Comment on hess-2021-549

Abstract

Evaporation forms a large loss term in the water balance of inland water bodies. During summer seasons, which are projected to become warmer with more severe and prolonged periods of drought, the combination of high evaporation rates and increasing demand on freshwater resources forms a challenge for water managers. Correct parameterisation of open water evaporation is crucial to include in operational hydrological models to make well supported predictions of the loss of water through evaporation. Here, we aim to study the controls on open water evaporation of a large lowland reservoir in the Netherlands. To this end, we analyse the dynamics of open water evaporation at two locations, i.e. Stavoren and Trintelhaven, at the border of Lake IJssel (1100 km2) where eddy covariance systems were installed during the summer seasons of 2019 and 2020. From these measurements we find that wind speed and the vertical vapour pressure gradient, but not available energy, can explain most of the variability of observed hourly open water evaporation. This is in agreement with Dalton's model which is a well-established model often used in oceanographic studies for calculating open water evaporation. At the daily timescale, we find that wind speed and water temperature are the main drivers in Stavoren. These observed driving variables of open water evaporation are used to develop simple data-driven models for both measurement locations. Validation of these models demonstrates that a simple model using only two variables, performs well both at the hourly timescale (R2 = 0.84 in Stavoren, and R2 = 0.67 in Trintelhaven), and at the daily timescale (R2 = 0.72 in Stavoren, and R2 = 0.51 in Trintelhaven). Using only routinely measured meteorological variables leads to well performing simple data-driven models at hourly (R2 = 0.78 in Stavoren, and R2 = 0.51 in Trintelhaven) and daily (R2 = 0.85 in Stavoren, and R2 = 0.43 in Trintelhaven) timescales. These results for the summer periods show that global radiation is not directly coupled to open water evaporation at the hourly or even daily timescale, but rather wind speed and vertical gradient of vapour pressure are variables that explain most of the variance of open water evaporation. However, when we extend the time series to a complete year, we find a distinct yearly cycle reflecting the yearly dynamics of global radiation. We find that the commonly used model of Penman (1948) produces results that resemble the yearly cycle of observed evaporation. However, at the diurnal scale estimated evaporation using Penman’s model disagrees with observed evaporation. Therefore, using the Penman equation to model open water evaporation for shorter periods of time is questioned. We would like to stress the importance of including the correct drivers in the parameterization of open water evaporation in hydrological models to adequately represent the role of evaporation in the surface-atmosphere interaction of inland water bodies.