Integration of the variable infiltration capacity model soil hydrology scheme into the community land model

Enhancements to the soil hydrology scheme in the NCAR Community Land Model version 3 (CLM3) are described, which are intended to improve the ability of CLM to represent land surface hydrologic processes. Specifically, the CLM3 soil hydrology scheme has been replaced with the scheme used in the Variable Infiltration Capacity model (VIC). While the modified model incorporates VIC soil hydrology, the original model structure in CLM3 is unchanged in most other respects, such as the representation of vegetation and its controls on surface energy exchanges. The modified version of CLM3 makes direct use of VIC soil parameters that have been developed for off‐line regional, continental, and global simulations. The performance of the new model and CLM3 are evaluated through a comparison with observations at three flux tower sites: ABRACOS, a pasture clearing in the Brazilian rain forest, HAPEX‐MOBILHY, a midlatitude agricultural site in France, and Valdai, a midlatitude grassland in Russia, as well as two large river basins (Arkansas‐Red and Colorado in the United States). Overall, the results show that the merged model better reproduces observed soil hydrological variability, in particular, the seasonal evolution and amplitude of soil moisture, as compared to CLM3. The river basin simulations show that in the merged model, total runoff is generally less than in CLM3 and better agrees with observations. Owing to the interaction of runoff and soil moisture, the simulation of total evapotranspiration (or latent heat) is also improved in the merged model.