Evaporation from northern latitude wetlands

Abstract. The atmospheric demand for evaporation in northern latitude ecosystems is expected to increase with increasing temperatures and a longer snow-free season. To understand how increased evaporative demand will affect ecosystems in this typically moisture-rich region, we need more knowledge about the factors that control evaporation and, furthermore, how evaporation modifies local hydrology. We used year-round evaporation estimates from four eddy-covariance wetland sites in Norway to quantify evaporation and identify its main controls along climatic gradients in temperature and precipitation. We found that ecosystem evaporation was indeed mainly controlled by atmospheric evaporative demand and spring snow-cover duration. Soil moisture remained high during the measurement period and likely never reached a level where it would impact evaporation. Annual evaporation ranged from 81 mm to 208 mm and increased with warm-season mean temperature along the spatial gradient. We found a large variation in the role of evaporation in the ecosystem water balance, with annual evaporation ranging from 9 % to 30 % of annual precipitation. In the warm season, evaporation was typically around 50 % of the seasonal precipitation, but reached a maximum of 72 %. Compared to other northern latitude sites in the FLUXNET2015-dataset, the evaporation from the Norwegian sites was lower than what would be expected from the site warm season mean temperatures. Our results show that evaporation is an important part of the northern latitude water balance, especially during the warm season and in parts of the region with low precipitation. Furthermore, our results indicate that earlier snow-cover melt-out and increased vapour pressure deficit have the potential to increase annual evaporation.