Assessing vertical diffusion in a stratified lake using a three‐dimensional hydrodynamic model

Abstract

Vertical turbulent diffusivity (K z ), which can be estimated from water temperature, is a key factor in the evolution of water quality in lentic waters. In this study, we analysed the capability of a three-dimensional hydrodynamic model (EFDC) to capture water temperature and vertical diffusivity in Lake Arendsee in the Northern German plain. Of particular interest to us is to evaluate the model performance for capturing the diffusion minimum within the metalimnion and analyse the response of the metalimnetic K z to meteorological forcing, namely changing wind speed and warming. The comparison confirmed that the calibrated model could reproduce both stratification dynamics and vertical diffusion profiles in the lake. The model was also shown to be able to capture the duration and vertical extent of the metalimnetic diffusion minimum. The scenario results illustrate that, compared to air temperature, wind velocity appeared to be the more influential meteorological variable on the vertical exchange within the metalimnion. While increasing wind velocities mostly affected the minimum values of K z in the metalimnion and thus led to intensified vertical exchange, the reduction of wind velocity mostly affected the depth of minimal K z , but not its absolute value.

Simulating thermal dynamics of the largest lake in the Caucasus region: The mountain Lake Sevan

“…The value of ce after calibration reached about 0.0026 instead of 0.0013 in the default, which is in the range of other studies using GLM (Ladwig et al, 2018;Rinke et al, 2010). Similarly, the hypolimnetic vertical diffusion coefficient post-calibration was at 6.79 10 -5 m 2 s -1 , consistent with the range in the literature (Dong et al, 2020). The wind factor WF slightly changed post-calibration to 0.97.…”

“…We applied the calibrated model in order to analyse the response of thermal dynamics to climate warming and changing wind conditions. Although climate change will ultimately affect all meteorological variables, air temperature and wind speed were shown to be of primary influence for lake physics (Dong et al, 2020;. It is therefore reasonable to focus an initial assessment of climate impact on lakes on these variables.…”

Variability in epilimnion depth estimations in lakes

“…Ultimately, these results suggest that the stratification period calculated in different studies or for different regions cannot be compared unless identical definitions are used. The method most appropriate for identifying the stratified period has been considered in other studies (Woolway et al, 2014;Engelhardt and Kirillin, 2014; however, our results offer some additional insights. The results suggest that the use of water density metrics, such as epilimnion depth estimates, in combination with traditional water-temperature-based definitions of stratification, is incompatible, given the non-linear relationship between temperature and density.…”

“…the Jassby and Powell (1975) heat-flux method, are restricted to use below the epilimnion and photic zone. Vertical turbulence profiles, however, as well as water temperature profiles, are estimated by some hydrodynamic lake models (Goudsmit et al, 2002, Dong et al, 2019. Such modelled data, therefore, offer a tool for assessing commonly used water temperatureor density-based methods in comparison to turbulence-based methods.…”

Bayesian Calibration Points to Misconceptions in Three‐Dimensional Hydrodynamic Reservoir Modeling

“…Stratification is particularly pronounced at warmer water temperatures when density differences can become greater. In addition, many other factors such as salinity and organic processes affect heat absorption and transfer in reservoirs and lakes (Dong et al., 2020; Sommer et al., 1986; Thackeray et al., 2013). Still, the heat transfer between air and water or in water by advection and diffusion is a slow process, which often takes several weeks and seasonally shifts more and more due to climate change (Woolway et al., 2021).…”