Study area and data

This study was carried out on twelve tributary catchments of the St. Lawrence River located in the province of Quebec, Canada with a natural hydrological regime (Fig. 1). Spring floods in the populated areas of the St. Lawrence valley cause frequent damages. For example, the recent major flooding events in the spring of 2017 and 2019 forced the evacuation of several thousands of people from flooded neighbourhoods in the Province and resulted in significant psychosocial and material damages (Benoit et al., 2022; Lin et al., 2019; Teufel et al., 2019) The area of the catchments varies between 367 and 4504 km2(Table 1). They are spatially distributed between the north and south shore of the St. Lawrence River and within four homogeneous hydrological regions used by the Quebec Expertise Center for Water Expertise (CEHQ) in charge of streamflow monitoring and forecasting. The Northwest St. Lawrence region (Batiscan, Bras du Nord and Matawin basins) is characterized by a continental climate; the Saint-Laurent Southwest region (Nicolet, Acadie) is characterized by a maritime climate; the Saint-Laurent Southeast region (York, Beaurivage, Bécancour, Famine, Etchemin, Ouelle) is characterized by a mix of maritime and continental climate and the Saint-Laurent Northeast region (Godbout) by a maritime climate (Assani et al., 2010a; Assani et al., 2010b; Mazouz et al., 2012). The studied basins also encompass three different physiographic regions (i) the St Lawrence Lowlands characterized by flat Proterozoic and Paleozoic sedimentary rocks covered by glaciomarine deposits and mostly clayed soils (Acadie); (ii) the Canadian Shield on the north shore of the St. Lawrence River, with its rugged Precambrian gneissic rocks and sandy soils concentrated in incised glacial valleys (Batiscan, Matawin, Bras du Nord and Godbout); (iii) the Appalachians on the south shore of the St. Lawrence, composed of tilted and folded Paleozoic sedimentary rocks, rolling hill topography, and loamy soils (Ouelle, York, Etchemin, Bécancour, Famine, and Nicolet) (Table 1). In terms of land cover, five basins (Batiscan, Godbout, Matawin, Ouelle, York and Bras du Nord) are nearly completely (>92%) forested basins, with the remaining area covered by agriculture and lakes. The southernmost Acadie basin is dominated by agriculture (72%) with only 25% forest cover, while the remaining basins (Nicolet, Etchemin, Bécancour, Beaurivage and Famine) have mixed covers (agriculture 12-39%, forest: 61-87%) (Table 1).
Daily historical streamflow observations at the outlet of the 12 basins were obtained from the Quebec Center for Water Expertise (CEHQ). The length of the observed flow data varies between basins, from 17 to 55 years (Table 1).
Having a good estimation of pre-flood snowpack conditions is one of the challenges to understand the contribution of snowmelt to peak flow variability. The difficulty of measuring snow depth and snow water equivalent (SWE) typically results in limited data being available both over time and space. While remote sensing can be used to estimate SWE in low vegetation areas, problems remain in forested areas (Bergeron et al., 2014; Brown, 2010; Larue et al., 2017; Sena et al., 2016). In Quebec, a network of snow survey sites has been installed in forested areas to measure snow depth and SWE every two weeks in the winter and spring seasons, but the spatial distribution and density of these stations is low (Nemri and Kinnard, 2019). Consequently, using outputs of snow and hydrological models seems the only solution to derive long and continuous SWE and soil moisture records. In a previous study conducted in the same basins by Nemri and Kinnard (2019), the GR4J hydrological model (Perrin et al., 2003) coupled to the Cemaneige snow model (Valéry et al., 2014b) was calibrated and validated in order to properly simulate basin-wide SWE, soil moisture and daily streamflow. The calibration methods and validation results are well described in Nemri and Kinnard (2019). They found that a multi-objective calibration strategy gave the best simulation of both streamflow and SWE, and the simulation results using this method were used in the present study. The GR4J-Cemaneige model was forced by daily precipitation and temperature date extracted from daily grids produced by the Atmospheric Environment Information Service (SIMAT) in collaboration with the CEHQ (Bergeron 2015). Historical SWE measurements at 12 measuring points of the Quebec snow survey network located in or very close to the selected basins were used in the calibration along with the observed streamflow (see Fig. 1 for locations). The Cemaneige snow module simulates the accumulation and snowmelt in five altitude bands. The precipitation phase (rain, snow) is determined using the mean temperature of each altitude band. The snow/rain fraction is calculated according to the function used in the Hydrotel model (Fortin et al., 2001) based on the minimum (Tmin) and maximum (Tmax) daily temperature at each altitude band: when Tmax ≤ 0 °C, all precipitation fall as snow, while if Tmin ≥ 0 °C all precipitation fall as rain, else the snowfall fraction is estimated as 1-Tmin/(Tmax- Tmin). These functions are well described by Valéry (2010) and Valéry et al. (2014b).
In addition, soil moisture measurements were not available for the study so that the soil moisture simulated by the GR4J model was used. In GR4J, the hydrological processes in the basin are simplified into two interconnected reservoirs. The soil reservoir has a maximum capacity (mm), which is a free parameter to be calibrated, and determines the amount of water in the basin according to the degree of soil saturation, which itself is a function of the ratio between the quantity of stored water and the maximum storage capacity. A summary of the calibrated model parameters and Nash-Sutcliffe performance criteria is given in Table 2.