Modeling terrestrial hydrological systems at the continental scale: Testing the accuracy of an atmospheric GCM

A global hydrological routing algorithm (HYDRA) that simulates seasonal riv er discharge and changes in surface water level on a spatial resolution of 5' long x 5' lat is presented. The model is based an previous work by M. T. Coe and incorporates major improvements from that work including 1) the ab ility to simulate monthly and seasonal variations in discharge and lake and wetland level, and 2) direct representation of man-made dams and reservoir s. HYDRA requires as input daily or monthly mean averages of runoff, precip itation, and evaporation either from GCM output or observations. As an example of the utility of HYDRA in evaluating GCM simulations, the mo del is forced with monthly mean estimates of runoff from the National Cente rs for Environmental Prediction (NCEP) reanalysis dataset. The simulated ri ver discharge clearly shows that although the NCEP runoff captures the larg e-scale features of the observed terrestrial hydrology, there are numerous differences in detail from observations. The simulated mean annual discharg e is within +/-20% at only 13 of 90 fluvial gauging stations compared. In g eneral, the discharge is overestimated for most of the northern high latitu des, midcontinental North America, eastern Europe. central and eastern Asia , India, and northern Africa. Only in western Europe and eastern North Amer ica is the discharge consistently underestimated. Although there appears to be a need for improved simulation of land surface physics in the NCEP prod uct and parameterization of how velocities within HYDRA, the timing of the monthly mean discharge is in fair agreement with the observations. Including lakes within HYDRA reduces the amplitude of the seasonal cycle of discharge and the magnitude of the annual mean discharge of the St. Lawren ce River system, in qualitative agreement with the observations. In additio n, including the wetlands of the Sudd reduces the magnitude of the simulate d annual discharge of the Nile River to values in better agreement with obs ervations. Finally, the impact of man-made dams and their reservoirs on the magnitude of monthly mean discharge can be explicitly included within HYDRA. As an ex ample, including darns and reservoirs on the Parana River improves the agre ement of the simulated mean monthly discharge with observations by reducing the amplitude of the seasonal cycle to values in good agreement with the o bservations. The results of this study show that, although improvements can be obtained through better representations of flow velocities and more accurate digital elevation models. HYDRA can be a powerful tool for diagnosing simulated te rrestrial hydrology and investigations of global climate change.