Adaptive reservoir operation considering water quantity and quality objectives: Application of parallel cellular automata and sub-seasonal streamflow forecasts

This paper presents a new framework for the adaptive reservoir operation considering water quantity and quality objectives. In this framework, using the European Centre for Medium-Range Weather Forecasts (ECMWF) database, the monthly precipitation forecasts, with up to 6-month lead time, are downscaled and bias corrected. The rainfall forecasts are used as inputs to a rainfall-runoff simulation model to predict sub-seasonal inflows to reservoir. The water storage at the end of a short-term planning horizon (e.g. 6 months) is obtained from some probabilistic optimal reservoir storage volume curves, which are developed using a long-term reservoir operation optimization model. The adaptive optimization model is linked with the CE-QUAL-W2 water quality simulation model to assess the quality of outflow from each gate as well as the in-reservoir water quality. At the first of each month, the inflow forecasts for the coming months are updated and operating policies for each gate are revised. To tackle the computational burden of the adaptive simulation-optimization model, it is run using Parallel Cellular Automata with Local Search (PCA-LS) optimization algorithm. To evaluate the applicability and efficiency of the framework, it is applied to the Karkheh dam, which is the largest reservoir in Iran. By comparing the run times of the PCA-LS and the Non-dominated Sorting Genetic Algorithms II (NSGA-II), it is shown that the computational time of PCA-LS is 95 % less than NSGA-II. According to the results, the difference between the objective function of the proposed adaptive optimization model and a perfect model, which uses the observed inflow data, is only 1.68 %. It shows the appropriate accuracy of the adaptive model and justifies using the proposed framework for the adaptive operation of reservoirs considering water quantity and quality objectives.

Influences of hydrodynamics on dissolved inorganic carbon in deep subtropical reservoir: Insights from hydrodynamic model and carbon isotope analysis

Dam construction significantly impacts river hydrodynamics, subsequently influencing carbon biogeochemical processes. However, the influence of hydrodynamic conditions on the migration and transformation of Dissolved Inorganic Carbon (DIC) remains uncertain. To bridge this knowledge gap, we integrated hydrochemistry, isotopic composition (δ13CDIC), and a hydrodynamic model (CE-QUAL-W2) to examine the distinctions, control mechanisms, and environmental effects of DIC biogeochemical processes in a typical large and deep reservoir (Hongjiadu Reservoir) under different hydrodynamic conditions. We evaluated hydrodynamic alterations through the Schmidt stability index and relative water column stability. The analysis disclosed that during weak hydrodynamics periods, the energy necessary for complete mixing the surface and deep water was 34 times higher (3615.32 J/m2 vs.106.86 J/m2), and stability was 13 times greater (312.96 vs. 24.69) compared to periods of strong hydrodynamics. Additionally, the spatiotemporal heterogeneity of DIC concentrations (1.4 % to -9.1 %) and δ13CDIC (-1.7 % to -19.5 %) from the dry to wet seasons reflected disparities in DIC control mechanisms under varied hydrodynamic conditions. Based on model simulations, our calculations indicate that during weak hydrodynamics periods, the enhancement of the biological carbon pump effect resulted in substantial sequestration of DIC, reaching up to 379.6 t-DIC·d-1 in the water. Conversely, during strong hydrodynamics periods, DIC retention capacity decreased by 69.2 t·d-1, resulting in reservoir CO2 emissions of 22.7 × 104 t, which were more than 7 times higher than during weak hydrodynamics periods (3.2 × 104 t). Our findings emphasize the discernible impact of hydrodynamic conditions on reservoir biogeochemical processes related to DIC. Considering the increasing construction of reservoirs globally, understanding and controlling hydrodynamic conditions are crucial for mitigating CO2 emissions and optimizing reservoir management.

