Long-term trends in water temperature and ice cover in the subalpine lake, vre Heimdalsvatn, and nearby lakes and rivers

Long-term data series of ice cover on lakes and river temperatures from the mountain areas of Norway are lacking. The present study analyses the last four decades of ice data from the subalpine lake, vre Heimdalsvatn, and water temperature data from its outlet river, Hinogla. These data are compared to water temperature data from three neighbouring, quite different locations, the glacier-fed rivers LeirungsAyeni and Sjoa, and the alpine lake, Bessvatn. The study also examines the air temperature/river temperature relationships, and the air temperature/ice freeze-up and break-up dates. During the months of July, August and September, the water temperature in Hinogla was well correlated to the air temperature, but the correlation was poor in the remaining months due to the ice cover and snow conditions. A significant temperature increase of 2-3A degrees C has been observed in Hinogla in the months August-October since 1984. There were only minor changes in the duration of the ice cover season during the last 40 years, but a delay of 9 days was found in the freeze-up date and a delay of 6 days in the break-up date, although the latter was not significant.