On the choice of soil hydraulic models in land-surface schemes

The uncertainties in soil hydraulic functions and soil hydraulic parameters affect the performance of land-surface schemes used in climate and weather prediction models. The soil hydraulic model of Clapp and Hornberger is mos t widely used in land-surface modelling, while other models favoured by soi l physicists are hardly used for the purpose. In this study, we give a summ ary of four soil hydraulic models and examine the impact of these models on the performance of a land-surface scheme. It is found that inconsistency i n soil hydraulic functions and parameters leads to different outcomes in la nd-surface modelling. We introduce a technique to match the soil hydraulic parameters for different models, so that the disagreement in the descriptio n of soil hydraulic properties among different models is reduced, while int rinsic differences in the soil hydraulic functions remain. The numerical te sts also show that the land-surface model has a degree of tolerance to the uncertainties in soil hydraulic models, at least in the case of off-line si mulations. The van Genuchten model performs well, but is numerically expens ive. The Brooks-Corey and Clapp-Homberger models are sufficiently accurate with numerical efficiency, and are therefore more suitable for land-surface schemes used in atmospheric models.