runoff models with realistic soil moisture values based upon currently available observations.