Effects of Floating Macrophytes on Evaporation Rates in Tropical Reservoirs

Abstract Reservoir evaporation represents one of the most significant forms of water loss in tropical and semi-arid areas, but methods to accurately quantify evaporation fluxes at large scales are currently missing. The objectives of this study were: (1) to estimate evaporation in two Brazilian tropical reservoirs by calibrating the Stephens-Stewart equation through remote sensing, and (2) to assess the relationship between macrophyte coverage and evaporation. The calibrated Stephens-Stewart equation provided similar results to class A pan reference measurements (root mean square error of 0.8 mm/day). The two reservoirs yielded an evaporation volume that would be sufficient to meet the water needs of almost a million people. Furthermore, due to the poor water quality of one of the reservoirs, an important percentage of its surface area was covered by macrophytes. Macrophyte coverage was negatively correlated with the evaporation flux (R²=0.76), and led on average to an 11% reduction of the estimated evaporation rate. Overall, the results of this study demonstrate the usefulness of using remote sensing products to estimate reservoir evaporation and outline the need to better understand the relationship between macrophyte cover and reservoir evaporation in tropical and semi-arid areas.