A land-surface hydrology parameterization with subgrid variability for general circulation models

Most of the existing generation of general circulation models (GCMs) use so-called bucket algorithms to represent land-surface hydrology. Biosphere-atmosphere models that include the transfer of energy, mass, and momentum between the atmosphere and the land surface are a recent alternative to this highly simplified representation of the land surface in GCMs. These models require estimation of a large number of parameters for which parameter estimation methods and supporting data remain to be developed. We describe a more incremental approach to generalizing the bucket representation of land-surface hydrology based on a model that represents the variation in infiltration capacity within a GCM grid cell. The variable infiltration capacity (VIC) model requires estimation of three parameters: an infiltration parameter, an evaporation parameter, and a base flow recession coefficient. The VIC model was explored through direct comparisons with the Geophysical Fluid Dynamics Laboratory (GFDL) bucket model for the French Broad River, North Carolina, and via sensitivity analysis for the GFDL R30 grid cell which contains the French Broad River. Generally, the bucket model runoff had much greater variability than the historic streamflows for short time scales (e.g., 1 day); the VIC model was much more similar to the observed flows in this respect. The results also showed that the bucket model tended to have unrealistically high short-term variability. The sensitivity analysis showed that the base flow parameter exerted the greatest influence on both the mean and variability of most of the hydrologic variables, especially winter runoff, summer evaporation, and summer and winter soil moisture.