A SENSITIVITY STUDY OF THE GROUND HYDROLOGIC MODEL USING DATA GENERATED BY AN ATMOSPHERIC GENERAL CIRCULATION MODEL

In this study, the Ground Hydrologic Model (GHM) developed by Lin, Alfano and Bock (1978) for use in an atmospheric general circulation model (GCM) has been refined. A series of sensitivity studies of the new version of the GHM were conducted for the purpose of understanding the role played by various physical parameters in the GHM.^ This version of the GHM has made the following refinements: (1) The GHM is coupled directly with the planetary boundary layer (PBL), using Deardorff\u27s (1972) parameterization. (2) A bulk vegetation layer is added with a more realistic large-scale parameterization. (3) The infiltration rate is modified using Green-Ampt\u27s (1911) formula.^ The GHM has been tested using input data derived from a GCM simulation run for eight North America regions for 45 days. The results are compared with those of the resident GHM in the GCM. The daily average of grid surface temperatures from both models agree reasonably well in phase and magnitude. However, large difference exists in one or two regions on some days. The daily average evaportranspiration is in general 10-30% less than the corresponding value given by the resident GHM.^ Sensitivity studies have been Conducted for: (1) Initial conditions and lower boundary conditions. The effect of different initial soil moisture conditions in the surface layer persists approximately one week, while in the lower layer at least as long as the operational period. Different lower boundary conditions only produce minor effects. (2) Vegetation density. For extreme cases of desertification and afforestation, the effects of the vegetation density on surface temperature, evaporation, sensible heat and soil moisture are most significant. (3) Canopy resistance. Mainly the daily averaged grid evapotranspiration and the soil moisture content are effected. (4) Surface albedo. Albedo exerts a large effect on the energy balance and thus temperature variation. (5) Depth of root zone and root density distribution. The effects from thse two parameters on the grid evaporation, sensible heat, moisture content and temperature are small.