THE COMPONENTS OF A SVAT SCHEME AND THEIR EFFECTS ON A GCMS HYDROLOGICAL CYCLE

'Bucket'-type land surface models are being replaced in some general c irculation model (GCM) climate studies by 'SVAT' (Surface Vegetation-A tmosphere Transfer) models, which feature an explicit treatment of veg etation control over the surface energy balance. The evaporation calcu lations of a typical SVAT model differ from those of a bucket model in at least four fundamental ways: (a) the SVAT model typically allows a greater variety of environmental stresses to limit evapotranspiration ; (b) it generally includes a canopy interception reservoir; (c) the c ontrol of the land surface over evaporation in a SVAT model is influen ced by the atmosphere; and (d) the land surface control in a SVAT mode l varies on short time scales. Global fields and grid square diurnal c ycles illustrate the hydrological cycle produced in a 20-year simulati on with an atmospheric GCM coupled to a SVAT model. A sensitivity anal ysis then examines the relative importance of the SVAT/bucket differen ces in terms of their effects on the simulated hydrological cycle. The interception reservoir exerts more control over global evaporation th an does either the vapor pressure deficit stress or the temperature st ress. The effect of the temperature stress is, in fact, insignificant. The time variability of land surface control over the surface energy balance in a SVAT model significantly increases moisture convergence o ver land.