a Two-Layer Variable Infiltration Capacity Land Surface Representation for General Circulation Models.

A simple two-layer variable infiltration capacity (VIC-2L) land surface model suitable for incorporation in general circulation models (GCMs) was developed. The model consists of a two-layer characterization of the soil within a GCM grid cell, and uses an aerodynamic representation of latent and sensible heat fluxes at the land surface. The effects of GCM spatial subgrid variability of soil moisture and a hydrologically realistic runoff mechanism are represented in the soil layers. In the upper layer, the spatial distribution of infiltration and soil moisture capacities is included. The lower layer is lumped spatially and uses a nonlinear drainage representation. The model partitions the area of interest into multiple land surface cover types; for each land cover type the fraction of plant roots in the upper and lower zone is specified. Evaporation occurs via canopy evaporation, evaporation from bare soil, and transpiration, which is represented using a canopy and architectural resistance formulation. The model was tested using long-term hydrologic and climatalogical data for Kings Creek, Kansas to estimate and validate the hydrological parameters. Surface flux data from three First International Satellite Land Surface Climatology Project Field Experiment (FIFE) intensive field campaigns in the summer and fall of 1987 in central Kansas, and from the Anglo-Brazilian Amazonian Climate Observation Study (ABRACOS) in Brazil were used to validate the model -simulated surface energy fluxes and surface temperature. In addition, a derived distribution approach which accounts for the effects of subgrid scale spatial variabilities of precipitation on surface energy fluxes, soil moisture, and runoff production was developed for an extended version of VIC-2L model. The derived distribution approach differs from pixel-based approaches which discretize precipitation over a spatial domain, and from previous statistical approaches that combine the point precipitation distribution with the point statistical distribution of selected land surface characteristics. The results of the derived distribution method are compared with those obtained using an exhaustive pixel-based approach, and the results obtained by applying uniform spatially averaged precipitation to the VIC-2L model. Under most conditions, the derived distribution approach gives good approximations to the pixel-based approach, and is superior to the constant precipitation approach for surface fluxes, surface temperature, runoff, and soil moisture. Finally, VIC-2L sensitivity of predictions to model parameters were explored for two different climate regimes using both fractional factorial and one-at-a-time sensitivity analyses.