Effects of the change from forest to agriculture land use on the spatial variability of summer extreme daily flow characteristics in southern Quebec (Canada)

Studies looking at the effects of forest land cover on streamflow in Quebec have shown that a decrease in forest cover in a watershed leads to an increase in minimum flows. However, in many watersheds, such a decrease in forest cover is primarily due to an increase in agricultural land use. The goal of the study was to analyze the impact of a change from forest to agriculture land cover on the spatial variability of maximum and minimum extreme flows measured in summer (May–October) in 36 watersheds with surface areas between 200 km2 and 4000 km2, over the period from 1960 to 1990 using the canonical correlation analysis and analysis of covariance methods. The main result of this study is the fact that, in watersheds characterized by relatively high agricultural land cover (>20%), daily minimum flows are relatively small. The decrease in daily minimum flows associated with an increase in agricultural land cover is likely the result of relatively extensive evaporation due to the low-permeability (clayey) and nearly flat substrate in the farmland-dominated St. Lawrence Lowlands, as well as by high summer temperatures and, at times, relatively long dry spells. Hence, in Quebec, agricultural land use results in an inversion of the hydrological effects of a decrease in forest cover on extreme minimum flows. As for daily maximum flows, an increase of agricultural land cover results in a significant increase in the variability (coefficient of variation) of their magnitude, although this is only observed during the last part of the summer season (September–October), likely as a result of the diversity of factors which cause floods (tropical cyclones, polar fronts, convection, etc.).