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.
General Comments:
The authors present multiple years of eddy covariance data from four northern sites across Norway and compare annual evapotranspiration totals and evaporative controls to other northern FLUXNET sites. The presented data is a valuable contribution to the field due to the scarcity of evaporation measurements at northern sites, particularly over both the snow-covered and snow-free seasons, and the authors present the interannual variability of evaporative totals and controls across their sites. It is a lot of work to compile eddy covariance data from that many site-years and I commend the authors for their efforts, presentation, and gap-filling techniques. I appreciate how well they have contextualized their sites within current (albeit fairly scarce) literature and evaporation data from other northern sites. Although the writing and figures are clear, I find the abstract, discussion (mainly sections 4.2 and 4.3) and main conclusions do not entirely explain the results of the study in enough detail as the paper is currently written. This comment may be a compliment to the paper, as there were interesting results and nuances among the sites that were presented in the results, yet not fully explained in the discussion. The authors often repeat themselves, explaining that a warmer climate and increased atmospheric demand for moisture will increase ET (which is already fairly well established), yet do not fully dive into the interesting results and differences between sites. The authors reiterate that a longer snow-free period will increase overall evaporation and that their sites had low sensitivity to the phenology and soil water content. While these are important conclusions, it also does appear that there are variations among these four sites presented that could be explored in more detail with even more added scientific value (i.e. different threshold responses to VPD, varying sensitivities to soil moisture, etc.). Additionally, the Penman vs. observed ET at these sites is a useful comparison to know how to deal with surface conductance terms in the future. Overall, this paper is a useful and well-presented contribution to furthering our understanding year-round northern wetland evaporation. However, the authors may want to consider reframing the primary messaging of the paper away from describing the ET totals within the context of the water balance (which is tricky when only comparing ET and precipitation and not even presenting snow accumulation data) to focusing on highlighting the different controls/sensitivities between these sites, even if subtle.
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