Simple method of estimating daily mean soil temperature by using the force-restore model

A simple model to estimate the daily mean soil temperature using routine meteorological data has been developed. The model is based on a heat budget equation connected with the Force-Restore Model, in which interdiurnal variation of the soil temperature at the surface layer is estimated. The model has some advantages. First, it is not necessary to set up soil thermometric parameters and initial values at the deep soil layer. Second, sensitivities of the soil thermometric parameters are low compared to the case of estimating diurnal variation of soil temperatures. Finally, the boundary condition is easy to determine because annual mean soil temperatures at different depths are nearly equal. The model was applied for a bare soil field and verified using the observed data by the heat balance method. Estimated values of daily mean soil temperature agreed well with the observed value, even if the soil thermometric parameters were set as approximated values. To adapt the model for any case, a way to estimate the initial value of soil temperature was examined. Although the soil moisture availability was required to estimate the evaporation in the model, the average value of long-time period gave a good result for estimation of soil temperature. In case the value of the actual soil moisture is unavailable, the model gave a good estimation of cumulative evaporation using routine meteorological data and soil temperature at the surface layer. The model could be used to evaluate the soil moisture availability.