Abstract
The GEWEX/ISLSCP Global Soil Wetness Project is an international initiative to enlist a spectrum of land-surface modeling centers and programs to use common global atmospheric forcing data sets to drive their respective land-surface models, in order to simulate and compare global soil wetness fields, and ultimately apply them in GCM sensitivity tests. As a participant in this project, we used the common ISLSCP atmospheric forcing data for 1987-1988 and our NCEP land-surface model (LSM) to generate global hydrological fields, including soil moisture, which cannot be obtained uniformly over large continental areas from scattered observations. We found that the simulated global hydrological cycle reasonably reflects the seasonal variation of those hydrological fields. Despite the relatively coarse 1×1 degree resolution of forcing data, the soil moisture field has a remarkable spatial variability, which is related not only to the spatial pattern of precipitation, but also to the spatial distribution of runoff and evaporation. Also, the spatial variability in vegetation and soil characteristic contributes to the variability in the surface hydrological cycle.
We also compared our simulated global land-surface hydrological cycle to the 1987-1988 NCEP/NCAR reanalysis of land-surface products. It seems that, in most regions, the reanalysis of soil moisture has a larger annual cycle amplitude than the GSWP soil moisture. A more remarkable difference is that in the mid- and high-latitudes of the northern hemisphere, the reanalysis has a more uniformly distributed and wetter soil moisture than the corresponding GSWP soil moisture, which reflected the artificial climatological soil moisture damping field applied in the reanalysis system.
Finally, we attempted to validate our soil moisture simulations with soil moisture measured by the Illinois Soil Moisture Network, and focused on the comparison of the area-averaged deep root zone soil moisture. It appears that the simulated soil moisture of 1987-1988 in the state of Illinois is able to reasonably capture not only the phase of the seasonal evolution, but also the amplitude of annual variation. The encouraging results indicate that a sound LSM can well simulate the natural soil moisture evolution, given accurate atmospheric forcing and reasonable specification of local vegetation and soil characteristics.
