Should canyon-shaped Three Gorges Reservoir be treated as an artificial lake in evaporation studies? Results from eight years floating pan observations

Reservoirs lose significant amounts of water through evaporation, particularly for large canyon-shaped reservoirs with dams on major rivers. The canyon-shaped reservoirs, characterized by their long, narrow, and deep water bodies within steep V-shaped or U-shaped valleys, differ significantly from lakes but are often mistakenly studied as ‘artificial lakes’, leading to substantial biases in evaporation estimates. Canyon-shaped reservoirs boast the largest storage capacities among reservoir types in China and globally. This study, utilizing eight years observations from a floating pan on the Three Gorges Reservoir (TGR)—China’s largest reservoir, explores the evaporation processes unique to canyon-shaped reservoirs from both the mass transfer and energy balance perspectives.

Regarding the energy balance, the results reveal that evaporation from the floating pan exhibits a bimodal pattern in August and December, contrasting sharply with the unimodal pattern observed in lakes or lake-type reservoirs. The December peak lags the net radiation by four months. The water body’s energy balance follows a seasonal trajectory, with a heat storage period from March to August and a heat release period from September to February. An energy budget analysis of seven cross-sections based on water temperature along the TGR’s main stream highlights the critical roles of heat storage and advected energy, which are influenced by varying water depth and flowing water.

In terms of mass transfer, we discovered that the evaporation rate over the TGR is intensified by temperature inversions within the boundary layer (negative water-to-air temperature differences), a characteristic hydrothermal feature of canyon-shaped reservoirs. Unlike lakes, the evaporation rate per unit water-to-air vapor pressure difference does not depend on horizontal wind speed but significantly increases during temperature inversions, primarily occurring from March to August. This phenomenon is attributed to river-valley breezes, which generate significant vertical air movements that drive evaporation. The enhancement in evaporation rate is roughly estimated to be 117 mm annually.

These findings underscore that canyon-shaped reservoirs should not be treated as artificial lakes when studying evaporation. The impacts of varying water depth and horizontal flow should be seriously considered when investigating the energy balance for evaporation, and the role of river-valley breezes must be carefully examined when studying the turbulent transfer for water evaporation over the water surface.