Year-round measurements of evaporation from northern latitude wetlands in Norway

As the atmosphere warms, atmospheric evaporative demand is expected to increase across many high-latitude ecosystems, while the duration of seasonal snow cover is projected to change. In Norway, a typically moisture-rich region, improved understanding of the controls on evaporation is needed to assess how these changes may affect ecosystem hydrology. In this study, we used year-round evaporation estimates from four eddy-covariance wetland sites in Norway to quantify evaporation and identify its main controls. To estimate monthly, seasonal, and annual evaporation, eddy-covariance data were gap-filled using a random forest model. The sites cover a latitudinal gradient from 60 to 78° N, a precipitation gradient from 218 to 968 mm per year and a gradient in mean temperature from −3.9 to 2.7 °C. To identify evaporation controls, we performed a factor analysis on observed time series in the snow-free and snow-covered season, separately. In addition, we compared the observed evaporation with the results of a Penman-Monteith model. We found that ecosystem evaporation was mainly controlled by atmospheric evaporative demand, both in the snow-free and the snow-covered season, whereas soil moisture likely never decreased to a level where it restricted evaporation. However, the sensitivity of the Bowen ratio to the vapour pressure deficit varied between sites, showing a decrease in the Bowen ratio beyond a vapour pressure deficit of 1 kPa at sites with a larger cover of open water and non-vascular vegetation compared to a site with a higher cover of vascular plants. Annual evaporation ranged from 80 to 208 mm, equivalent to 9 % to 30 % of the total precipitation. In the warm season, evaporation was typically around 50 % of the seasonal precipitation, reaching a maximum of 72 %. Sites and years with long-duration snow-cover had lower annual evaporation. Compared to other northern latitude sites in the FLUXNET2015 data set, evaporation was lower than expected from the mean temperature of the warm season. Our results show that evaporation is an important part of the northern latitude water balance, especially during the warm season and in regions 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.