Effects of simulated climate change on the hydrology of major river basins

Changes in the climatology of precipitation, evapotranspiration, and soil moisture lead also to changes in runoff and streamflow. The potential effects of global warming on the hydrology of 23 major rivers are investigated. The runoff simulated by the Canadian Cetre for Climate Modeling and Analysis (CCCma) coupled climate model for the current climate is routed through the river system to the river mouth and compared with results for the warmer climate simulated to occur towards the end of the century. Changes in mean discharge, in the amplitude and phase of the annual streamflow cycle, in the annual maximum discharge (the flood) and its standard deviation, and in flow duration curves are all examined. Changes in flood magnitudes for different return periods are estimated using extreme value analysis. In the warmer climate, there is a general decrease in runoff and 15 out of the 23 rivers considered experience a reduction in annual mean discharge (with a median reduction of 32%). The changes in runoff are not uniform and discharge increases for 8 rivers (with a median increase of 13%). Middle- and high- latitude rivers typically show marked changes in the amplitude and phase of their annual cycle associated with a decrease in snowfall and an earlier spring melt in the warmer climate. Low-latitude rivers exhibit changes in mean discharge but modest changes in their annual cycle. The analysis of annual flood magnitudes show that 17 out of 23 rivers experience a reduction in mean annual flood (a median reduction of 20%). Changes in now duration curves are used to characterize the different kinds of behavior exhibited by different groups of rivers. Differences in the regional distribution of simulated precipitation and runoff for the control simulation currently limit the application of the approach. The inferred hydrological changes are, nevertheless, plausible and consistent responses to simulated changes in precipitation and evapotranspiration and indicate the kinds of hydrological changes that could occur in a warmer climate.