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Surface Runoff Parameterization

The CLM uses a TOPMODEL-based approach to model surface runoff. The key concept is the fractional saturated area (fsat) of a grid cell, which determines the saturation excess (Dunne) runoff.

fsat is calculated as:
fsat = fmax * exp(-0.5*fover*ztau)
Where fmax is the maximum potential fsat, fover is a decay factor, and ztau is the water table depth.

fmax is calculated from the cumulative distribution function of the compound topographic index (CTI) at 0.125° resolution, excluding pixels with CTI above the 95th percentile. For regions without CTI data, a global mean fmax is used.

The decay factor fover was determined through sensitivity analysis to be 0.5 m^-1 for global simulations.

This TOPMODEL-based approach parameterizes surface runoff generation based on the spatial distribution of soil moisture and topographic characteristics within a grid cell.