Predicting the discharge of global rivers

The ability to simulate coupled energy and water fluxes over large continen tal river basins, in particular streamflow, was largely nonexistent a decad e ago. Since then, macroscale hydrological models (MHMs) have been develope d, which predict such fluxes at continental and subcontinental scales. Beca use the runoff formulation in MHMs must be parameterized because of the lar ge spatial scale at which they are implemented, some calibration of model p arameters is inevitably necessary. However, calibration is a time-consuming process and quickly becomes infeasible when the modeled area or the number of basins increases. A methodology for model parameter transfer is describ ed that limits the number of basins requiring direct calibration. Parameter s initially were estimated for nine large river basins. As a first attempt to transfer parameters, the global land area was grouped by climate zone, a nd model parameters were transferred within zones. The transferred paramete rs were then used to simulate the water balance in 17 other continental riv er basins. Although the parameter transfer approach did not reduce the bias and root-mean-square error (rmse) for each individual basin, in aggregate the transferred parameters reduced the relative (monthly) rmse from 121% to 96% and the mean bias from 41% to 36%. Subsequent direct calibration of al l basins further reduced the relative rmse to an average of 70% and the bia s to 12%. After transferring the parameters globally, the mean annual globa l runoff increased 9.4% and evapotranspiration decreased by 5.0% in compari son with an earlier global simulation using uncalibrated parameters. On a c ontinental basis, the changes in runoff and evapotranspiration were much la rger. A diagnosis of simulation errors for four basins with particularly po or results showed that most of the error was attributable to bias in the Gl obal Precipitation Climatology Project precipitation products used to drive the MHM.