A hydrological modelling of the flood response of two watersheds to climate change are presented. The two study watersheds are the Upper Campbell and the Illecillewaet watersheds located in British Columbia. The first watershed is a maritime watershed located on the east slopes of the Vancouver Island mountains whereas the second watershed is located in the Selkirk Mountains in Eastern British Columbia. The Canadian Climate Centre General Circulation Model (CCC GCM) has been used for the estimation of the effect of the climate change on meteorological parameters. The CCC GCM is a steady state model and the output of the 1991 run has been used. In addition to the changes in the amounts of precipitation and temperature usually assumed in hydrological climate change studies, other meteorological and climatic parameters are also considered; specifically, the effect of climate on the spatial distribution of precipitation with elevation, and also on cloud cover, glaciers, vegetation distribution, vegetation biomass production, and plant physiology. The results showed that the mean annual temperature in the two watersheds could increase by more than 3°C and the annual basin-wide precipitation could increase by 7.5% in Upper Campbell watershed and by about 17% in the Illecillewaet watershed. As a result, the mean annual runoff will increase by 7.5% in the Upper Campbell watershed and 21% in the Illecillewaet basin. For the study of floods, nine flood parameters have been investigated, the total number of flood episodes, the flood days per year, the duration of flood events, the annual flood volume, the mean flood flow, the mean flood peak, the annual maximum flood peak, the day of occurrence of the centroid of flood volume, and the day of occurrence of annual maximum flood peak. These nine parameters were extracted from the hydrographs of the two study watersheds using the double long-term mean daily flow. The study showed that, under the climate change scenario, the floods in the maritime Upper Campbell watershed would increase, on average, in magnitude by 14%, in volume by 94%, in frequency by 11%, and duration by 44%. The timing of the floods would remain almost unchanged, and the centroid of flood volume would shift earlier by only 2 days. In contrast, in the interior mountain Illecillewaet watershed, the floods would decrease, on average, in magnitude by 7%, in volume by 38%, and frequency by 23%. The duration of flood events, under the altered climate scenario, would remain, essentially, unchanged increasing by only 2.6%. Also, the study showed that in the Illecillewaet watershed the largest change between the altered climate and the present climate scenarios would be the timing of floods since the centroid of flood volume would occur 20 days earlier. The above changes in the flood response of the two study watersheds can be explained by the changes in the distribution and form of annual precipitation. These results indicate that different management procedures will be needed to minimize the effects of climate change on the flooding of the two climatically different watersheds and the regions that they represent.

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