Three-dimensional hydrodynamic and ecosystem modeling of warm-monomictic reservoirs (Maroon and Abolabbas, Iran) under the impact of 21st century climate change

Lakes and reservoirs, as the endpoint of their upstream catchment, collect their water as the resultant of the hydrological and ecosystem processes and, thus, act as a gauge of their catchment’s management. Physical processes, e.g. stratification and mixing and propagation of density currents, are the main drivers of the biochemical factors in lakes and reservoirs and are prone to be altered and affected by climate change. Hydrodynamic models are the tools to simulate the behavior of the water body under various forcing conditions. In this dissertation, hydrodynamics and limnological processes of the Maroon reservoir and the planned Abolabbas reservoir in southwest Iran were studied in a physical (the former) and biochemical (the latter) respect using 3D hydrodynamic (and biochemical) modeling. We used two 3D models and compared them in capturing hydrodynamics of the Maroon reservoir, studied the role of the reservoir’s morphology in its hydrodynamics, simulated nutrients cycle of the nearby Abolabbas reservoir and have an insight about these processes in the Maroon reservoir and eventually identified and predicted the effects of different climate change scenarios on the Maroon reservoir’s hydrodynamics and thermal regime in the 21st century. This dissertation is presented, based on the perspectives mentioned above, in four individual research studies, which have been published or are in press as follows: Chapter 1. Introduction The research on physical limnology and the concept of hydrodynamic modeling in lakes and reservoir are briefly introduced. The importance of climate change in limnology and its shown impacts on lakes and reservoirs in the literature are summarized, and finally the purpose and motivation of the research forming this thesis is presented. Chapter 2. Comparison between two hydrodynamic models in simulating physical processes of a reservoir with complex morphology: Maroon reservoir Two 3D hydrodynamic models AEM3D and MIKE3 are compared in simulating hydrodynamics of the Maroon reservoir. The reservoir has a complex bathymetry with steep walls which makes it a good case for studying the performance of hydrodynamic models. The results indicated that the AEM3D model, by using a finite difference scheme with a purely z-level vertical discretization, shows better consistency with observations so that the AME and RMSE of the model remain below 1℃. The MIKE3 model showed overall higher errors from 56 to 130% larger than AEM3D and the level of error strongly depends on its vertical discretization method and the turbulence model. The lowest errors by MIKE3 were seen by the k-ε turbulence model with a hybrid z-sigma discretization, while the highest errors were generated by the sigma discretization. The stand-alone vertical mixing model in AEM3D model, used instead of the constant-eddy-viscosity or k-ε formulation, showed a better performance in modeling vertical mixing and wind mixed layer, which is another reason of observing better results by this model than MIKE3. Chapter 3. Effects of morphology in controlling propagation of density currents in a reservoir using uncalibrated three-dimensional hydrodynamic modeling Effects of basin morphology are shown to affect density current hydrodynamics of the Maroon reservoir using the AEM3D hydrodynamic model. The model results were validated with measured water temperature data at five locations in the reservoir. The Maroon reservoir consists of upper and lower basins that are connected by a deep and narrow canyon. Analyses of simulations indicate that the canyon strongly affects density current propagation and the resulting differing limnological characteristics of the two basins. The evolution of Wedderburn Number, Lake Number, and Schmidt Stability Number are shown to be different in the two basins, and the difference is attributable to the morphological separation by the canyon. Investigation of the background potential energy (BPE) along the length of the canyon indicated that a density front passes through the upper section of the canyon but is smoothed into simple filling of the lower basin. Chapter 4. Pre-impoundment assessment of the limnological processes and eutrophication in a reservoir using three-dimensional modeling: Abolabbas reservoir, Iran The 3D hydrodynamic and ecosystem model ELCOM-CAEDYM (former name of AEM3D) is used to simulate the oxygen and nutrient cycles (eutrophication processes) in the planned Abolabbas reservoir with three (one normal and two drought) reservoir scenarios. To evaluate the eutrophication, the Trophic State Index (TSI) and Vollenweider’s model are applied to the model outputs for the total phosphorus (TP). The results show that under normal conditions the reservoir will be oligotrophic, whereas the drought scenarios cause a general lowering of the water quality indices and the development of a mesotrophic-eutrophic or even a fully eutrophic state. Under drought conditions the reservoir might suffer from severe oxygen depletion, especially in the hypolimnion. The sensitivity analysis indicates that the wind drag coefficient, light intensity, and sediment oxygen exchange rate exhibit the strongest influence on the modeled eutrophication state in the planned reservoir. Chapter 5. A potential tipping point in the thermal regime of a warm-monomictic reservoir under climate change using three-dimensional hydrodynamic modeling We modeled the response of the warm-monomictic Maroon reservoir to possible 21st century climate change, using projections of three CMIP5 GCMs under RCP4.5 and 8.5 scenarios. The raw GCM projections are bias-corrected by novel quantile mapping approaches to provide the boundary conditions for the model AEM3D model. Prior to the predictive modeling, a new so-called overturn bias analysis was applied to evaluate possible bias in the GCM predictors for properly simulating the lake dynamics in the observational (historic) time window and found to be negligible. The modeling results indicate for RCP4.5 a continuous reduction with a complete suppression of the lake’s winter mixing by the end of the 21st century, implying a switch from monomictic to weakly oligomictic behavior. Under the more extreme RCP8.5, such a transition occurs abruptly in the late 2050s in the form of a tipping point, followed within a decade - with yet some short periods of winter mixing - by a conversion to complete meromixis. This happens, because surface and mixing temperatures significantly increase due to climate warming, whereas the hypolimnion is less altered, partly due to cold river inflow (underflow) in the future winter.