AC: Response to referee comments on essd-2020-219

Abstract

Mountain snow and ice greatly influence the hydrological cycle of alpine regions by regulating both the quantity and seasonal variations of water availability downstream. This study considers the combined impacts of climate and glaciers changes due to recession on the hydrology and water balance of two high-elevation basins in the Canadian Rockies. A distributed, physically based, uncalibrated glacier hydrology model developed in the Cold Regions Hydrological Modelling platform (CRHM) was used to simulate the glacier mass balance and basin hydrology of Peyto and Athabasca Glacier basins in Alberta. Bias-corrected reanalysis data were used to drive the model. The model calculates the water balance of a glacierized basin, influenced by the surface energy and mass balance, and considering redistribution of snow by wind and avalanches. It was set up using hydrological response units based on elevation bands, surface slope and aspect, as well as changing land cover. Aerial photos, satellite images and Digital Elevation Models (DEM) were assimilated to represent the changing configurations of glacier area and the exposure of ice and firn. Observations of glacier mass balance, snow and glacier ice surface elevation changes at glacier and alpine tundra meteorological stations and streamflow discharge at the glacier outlets were used to evaluate the model performance. Model results indicated that both basins have undergone continuous glacier loss over the last three to five decades, leading to a 6–31 % reduction in glacierized area, a 78–109 % increase in ice exposure, and changes to the elevation and slope of the glacier surfaces. Diurnal temperature ranges are increasing, mainly due to increasing summer maximum daily temperatures. Annual precipitation is not changing much, but rainfall ratios are increasing. Basin hydrology was simulated over two periods, 1965–1975 and 2008–2018, using observed glacier configurations. The results show that changes in both climate and glacier configuration caused changes in melt rates and runoff, and a shift of peak flows from August to July. Glacier melt contributions increased from 27–61 % to 43–59 % of annual discharges. Recent discharges were 3–19 % higher than in the 1960s and 1970s. The results suggest that increased exposure of glacier ice and lower surface elevation due to glacier thinning were less influential in increasing streamflow than climate warming. Streamflow from these glaciers continues to increase.