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

• Published in: Hydrological processes 2021-08, Vol.35 (8), p.n/a
• Main Authors: Fournier, Judith, Thiboult, Antoine, Nadeau, Daniel F., Vercauteren, Nikki, Anctil, François, Parent, Annie‐Claude, Strachan, Ian B., Tremblay, Alain
• Format: Article
• Language: English
• Subjects: boreal reservoirs; bulk transfer; Cold season; Covariance; Depth perception; Eddy covariance; Empirical equations; Estimates; Evaporation; Hydroelectric power; Hydrologic data; Hydrologic models; Hydrology; Ice breakup; Ice cover; Meteorological observations; Morphometry; Radiation; Reservoir evaporation; Reservoir management; Reservoirs; scarce data; Seasons; Sensitivity analysis; Surface temperature; Temperature requirements; Variance analysis; Vortices; Water; Water temperature; Wind measurement; Wind speed
• ISSN: 0885-6087; 1099-1085
• DOI: 10.1002/hyp.14335

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. Using eddy covariance observations as reference, six evaporation models, categorized as empirical, combined and mass transfer, are tested on two boreal hydropower reservoirs of contrasting morphometry, while considering the scarce data context in which these reservoirs are typically managed. The Bulk transfer model with a constant Dalton coefficient of 1.2 × 10−3 worked best for estimating evaporation, especially if water surface temperature is available.