A variable velocity flow routing algorithm for GCMs

A global hydrological routing algorithm is developed for use in the coupled general circulation model (GCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA). Unlike existing schemes that use a constant velocity in surface flow routing calculations, this approach uses a time-evolving velocity that depends on the amount of runoff generated in the GCM land grid cell. The scheme uses Manning's equation to estimate flow velocities for a river channel with rectangular cross section. The river flow directions at the required GCM resolution are obtained from 1 degrees river flow directions using an algorithm that requires minimal manual intervention. With the use of a suitable roughness coefficient for natural channels in Manning's equation and all other parameters determined from the existing literature, the scheme does not require calibration of parameters and can be used directly in GCMs. Simulated streamflow is compared with observed hydrographs at the mouths of major rivers to show that the scheme performs satisfactorily.