Evaporation from a large lowland reservoir – observed dynamics during a warm summer

Distinct differences in surface characteristics between a water body and a land surface result in different drivers of evaporation and therefore its dynamics. It is essential to include and represent this difference in the parameterization of open water evaporation (E_water) to improve operational hydrological models. Additionally, more accurate parameterization becomes even more crucial to predict potential changes in quantity and dynamics of E_water in a changing climate in support of optimal water management now and in the future.

For this purpose, we performed a long-term measurement campaign to measure E_water and related meteorological variables over a large lowland reservoir in the Netherlands. During the summer seasons of 2019 and 2020 eddy-covariance systems were applied at two locations at the border of lake IJsselmeer in the Netherlands. These high temporal resolution measurements gave us the opportunity to explore the dynamics and identify the underlying driving mechanisms of E_water. Using the data collected during the summer of 2019 we were able to develop a simple regression model for both measurement locations. Combinations, both sums and products, of the following independent variables were considered: global radiation, wind speed, water skin temperature, vapour pressure deficit, and vertical vapour pressure gradient. The product of wind speed and vertical vapour pressure gradient best explained the observed hourly E_water rates, which is consistent with the commonly used aerodynamic approach. The model was validated using the data of 2020. Additionally, we compared measured E_water to E_water computed with Makkink’s equation, which is currently used in the Dutch operational hydrological models to estimate E_water. Although a correction factor is applied to account for the difference between land evaporation and E_water, Makkink is not able to capture the dynamics of E_water. This was reflected in the timing and shape of the evaporation peak at both daily and monthly scales. The disagreement of E_water dynamics found between the measured and simulated E_water even more demonstrates the value and need of a correct parameterization of E_water.