Mi et al. Environ Sci Eur (2019) 31:19 https://doi.org/10.1186/s12302-019-0202-4 RESEARCH Open Access Variable withdrawal elevations as a management tool to counter the efects of climate warming in Germany’s largest drinking water reservoir Chenxi Mi1,2* , Amir Sadeghian3, Karl‑Erich Lindenschmidt3 and Karsten Rinke1 Abstract Background: Thermal stratifcation in reservoirs is a signifcant factor afecting water quality, and can be strongly infuenced by climate change and operational strategies. Reservoirs in the temperate zone react most sensitively to climate warming during winter as ice cover and inversed stratifcation are about to disappear in a warmer world. In this study, two well‑established hydrodynamic models, the one‑dimensional General Lake Model (GLM) and the two‑ dimensional CE‑QUAL‑W2 (W2), were used to investigate the response of winter inversed stratifcation in the Rapp‑ bode Reservoir to future climate warming, combined with diferent water withdrawal elevations. Results: Under increased air temperature, the duration of inversed stratifcation is reduced and the inversion phe‑ nomenon will entirely disappear under current management if the air temperature is increased high enough (more than 4.5 K) in the future. Under strong climate warming, the Rappbode Reservoir will therefore change from a dimictic to a monomictic mixing type. Changing the reservoir management from deep withdrawal (e.g., below 350 m a.s.l.) to shallow withdrawal elevations (e.g., above 390 m a.s.l.) reduces internal heat energy stored in the reservoir in summer and prolongs the inversed stratifcation period in winter. This strategy can retain the dimictic behavior even under strong warming. Conclusions: Our study indicates that adjusting the withdrawal elevation is an efective management instrument to control the winter conditions and can, in fact, mitigate climate warming efects on winter hydrodynamics by stabiliz‑ ing the dimictic mixing type. Keywords: Rappbode Reservoir, Inversed stratifcation, Climate warming, Water withdrawal, Model comparison Introduction and background is suppressed giving rise to a nutrient-rich but colder Termal stratifcation is the phenomenon referring to hypolimnion and a nutrient-poor but warmer epilim- a variation in the water temperature at diferent depths nion [34]. Due to the weakened connectivity, it is difcult in a water body, and is the result of changes in water to supplement the hypolimnion with oxygen from the density with temperature [4]. Te stratifcation is a sig- atmosphere, which can lead to a serious environmental nifcant factor for the evolution of water quality in len- problem of anoxia in the bottom layer [42]. Additionally, tic waters [5]. For example, because of the stratifcation, the enhanced stratifcation throughout summer can lead connectivity between the bottom and surface layers to an increased occurrence of toxin-producing cyano- bacteria in the epilimnion, posing a signifcant threat to water quality [31]. Furthermore, by infuencing the *Correspondence: [email protected] greenhouse gas exchange between the water and atmos- 1 Department of Lake Research, Helmholtz Centre for Environmental phere, stratifcation can afect the role of aquatic systems Research, Magdeburg, Germany Full list of author information is available at the end of the article in the global carbon cycle [14, 47]. Terefore, concerning © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Mi et al. Environ Sci Eur (2019) 31:19 Page 2 of 15 the importance of stratifcation for aquatic ecosystems, a for aquatic systems, there is a need to comprehensively large amount of investigations has focused on this phe- illustrate how the inversion responds to such a noticeable nomenon and its infuencing factors. Reservoirs ofer warming trend and whether anthropogenic management opportunities to afect stratifcation by operational strat- practices can infuence the inversion so as to mitigate the egies, which are not available for natural lakes, and mod- negative efect caused by global warming. els can be used to explore the available options. Among various management strategies for waters, Based on the stratifcation patterns [5], most of the selective water withdrawal is considered to be an efec- freshwater systems in temperate regions are either dimic- tive method to control the thermal stratifcation for tic or warm monomictic: a dimictic water body can be meeting the requirements of diferent usages. For exam- ice covered in winter and is generally stratifed both in ple, Casamitjana et al. [10] applied a one-dimensional summer (direct) and winter (inverse) with turnover mix- model (DLM) to elucidate the response of the thermal ing in spring and autumn; a warm monomictic water is structure of Boadella Reservoir to diferent withdrawal never ice covered and undergoes only one mixing period scenarios. Te results show that the withdrawal location in winter in addition to being stratifed during the rest determines the thermocline depth and the hypolimnion of the year. Currently, when evaluating the stratifcation volume; Çalıskan and Elçi [9] investigated the infuence dynamics in this area, most studies focus on the summer of selective withdrawal on the hydrodynamics of Tahtali stratifcation with three identifable layers, epilimnion, Reservoir and concluded that hypolimnetic withdrawal metalimnion and hypolimnion, and distinct water tem- is the most efective choice to encourage water mixing perature diferences along the vertical direction during and reduce anoxia; Weber et al. [45] developed an opti- this period [18, 22, 23, 30, 37]. However, little attention mization withdrawal strategy which can automatically has been specifcally given to the winter inversed stratif- determine the withdrawal elevation to modify the ther- cation in dimictic waters and its infuencing factors. mal structures for diferent water usages. However, all For dimictic water bodies, the surface water tempera- such previous studies focused on the efect of selective ture in winter is lower than the bottom layer which causes withdrawal on summer stratifcation and did not take the the winter inversed stratifcation [27]. Tis inversed strat- winter inversed stratifcation into account. ifcation is a prerequisite for the formation of an ice cover In this study, two well-established hydrodynamic mod- [5, 29]. Te winter inversed stratifcation (and prolonged els (i.e., General Lake Model and CE-QUAL-W2) were ice cover) can cause low oxygen and fsh kills during long selected to elucidate the response of winter inversed winters, which then can lead to increased grazing of zoo- stratifcation in the Rappbode Reservoir to future cli- plankton on phytoplankton in the following period (and mate warming combined with diferent water withdrawal thus increased water quality). Moreover, the existence elevations. Considering the importance of ice phenology of the inversion is important evidence in distinguish- for the temperature inversion, the ice cover (duration and ing between dimictic and monomictic waters. Due to thickness) is also included in the scenarios. Rappbode global warming trends over the last decades, the winter Reservoir is the largest drinking water reservoir in Ger- inversed stratifcation and ice cover have been shown to many and supplies drinking water to more than 1 mil- be reduced or have even disappeared in numerous lakes lion people. A previous study predicted that by the end around the world, leading to potential changes of mix- of this century, the winter stratifcation will completely ing patterns from dimictic to monomictic types [1, 17, disappear in most of the deep waters in this region as a 41]. By employing a 1D hydrodynamic model, Peeters result of global warming, which will severely threaten et al. [33] simulated the thermal structure of Lake Zurich their aquatic ecosystems [24]. It is expected that our over half a century and predicted that under substan- study will help reservoir operators efectively control the tially increasing air temperatures in the future, the winter winter inversed stratifcation so as to mitigate the nega- inversion will disappear resulting in the lake shifting from tive infuence caused by the increase in air temperature. being generally dimictic to generally monomictic. Te Additionally, the two models (General Lake Model and disappearance of the winter inversion and the subsequent CE-QUAL-W2) are widely used in research to analyze changes in thermal patterns due to climate warming have the vertical stratifcation and thermal dynamics and both a negative infuence on the hypolimnetic oxygen con- are distributed as open-source software. Tis study pro- centration and can even change the available habitat for vides an excellent opportunity to compare and evaluate aquatic organisms [17, 33]. Under high greenhouse gas the models’ performances in detail, which will beneft emissions in the absence of climate mitigation polices, researchers for model selection and application. Further- the global average air temperature may increase by 5 °C more, simulation results are more reliable if they are sup- by the end of the twenty-frst century [15]. Concern- ported by several models, so that