Hydrological response to warm and dry weather: do glaciers compensate?

Abstract. Warm and dry summer days can lead to low streamflow due to a lack of rainfall and increased evaporation. In glacierized catchments, however, such periods can lead to a very different hydrological response as glaciers can supply an increased amount of meltwater, thereby compensating for the rainfall deficits. Here, we analyzed glacier-fed streamflow responses to warm and dry periods (WD) in long-term streamflow observations (> 50 years). WD events during summer (June–September) were analyzed for catchments with varying glacier cover in Canada, Norway and the European Alps. WD events were defined by days with temperatures above a daily varying threshold, based on the 80th percentile of the respective long-term temperature data for that day in the year, and daily precipitation sums below a fixed threshold (< 2 mm/d) for a minimum duration of seven days. Streamflow responses to these WD events were expressed as level of compensation (C) and were calculated as the event streamflow relative to the long-term streamflow regime. C ≥ 100 % indicates that increased melt could compensate, or even overcompensate, the rainfall deficit and increased evaporation. Results showed a wide range of compensation levels, both between catchments and between different WD events in a particular catchment. C was, in general, higher than 100 % for catchments with a higher relative glacier cover. June was the month with highest compensation levels, but this was likely more influenced by snowmelt than by glacier melt. For WD events in September, C was still higher than 100 % in many catchments, which indicated the importance of glacier melt as streamflow contributor in late summer. There was a considerable range in C of different WD events for groups of catchments with similar glacier cover. This could be partly explained by antecedent conditions, such as the snow fallen in the previous winter and the streamflow conditions thirty days before the WD event. Some decreasing trends in C were evident, especially for catchments in Canada and the European Alps. Overall, these results suggest that glaciers do not compensate straightforwardly. The different streamflow contributions and their variations are important for the buffering capacity and the compensating effect of glaciers in these high mountain water systems.