Scenarios for alterations in water temperature regime introduced by increased hydro-peaking

The regulation of the Aurland water course due to hydropower development has significantly altered the hydrological regime (both flow and water temperature) of the Vassbygdvatn lake and the downstream Aurlandselva river. The water released from the hydropower plant is directly discharged in Vassbygdvatn providing much colder water than prior to regulation, especially in late spring, summer and early autumn. As a consequence Vassbygdvatn supplies Aurlandelsva with this colder water, giving impacts on the biological system held by the river. An increase in water temperature of Aurlandselva would, in specific, better meet the fish population’s need for ambient water temperature. The analysis of the water temperature regime under the present manoeuvring of the hydropower system showed that enhancing the formation of temperature stratification in Vassbygdvatn during the summer period is the prioritised strategy to increase the water temperature in Aurlandselva. A continous production of the hydropower plant in late spring and summer seems to affect the development of the seasonal stratification in Vassbygdvatn, as the negative heat flux of the production water cools down the uppermost layers and destabilize the vertical water temperature gradient developed by solar radiation.