Evaporation from boreal reservoirs: A comparison between eddy covariance observations and estimates relying on limited data

Hydrological models used for reservoir management typically lack an accurate representation of open-water evaporation and must be run in a scarce data context. This study aims to identify an accurate means to estimate reservoir evaporation with simple meteorological inputs during the open-water season, using long-term eddy covariance observations from two boreal hydropower reservoirs with contrasting morphometry as reference. Unlike the temperate water bodies on which the majority of other studies have focused, northern reservoirs are governed by three distinct periods: ice cover in the cold season, warming in the summer and energy release in the fall. The reservoirs of interest are Eastmain-1 (52°N, mean depth of 11 m) and Romaine-2 (51°N, mean depth of 42 m), both located in eastern Canada. Four approaches are analysed herein: a combination approach, a radiation-based approach, a mass-transfer approach, and empirical methods. Of all the approaches, the bulk transfer equation with a constant Dalton number of 1.2 x 10⁻³ gave the most accurate estimation of evaporation at hourly time steps, compared with the eddy covariance observations (RMSE of 0.06 mm h⁻¹ at Eastmain-1 and RMSE of 0.04 mm h⁻¹ at Romaine-2). The daily series also showed good accuracy (RMSE of 1.38 mm day⁻¹ at Eastmain-1 and RMSE of 0.62 mm day⁻¹ at Romaine-2) both in the warming and energy release phases of the open-water season. The bulk transfer equation, on the other hand, was incapable of reproducing condensation episodes that occurred soon after ice breakup. Basic and variance-based sensitivity analyses were conducted, in particular to measure the variation in performance when the bulk transfer equation was applied with meteorological observations collected at a certain distance (~10–30 km) from the reservoir. This exercise illustrated that accurate estimates of open water evaporation require representative measurements of wind speed and water surface temperature.     

The Impact of Mangrove Prop-Root Epibionts on Juvenile Reef Fishes: A Field Experiment Using Artificial Roots and Epifauna

A field experiment was established in Bocas Del Toro, Panama to examine the relationship between sessile organisms living on mangrove prop roots and fish communities. Artificial mangrove roots (AMR) with different sets of artificial (AE) or real epibionts were established in five different locations in two separate years. Fish species in each plot were identified, counted, and their size estimated by visual census for 15 days in each replicate. In the artificial mangrove plots, the treatments with the most heterogeneous structure had significantly greater abundance of most families and species richness of fish in both years of the experiment. AMR plots with AEs attracted a more abundant and diverse fish assemblage than those with live epibionts, which had lower three-dimensional structure. All of the AMR plots had significantly greater fish abundance than comparable plots of sea grass alone. The location of the replicate also made a significant difference to fish abundance. The data indicate that prop-root epibionts can enhance fish abundance and diversity in mangroves, although the relationship may depend on the specific nature of the epibionts and fishes present.     

Effect of Secondary Circulations on the Surface–Atmosphere Exchange of Energy at an Isolated Semi-arid Forest

Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface–atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface–atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface–atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface–atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.