Climate change in the Fraser River watershed: flow and temperature projections

Abstract

An analysis of the historic flows and water temperatures of the Fraser River system has detected trends in both the annual flow profile and the summer temperatures. This study was undertaken to determine if these trends are likely to continue under the conditions predicted by various global circulation models. To do this, existing flow and temperature models were run with weather data that were derived from actual weather observations, but modified using changes predicted by the global circulation models.The validity of the flow model results is supported by very close agreement with the historical record. The differences between model output and the historical record for mean flow, mean peak flow, mean minimum flow and peak flow day were not statistically significant; furthermore, there was only a 3–4 day shift in the occurrence of cumulative flow milestones. The temperature model's mean water temperature was only 0.2°C higher than the historical record.For the period 2070–2099, the flow model predicted a modest 5% (150m3/s) average flow increase but a decrease in the average peak flow of about 18% (1600m3/s). These peaks would occur, on average, 24 days earlier in the year even though for 13% of the years the peak flow occurred much later as a result of summer or fall rain, instead of the currently normal spring freshet. In the same period, the summer mean water temperature is predicted to increase by 1.9°C. The potential exposure of salmon to water temperatures above 20°C, which may degrade their spawning success, is predicted to increase by a factor of 10.Trends in both flow and temperature in this study closely match the trends in the historical record, 1961–1990, which suggests that the historical trends may already be related to climate change. While the mean flow of 2726m3/s does not show a statistically significant trend, the hydrological profile has been changing.