River basin hydrology in a global off-line land-surface model

Land-surface hydrology has been examined for the Off-line Land-surface GEOS Assimilation (OLGA) system and Goddard Earth Observing System (GEOS-1) data assimilation system using a river routing model, The GEOS-1 land-surface parameterization is very simple, using an energy balance prediction of surface temperature and prescribed soil water, OLGA uses atmospheric data from GEOS-1 to drive a more comprehensive parameterization of the land-surface processes. Using a global river routing model, OLGA's hydrology is evaluated against GEOS-1 (which serves as a control case) and observations. The river routing model moves monthly mean climatologic source runoff: through river networks to simulate the river discharge from many river basins around the world, which can be compared to observed climatologic river discharge. Because of the soil hydrology, the OLGA system shows a general improvement in the simulation of river discharge, compared to the GEOS-1, by slowing the discharge of water. Snowmelt processes included in OLGA also have a positive effect on the annual cycle of river discharge and source runoff. The timing of the snowmelt in river discharge, however, still needs improvement in the annual cycle. Preliminary tests of a coupled land-atmosphere model indicate improvements to the hydrologic cycle compared to the uncoupled system. The river routing model has provided a useful tool in the evaluation of the GCM hydrologic cycle and has helped quantify the influence of the more advanced land-surface model.