Improved terrestrial hydrologic representation in mesoscale land surface models

This study addresses several deficiencies in the existing formulations for terrestrial hydrologic processes in the Common Land Model (CLM) and presents improved solutions, focusing on runoff prediction. In particular, this paper has 1) incorporated a realistic geographic distribution of bedrock depth to improve estimates of the actual soil water capacity; 2) replaced an equilibrium approximation with a dynamic prediction of the water table to produce more reasonable variations of the saturated zone depth; 3) used an exponential decay function with soil depth for the saturated hydraulic conductivity to consider the effect of macropores near the ground surface; 4) formulated an effective hydraulic conductivity of the liquid part at the frozen soil interface and imposed a maximum surface infiltration limit to eliminate numerically generated negative or excessive soil moisture solution; and 5) examined an additional contribution to subsurface runoff from saturation lateral runoff or baseflow controlled by topography. To assess the performance of these modifications, runoff results from a set of offline simulations are validated at a catchment-scaled study domain around the Ohio Valley region. Together, these new schemes enable the CLM to capture well the major characteristics of the observed total runoff variations. The improvement is especially significant at peak discharges under high flow conditions.