Diurnal variation characteristics of thermal structure in a deep reservoir and the effects of selective withdrawal

Thermal structure significantly impacts the reservoir ecology and environment. The diurnal temperature variation (DTV) influences the photosynthesis and respiration of phytoplankton in day and night, and meanwhile changes the vertical density convection and current. Selective withdrawal is practical to control the withdrawal elevation and outflow temperature, and can also influence the thermal structure of reservoir area. Previous research mainly focusses on the long-term change pattern of thermal structure in reservoir, while the diurnal thermal dynamics is less studied, and the effects of selective withdrawal on the DTV is still unknow. Hence, this paper aims to illustrate the diurnal variation characteristics of thermal structure in reservoir, and moreover to reveal the corresponding effects of selective withdrawal. Taking the Sanbanxi Reservoir as study case, a hydrodynamic-temperature numerical model with hourly simulating resolution was built and validated using measured temperature profile during August 2015 to August 2016, based on the CE-QUAL-W2. The effects of selective withdrawal schemes, including different withdrawal elevation and withdrawal types, were illuminated. The discrete Fourier transform (DFT), energy spectrum and statistical analysis were used. This paper showed the following: (1) In the reservoir area, there were three main regions with significant DTV, i.e., the surface layer, the 10-m water layer and the 60-m water layer (withdrawal layer). For the baseline (existing intake), the average DTVs were 0.657, 0.497 and 0.174 °C, respectively, at the surface layer, the 10-m-depth layer and the 60-m-depth layer. (2) From the upstream to the downstream channels of reservoir, the DTV kept unchanged at the surface layer, and increased at the 10-m-depth layer and 60-m-depth layer. (3) A higher withdrawal elevation and the internal weir schemes (stoplog gate/temperature-control curtain) could increase the epilimnetic DTV and energy density, and were suggested to mitigate the hypoxia and nutrient enrichment, compared with a lower withdrawal elevation and the multi-level intake scheme. The results could provide technical support for the ecological management of reservoir and engineering design of the selective withdrawal.

Influences of water level fluctuation on water exchange and nutrient distribution in a bay: Evidence from the Xiangxi Bay, Three Gorges Reservoir

Following the Three Gorges Reservoir (TGR) impoundment, many tributaries were turned into bays; hydrodynamic conditions of TGR profoundly changed the residence time, temperature, and nutrient distributions of bays, and nutrient enrichment occurred in these bays. However, little research has been done on the effects of water level qqfluctuations (WLFs) of TGR on the bay. In this study, Xiangxi Bay (XXB), one of the tributaries of TGR, was selected as the delegate to construct and calibrate a two-dimensional hydrodynamic-temperature-tracer-water quality model based on the CE-QUAL-W2. The results were the following: 1) In spring, as total nitrogen (TN) in the TGR tended to be higher than that in the XXB, the downward WLF increased water exchange, TGR-XXB nutrient flux and TN in the epilimnion of the XXB, and decreased the water exchange and TN in the hypolimnion of the XXB. The upward WLF did the opposite. The situation would be reversed in autumn. 2) Under a larger magnitude or a shorter period of WLF, its corresponding effects on the water exchange and TN increased. 2) Both the downward and upward modes of WLF helped to decrease the thermal stratification of XXB. 4) The upward/downward WLF could be used to decrease the epilimnetic TN of XXB in spring/autumn, and was suggested to reduce the local algal bloom. The WLFs by the TGR regulation could profoundly change the water exchange and nutrient distribution in the bay, which helped to control nutrient concentrations and prevent algal blooms.

