Evaluation of Global Soil Wetness Project soil moisture simulations

To produce a global soil moisture "data set," ten different land surface mo dels were forced with meteorological observations for a two-year period (19 87-88) for the Global Soil Wetness Project (GSWP). We compare observed plan t-available soil moisture in the top l-m soil layer to the same quantity ge nerated by the models. Our soil moisture observations are from grasslands a nd agricultural regions in Russia, Illinois (USA), China, and Mongolia. None of the models does a good job of producing the actual soil moisture va lue 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 mo dels do a fairly good job of reproducing the seasonal cycle of soil moistur e for the various areas. The model biases are different in different locati ons, so correcting them with a simple adjustment of the mean will not produ ce correct results. Better specification of parameters, or better represent ation of physical and biological processes, is still needed to improve thes e models. Future GSWP experiments should be conducted for a longer time period. They should emphasize catchment scale validation and higher time resolution of m odel output. Increased soil moisture observations, possibly incorporating s atellite measurements, also would greatly improve a second project.