Evaluation of evaporation estimation methods for a covered reservoir in a semi-arid climate (south-eastern Spain)

SummaryThe main purpose of this study was to evaluate different methods of evaporation estimation for covered water reservoirs. A reservoir equipped with a suspended cover was fully monitored to register the evaporation rate and microclimate below the cover. The datasets were used to evaluate the performance of commonly used evaporation methods, namely energy budget, mass-transfer, combination (Penman and FAO-56 Penman–Monteith) and floating class-A pan. The mass-transfer formula based on the Sherwood number proposed for free convection conditions, which were observed to prevail below the cover, supplied reasonably good estimates of covered reservoir evaporation and it is a good option from a practical point of view, with low input data requirements. Detailed input data and modifications in the calculation of energy fluxes are required to get good evaporation estimations of covered surfaces with the energy budget and FAO-56 Penman–Monteith methods. Besides, some of the standard meteorological input data (such as wind speed at 2 m height) cannot be registered below the cover. Penman equation presented a poor performance related to the overestimation of the advective component for free convection conditions. The pan evaporation was found to be substantially higher than the reservoir evaporation, due to the particular characteristics of the tank, that increased surface temperature and hence evaporation rate. A simplified empirical mass-transfer formula was also proposed to estimate evaporation of covered water bodies from the only knowledge of the surface-to-air mixing ratio gradient.

► Free convection regime prevails in covered reservoirs.
► Open-water evaporation methods need to be adapted to covered conditions.
► Sherwood-based formula provides accurate evaporation estimations of covered surfaces.
► Floating class-A pans are not recommended to measure covered-reservoir evaporation.