Evidence of a rapid phosphorus-induced regime shift in a large deep reservoir

Ecological regime shift studies in freshwater systems are mainly limited to shallow lakes and reservoirs, while abrupt changes in deeper lakes are often attributed to climate change. Here, we demonstrate the application of regime shift theory to one of California's newest and deepest reservoirs, Diamond Valley Lake (DVL), which in recent years showed an unexpected rapid departure from its water quality conditions of the previous decade. The reservoir shifted from a well oxygenated condition with low phytoplankton growth to a hypoxic, phytoplankton-dominated turbid system. We statistically identified the critical stressor (phosphorus (P)), switch points, and its load threshold and characterized its transition to an alternative stable state and the stabilizing mechanisms contributing to hysteresis. We analyzed long-term environmental, chemical and flow data, conducted a hydrographic survey, and developed a hydrodynamic model to characterize the factors that contributed to regime shift and to evaluate different management strategies that might reverse this shift. Our findings indicate that large deep systems exhibit different transition dynamics in the presence of an acute stressor compared to regime shifts in shallow systems. A cumulative external TP load threshold of 4.6 mg m^-2 d^-1 added to the reservoir over nearly 11 months was identified as the critical stressor. For large deep systems, inherent morphometric features such as large relative depth combine with external stressors to drive regime shifts. Light winds, morphometric conditions impeding deep mixing, and a stable stratification that lasts up to 9 months makes DVL more susceptible to hypolimnetic hypoxia, an intrinsic factor accelerating regime shift. Results also suggest regime shift occurred in 2013, when new limnological processes were established to reinforce the new alternative stable state and existing ecosystem services were impaired. Interactions between hypoxia, internal P loading (~2.1 mg m^-2 d^-1), and seasonal cyanobacterial blooms were identified as mechanisms perpetuating the new alternative state.

The impacts of changing climate and streamflow on nutrient speciation in a large Prairie reservoir

Climate mediated warming water temperature, drought and extreme flooding are projected to shift the phenology of nutrients in receiving lakes and reservoirs further intensifying eutrophication and algal blooms, especially in temperate reservoirs. An emerging issue in reservoir management is the prediction of climate change impacts, a necessity for sound decision making and sustainable management. Lake Diefenbaker is a large multipurpose reservoir in the Canadian Prairies. In this study, the impact of climate change on nutrient speciation in Lake Diefenbaker is examined using loosely linked SpAtially Referenced Regression On Watershed attributes (SPARROW) and CE-QUAL-W2 models. Two climate mediated scenarios, RCP 8.5 representing the most extreme climate change, and climate induced streamflow were modelled. Nutrient levels are anticipated to double under the climate change and streamflow scenarios. Winter and spring were identified as hot moments for nitrogen pollution with a plausible saturation of nitrous oxides in the future. Of concern is a plausible recycling of nitrate through dissimilatory nitrate reduction to ammonium. Summer and fall on the other hand represent the period for phosphorus enrichment and internal loading with a probable succession of cyanobacteria in the summer.

Automatic calibration of the two-dimensional hydrodynamic and water quality model using sequential uncertainty fitting approach

Reservoir hydrodynamic and water quality modeling, in conjunction with the monitoring programs, is one of the essential tools for controlling the pollution of these types of water bodies. The complexity of the model, data scarcity, and the variable nature of natural phenomena lead to uncertainty in models, which should be considered in the calibration process of these models. Uncertainty-based automatic calibration is one of the methods that can be effective in achieving a high-reliability model. In this paper, the Sequential Uncertainty Fitting (SUFI-2) algorithm was used for the automatic calibration of the two-dimensional hydrodynamic and water quality model (CE-QUAL-W2) for the reservoir under parameter uncertainty conditions. To this end, the CE-QUAL-W2 model was developed to simulate the temperature and water surface elevation of the Karkheh Dam reservoir (western Iran). The parameters affecting temperature were regarded as uncertain parameters in the calibration process, including the coefficients of longitudinal eddy viscosity, longitudinal eddy diffusivity, Chezy coefficient or Manning, wind sheltering, solar radiation absorbed in the surface layer, extinction coefficient for pure water, and the experimental coefficients of wind speed function. The developed method demonstrated a high potential for matching the simulated temperature and water surface elevation for the reservoir with the measured data. Averagely, 69% of the simulated temperature and 90% of the simulated water surface elevation were located within the 95% confidence interval. The SUFI-2 algorithm also showed better performance in terms of the convergence rate compared with the particle swarm optimization (PSO) algorithm, which indicated a lower number of calls (80 calls compared to 2000 calls) and could reduce the total root-mean-square error by 9.6%.

