CALCULATIONS OF RIVER-RUNOFF IN THE GISS GCM - IMPACT OF A NEW LAND-SURFACE PARAMETERIZATION AND RUNOFF ROUTING MODEL ON THE HYDROLOGY OF THE AMAZON RIVER

This study examines the impact of a new land-surface parameterization and a river routing scheme on the hydrology of the Amazon basin, as de picted by the NASA/Goddard Institute of Space Studies (GISS) global cl imate model (GCM). The more physically realistic land surface scheme i ntroduces a vegetation canopy resistance and a six-layer soil system. The new routing scheme allows runoff to travel from a river's headwate r to its mouth according to topography and other channel characteristi cs and improves the timing of the peak flow. River runoff is examined near the mouth of the Amazon and for all of its sub-basins. With the n ew land-surface parameterization, river runoff increases significantly and is consistent with that observed in most basins and at the mouth. The representation of the river hydrology in small basins is not as s atisfactory as in larger basins. One positive impact of the new land-s urface parameterization is that it produces more realistic evaporation over the Amazon basin, which was too high in the previous version of the GCM. The realistic depiction of evaporation also affects the therm al regime in the lower atmosphere in the Amazon. In fact, the lower ev aporation in some portions of the basin reduces the cloudiness, increa ses the solar radiation reaching the ground, increases the net radiati on at the surface, and warms the surface as compared to observations. Further GCM improvement is needed to obtain a better representation of rainfall processes.