Evaluation of Global Soil Wetness Project soil moisture simulations

To produce a global soil moisture "data set," ten different land surface models were forced with meteorological observations for a two-year period (1987-88) for the Global Soil Wetness Project (GSWP). We compare observed plant-available soil moisture in the top l-m soil layer to the same quantity generated by the models. Our soil moisture observations are from grasslands and agricultural regions in Russia, Illinois (USA), China, and Mongolia. None of the models does a good job of producing the actual soil moisture value for any of the regions. Thus, GSWP has not yet demonstrated the ability to produce global soil wetness data sets. Once the bias is removed, the models do a fairly good job of reproducing the seasonal cycle of soil moisture for the various areas. The model biases are different in different locations, so correcting them with a simple adjustment of the mean will not produce correct results. Better specification of parameters, or better representation of physical and biological processes, is still needed to improve these models. Future GSWP experiments should be conducted for a longer time period. They should emphasize catchment scale validation and higher time resolution of model output. Increased soil moisture observations, possibly incorporating satellite measurements, also would greatly improve a second project.