Journal of the Meteorological Society of Japan, Vol. 77, No. 1B, pp. 235-255, 1999 235 Assessment of Annual Runoff from Land Surface Models Using Total Runoff Integrating Pathways (TRIP) By Taikan Oki Institute of Industrial Science, Univ. of Tokyo, Japan Teruyuki Nishimura Frontier Research System for Global Change, Tokyo, Japan and Paul Dirmeyer Center for Ocean-Land-Atmosphere Studies (Manuscript received 5 April 1998, in revised form 15 October 1998) Abstract 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 1x1 mesh for 1987 and 1988 was simulated in an offlinemode 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 1x1 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 1987and 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. 1. Introduction General circulation models (GCMs) are used for weather forecasting,climate prediction, and climatic simulations. For these numerical simulations, it is necessary to describe well the boundary conditions of the atmosphere: these are the surface conditions of land and sea. In the case of the land surface as a boundary condition of the atmosphere, surface soil moisture and snow accumulation are believed to have a sig- nificant effect on atmospheric circulations, and the surface temperature is considered to be a result, and not a cause, of land-atmosphere interactions. Numerical models that describe the flux between land and atmosphere and the temporal change of state variables at land surface such as soil moisture, snow accumulation, etc., are used with GCMs, and called Land Surface Models (LSMs). LSMs gener- Corresponding author: Taikan Oki, IIS, Univ. of Tokyo, Minato-ku, Roppongi, Tokyo 106-8558, Japan. E-mail: taikan@iis.u-tokyo.ac.jp (c)1999, Meteorological Society of Japan 69