As one effort to estimate the global soil moisture distribution, the Global Soil Wetness Project (GSWP) was conceived. Under the GSWP, the global soil moisture distribution on 1°×1° mesh for 1987 and 1988 was simulated in an offline mode by 11 land surface models (LSMs). Even though the forcing conditions are mostly based on observations, validation studies are necessary because LSMs may not simulate accurately the partitioning of water at the surface of the earth between runoff, evaporation, and changes in soil moisture. A gridded 1°×1° global river channel network, named Total Runoff Integrating Pathways (TRIP) is used to calculate mean runoff estimated by the LSMs for drainage areas upstream of 250 operational gauging stations. Runoff observations from these stations in 150 major river basins of the world have been collected for 1987 and 1988, and were compared with the LSM products. It was found that LSMs estimated annual runoff fairly well, with a relative root mean square error of 40% for drainage areas with a fairly high density of raingauge observations (≥30/106km2), which was used to prepare the forcing precipitation. The error corresponds to approximately 18% of annual evapotranspiration. LSMs are also found to have a tendency to underestimate the annual runoff. This may be caused by underestimation of raingauges under strong wind conditions, especially for snow, because all of the LSMs underestimated the runoff for most of the drainage areas located in higher latitudes. A linear river routing model was applied for the global runoff products from the LSMs and analyzed at 250 gauging stations. The correlations between observed and simulated monthly runoff were improved for most of the LSMs by introducing the routing. River runoff information was found to be effective for the validation of water cycles on the continental scale.
Meteorological Society of Japan