Impact of climate change on the hydrology of St. Lawrence tributaries

Changes in temperature and precipitation projected for the next century will induce important modifications into the hydrological regimes of the St. Lawrence tributaries (Quebec, Canada). The temperature increase anticipated during the winter and spring seasons will affect precipitation phase and consequently the snow/precipitation ratio and the water volume stored into snow cover. The impact on northern river hydrology and geomorphology will be significant. In this study we aim to assess the magnitude of the hydrological alteration associated with climate change; to model the projected temporal shift in the occurrence of winter/spring center-volume date; to assess the sensitivity of the winter/spring center-volume date to changes in climatic variables and to examine the latitudinal component of the projected changes through the use of five watersheds on both shores of the St. Lawrence. The study emphasizes changes in the winter and spring seasons. Projected river discharges for the next century were generated with the hydrological model HSAMI run with six climate series projections. Three General Circulation Models (HadCM3, CSIRO-Mk2 and ECHAM4) and two greenhouse gas emissions scenarios (A2 and B2) were used to create a range of plausible scenarios. The projected daily climate series were produced using the historical data of a reference period (1961-1990) with a perturbation factor equivalent to the monthly mean difference (temperature and precipitation) between a GCM in the future for three 30 year horizons (2010-2039, 2040-2069; 2070-2099) and the reference period. These climate projections represent an uncertainty envelope for the projected hydrologic data. Despite the differences due mainly to the GCM used, most of the hydrological simulations projected an increase in winter discharges and a decrease in spring discharges. The center-volume date is expected to be in advance by 22-34 days depending on the latitude of the watershed. The increase in mean temperature with the simultaneous decrease of the snow/precipitation ratio during the winter and spring period explain a large part of the projected hydrological changes. The latitude of the river governed the timing of occurrence of the maximum change (sooner for tributaries located south) and the duration of the period affected by marked changes in the temporal distribution of discharge (longer time scale for rivers located at higher latitudes). Higher winter discharges are expected to have an important geomorphological impact mostly because they may occur under ice-cover conditions. Lower spring discharges may promote sedimentation into the tributary and at their confluence with the St. Lawrence River. The combined effects of modifications in river hydrology and geomorphological processes will likely impact riparian ecosystems. (C) 2010 Elsevier By. All rights reserved.