The reservoir Sundsbarmvatn, in Southern Norway, is used for electricity production from November to May. Sundsbarmvatn has two main basins. Water from the upper basin, Mannerosfjorden, flows into the lower basin, Gullnesfjorden. The two basins are separated by a narrow sound with a sill. The regulation interval for Sundsbarmvatn is 612-574 m a.s.l., but the sill prevents Mannerosfjorden from being lowered below 580 m a.s.l. The water intake in Gullnesfjorden is 571 m a.s.l.
The water temperature conditions has been studied during two winters when the reservoir water was released. This study shows that a marked thermocline was gradually developed at the depth of withdrawal in Gullnesfjorden. In the epilimnion layer the temperature is gradually lowered through the winter, but in the hypolimnion layer the temperature seems to stay constant through the winter. In Mannerosfjorden, however, we find no clear thermocline at the end of the winter. The remaining water was relatively warm with temperatures mainly above 3 °C.
The sill between the two basins seems to have a strong influence on which depth the water is flowing out of Mannerosfjorden and hence on the temperature and circulation pattern in Gullnesfjorden. At the end of the winter season this flow is strengthening and initiates a homogeneous flow layer in Gullnesfjorden. This layer is dipping downwards towards the outlet tunnel. For this reason the temperature of the water leaving the power station is 0.4-1.2 °C colder than the hypolimnion temperature in the reservoir at the tunnel depth.