Southern Quebec environmental flow assessments: spatial and temporal scales sensitivity

Faced with increasing demands for water withdrawals and a changing climate, the Quebec Department of Environment and Fight Against Climate Change is reviewing its water withdrawal guidelines to protect riverine ecosystems. For Southern Quebec, guidelines currently limit water withdrawals to a maximum of 15% of the 7Q2 (mean 7-day low flow with a return period of two years) during low flow periods. In this context, one of the issues raised is to investigate measures that help to preserve riverine ecosystems during low flow periods by establishing cut-off flow restrictions. This study compared eight low flow metrics to investigate which can be considered useful metrics to assess environmental flow in Southern Quebec rivers. Using 98 hydrometrics stations with a minimum of 20 years of daily flow data from eight hydrological regions, those low flow metrics were compared to three thresholds based on Tennant Method for monthly and annual temporal scales. The relevance of current hydrological regions delineation was investigated by looking at results within these regions, compared to six groups of stations defined using multivariate analyses. This study emphasizes that assessing environmental flows is linked to the hydrological context of the area of interest, the temporal scale of the historical data available, and the catchment size. The results showed that (1) winter low flows were lower than summer low flows; (2) 23% to 26% of the values were under the conservative thresholds for all the metrics depending of the time scale; and (3) the 7Q2, 7Q10 (mean 7-day low flow with a return period of ten years), Q95 and Q90 (95th and 90th percentile on the flow duration curve) are the less conservative for rivers having a low regime flow. To conclude, assessing several regionally adapted environmental flow metrics is recommended rather than systematically using the 7Q2 for Southern Quebec.