Evaporation from a reservoir with fluctuating water level: Correcting for limited fetch

SummaryOperational water reservoirs are used to manage the supply of water according to the local demand and availability. Hence, such reservoirs are characterized by large variability in the water level due to variations in inflow and outflow rates, thereby complicating the measurement and modeling of evaporation. The present study investigated evaporation from an open water body characterized by a rapidly fluctuating water level using an eddy covariance system (ECS) constrained by limited fetch. The aim is to identify the evaporation model which is in best agreement with the directly measured evaporation. The water vapor source area within the footprint of the ECS is of a dual nature, comprising the water surface and surrounding soil. Hence this study explored correcting the ECS evaporation rate by a footprint model that accounts for both the variable water level and the limited fetch. This correction significantly improved the reservoir energy balance closure and the agreement between measurements and models that primarily rely on the energy balance approach (e.g. Penman type models). Mean daily evaporation during 104 days (not continuous) from May to August 2008 was 6.73 ± 1.16 mm day−1. A reduction in daily evaporation from June to August was associated with the decrease in net radiation from its peak value in June. The Penman–Brutsaert model agreed best with the modified ECS measurements in providing long-term (104 days) prediction while on a daily basis the Penman model performed best.

► Evaporation from a reservoir with a highly fluctuation water level was measured by the eddy covariance technique.
► The measured evaporation was corrected for limited fetch using a footprint model.
► The corrected evaporation provided good energy balance closure and was in good agreement with evaporation models.
► Over a long time period of 104 days, the Penman-Brutsaert model was in best agreement with the measurements.
► On the daily time scale the Penman model provided best agreement with measurements.