Impacts of climatic changes on the hydrological regime: Moisie river case
The output of the Canadian Climate Center (CCC) General Circulation Model (GCM), coupled with the hydrologic deterministic modes CEQUEAU is used to evaluate the possible impact of a doubling of atmospheric C02 on the hydrologic regime of the Moisie river on the North Shore of the St Lawrence. In regions where snow plays an important contribution to the annual runoff and the ground is covered with snow for periods from 4 to 6 months, the seasonal variations of climatic changes under a 2 x C02 scenario may have very different impact on hydrologic regimes; Figures 1 and 3 show the annual and winter distributions of temperature changes for Quebec under the CCC 2 x C02 scenarios, white figure 2 shows the possible changes in annual precipitation. The annual temperature will increase around 4 to 5° C, white winter temperatures may increase as much as 6 to 9° C ; annual precipitation will increase by 15 % to 20 %. The CEQUEAU hydrologic model (MORIN et al., 1981 ; MORIN et COUILLARD, 1990) is a deterministic model which takes into account number of physiographic characteristics of the drainage basin (such as elevation and percent of forest and lake area) as defined in number of square grids. The model uses for input daily minimum and maximum temperatures and daily solid and liquid precipitation (rainfall and snowfall). As such meteorological information is usually available for a limited number of stations, the values are interpolated to each grid element of the drainage basin. The CEQUEAU deterministic model uses the degree-days method (CORPS OF ENGINEERS, 1960) to estimate daily snowmelt under forest canopy and in the open and Thornthwaite equation to calculate daily evapotranspiration, for each square grid of the basin. Daily water budget, using linear reservoir storage for soil moisture and ground water storage is then used on each square grid element, to estimate daily runoff production ; this daily runoff production on each grid is then routed downstream to the basin outlet. The CEQUEAU model is applied to the drainage basin of the Moisie river on the North shore of the Saint-Lawrence river. The Moisie river drainage basin covers an area of 19 248 km2, and we have used square grids of 20 km by 20 km to model this basin which is oriented roughly north to south with a length of 320 km and a width of 70 km. Climatic Normals for the 1951-1980 period for the Sept îles and Wabush Lake climate stations are used to calculate the coefficients of the Thornthwaite formula for the present conditions. Daily temperatures and precipitation for the Sept îles and Wabush Lake climate stations are then used as input to the CEQUEAU model, la calculate the flows for the 1966-1989 period. Even if calculations of flows have been made on a daily scale monthly values are used for the analysis of results. Table 1 presents a comparison of monthly and annual observed and calculated runoff for this period and show that the model satisfactorily reproduces the observed flows ; the annual difference is only - 1.2 % white monthly differences vary tram - 7 % to + 6 %. Monthly flows (fig. 4) adequately represent the annual cycle and there is no important under or over estimation of mean monthly values. Frequency analysis of ail monthly flows (fig. 5) show that the observed sequence is adequately simulated even for extreme values. The coefficient of correlation between annual observed and calculated flows is 0.88 and the error of estimation of calculated values is only 48 mm. Monthly changes in temperature and precipitation as output by the Canadian GCM under a 2 x C02 scenario are then used to simulate daily flows under changed climate conditions. The values at the grid points of the GCM are interpolated to the Sept-Iles and Wabush Lake Stations. Climate normals for the two stations, table 2, are first modified by these monthly changes in precipitation and temperature to reflect new conditions for the application of the Thornthwaite formula under a 2 x C02 scenario. Then daily temperature and precipitation values for the 1966-1989 period are modified by these monthly changes and used as input to the CEQUEAU model. The hydrologic results of this 2 x C02 scenario are presented in table 3 and figures 7 to 10. White annual runoff is reduced by only about 5 %, monthly values show much larger variations. Figure 7 and table 3 present a comparison of average monthly flows under present and the 2 x C02 scenario. Winter flows are significantly increased (from 19 % to 210 %) due to much more frequent snow melts in the beginning and end of winter, and winter low flows are significantly higher. The monthly spring flow is about the same (fig. 7), but is concentrated in a shorter period. Summer fiows (June to October) decrease by 25 % to 40 % due to increase in temperature and therefore evapotranspiration but are still higher than winter flows. Figure 8 shows the coefficients of variation of monthly flows. Like mentioned previously one notices a significant increase for winter months (December to Match), due to more frequent snow melts, white for the month of April the coefficient of variation is significantly reduced. For the summer months (May to October) the variation does not change very much. Probability distribution of simulated annual flows (fig. 9) show that for wet years the probability of occurrence does not change significantly, white for dry years, i.e. runoff of about 600 mm, corresponding to the driest years in the 1966-1989 period, the probability of not exceedence goes from 0, 12 to 0,28; i.e. the years of low runoff will happen much more frequently. For summer months the situation will be different as it is the wet years that will be more affected by changes in temperature; as an example for the month of August (fig. 10) the probability of occurences of dry years will not change as drastically as the frequency of wet years; increases in temperature and therefore possible evapotranspiration, will have no effect if no sufficient soil moisture is available.