Growing Season Evapotranspiration from a Subalpine Wetland
Our wetland had lower ET (157.7 mm) over the summer compared to other subalpine meadows and wetlands because of its shorter growing season and the impact of horizon shade (Table 3). Past studies reported that subalpine wet meadows (Groeneveld, Baugh, Sanderson, and Cooper, 2007; Wu et al. 2015) and wetlands (Sanderson & Cooper, 2008) have a large range in annual ET that reach up to 994 mm, with growing season (May to October) contributions of 200 – 657 mm. Large seasonal differences in ET are often caused by localised environmental variables that alter season length, such as solar radiation, air temperature, precipitation, water table depth, and soil moisture content (Flerchinger, Marks, Reba, Yu, and Seyfred, 2010; Wu et al. 2015). In our study, low cumulative ET at the wetland was due to the strong relationship between ET, available energy, and horizon shade. Throughout the measurement period, shade supported a thick snowpack, delayed transpiration contributions to ET, and limited the available energy during critical daily and seasonal growth periods. The influence of shade was evident when our site was compared to studies conducted at other subalpine wetlands with similar climates, vegetation structure, soil moisture, and water table depth. A playas wetland (2,296 m.a.s.l.) in San Luis Valley (SLV), Colorado (Sanderson & Cooper, 2008) had a higher annual ET range, from 352 to 571 mm (although, over a longer period, from April to October). The playas wetland in the SLV had similar environmental controls as our wetland, with mild summer temperatures and cold winters (July mean = 17 °C, January mean = -9 °C), low summer precipitation (128-189 mm), fine-grained soils, a low water table (>1 m), and sparse vegetation dominated by sedge and grass; indicating that one of the limiting variables for low ET at our wetland was the growing influence of horizon shade in the growing season. This impact of horizon shade was visible when observing the hourly and daily patterns of water fluxes across the season at our site and was supported by our GAM-modelling analysis.
We found similar seasonal ET patterns to other subalpine wetlands, defined by an increase through Snow Melt , peak ET duringGreen Up, and a steady decline throughout Peak Growing Season into Late Growing Season (Cooper, Sanderson, Stannard, and Groeneveld, 2006; Sanderson & Cooper 2008; Wang, Wang, Zheng, and Guo, 2012). ET was greatest during Green Up because of large inputs from snowmelt evaporation and transpiration from greening wetland vegetation, including trees and shrubs. Throughout Green Up,incoming solar (Rg) and net radiation (Q*) remained high following the solar maximum and provided large energy contributions to sustain latent (Qe) and sensible (Qh) heat fluxes, as was shown in Hrach et al. (2021). There were also noticeable spikes in ET during the midday hours (12:00 to 17:00) of early June (Figure 6), because of evaporation from snowmelt runoff. Following this, ET continued to increase until the seasonal maximum, which lasted from 12 – 17 July, two weeks before the max ET reported in other studies (Cooper et al. 2006; Wu et al. 2015). Maximum seasonal ET occurred earlier at our site because of the growing influence of shade as the season progressed.
We found that the seasonal water fluxes were strongly influenced by the relationship between available energy and horizon shade. Hrach et al. (2021) found that each hourly increase of shade during Dynamic Shade reduced daily Rg and Q* by 13% and 16%, respectively. Since ET was largely controlled by the available energy, we expected that shade would also strongly influence ET. During Dynamic Shade , each hourly interval of shade per day decreased ET by 17%, similar to other studies, which found that lower ET aligned with periods of reduced available energy and higher shade . Shade was an important control mechanism on the water and energy fluxes at Bonsai over the summer months.