Choosing an appropriate water quality model-a review

Water quality models are quite complex to use even for scientists, requiring knowledge in different areas such as biology, chemistry, physics, and engineering. Hence, the use of these models by a non-specialist is quite complicated, demanding considerable time and research, particularly to choose which model is the most appropriate for a given situation. In this study, a comparative guide is suggested, which can help users select the appropriate water quality model for certain systems and variables. Five models were considered as follows: AQUATOX, CE-QUAL-W2, Spatially Referenced Regression Model on Watershed Attributes (SPARROW), Soil and Water Assessment Tool (SWAT), and Water Quality Analysis Simulation Program 7 (WASP7), which have been widely used during the last 5 years. All of these selected models are free and easily available. It was verified that each model has its particularities and applications; however, the AQUATOX model has several advantages compared with the other models analyzed. In addition, to illustrate the availability of the proposed comparative guide, a case study was carried out to demonstrating the selection process of the selected models.

Ensemble warming projections in Germany's largest drinking water reservoir and potential adaptation strategies

The thermal structure in reservoirs affects the development of aquatic ecosystems, and can be substantially influenced by climate change and management strategies. We applied a two-dimensional hydrodynamic model to explore the response of the thermal structure in Germany's largest drinking water reservoir, Rappbode Reservoir, to future climate projections and different water withdrawal strategies. We used projections for representative concentration pathways (RCP) 2.6, 6.0 and 8.5 from an ensemble of 4 different global climate models. Simulation results showed that epilimnetic water temperatures in the reservoir strongly increased under all three climate scenarios. Hypolimnetic temperatures remained rather constant under RCP 2.6 and RCP 6.0 but increased markedly under RCP 8.5. Under the intense warming in RCP 8.5, hypolimnion temperatures were projected to rise from 5 °C to 8 °C by the end of the century. Stratification in the reservoir was projected to be more stable under RCP 6.0 and RCP 8.5, but did not show significant changes under RCP 2.6. Similar results were found with respect to the light intensity within the mixed-layer. Moreover, the results suggested that surface withdrawal can be an effective adaptation strategy under strong climate warming (RCP 8.5) to reduce surface warming and avoid hypolimnetic warming. This study documents how global scale climate projections can be translated into site-specific climate impacts to derive adaptation strategies for reservoir operation. Moreover, our results illustrate that the most intense warming scenario, i.e. RCP 8.5, demands far-reaching climate adaptation while the mitigation scenario (RCP 2.6) does not require adaptation of reservoir management before 2100.

The formation of a metalimnetic oxygen minimum exemplifies how ecosystem dynamics shape biogeochemical processes: A modelling study

Metalimnetic oxygen minima are observed in many lakes and reservoirs, but the mechanisms behind this phenomena are not well understood. Thus, we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir with a well-established two-dimensional water quality model (CE-QUAL-W2) to systematically quantify the chain of events leading to its formation. We used high-resolution measured data to calibrate the model, which accurately reproduced the physical (e.g. water level and water temperature), biogeochemical (e.g. nutrient and oxygen dynamics) and ecological (e.g. algal community dynamics) features of the reservoir, particularly the spatial and temporal extent of the MOM. The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes. The occurrence of the cyanobacterium Planktothrix rubescens in the metalimnion delayed and slightly weakened the MOM through photosynthesis, although its decaying biomass ultimately induced the MOM. Our research also confirmed the decisive role of water temperature in the formation of the MOM since the water temperatures, and thus benthic and pelagic oxygen consumption rates, were higher in the metalimnion than in the hypolimnion. Our model is not only providing novel conclusions about the drivers of MOM development and their quantitative contributions, it is also a new tool for understanding and predicting ecological and biogeochemical water quality dynamics.

Development of the delta-normal stress combining CE-QUAL-W2 as a novel method for spatio-temporal monitoring of water quality in Karkheh Dam Reservoir

Continuous monitoring of water quality in dam reservoirs is a typically difficult and costly operation. In this study, the results of computer modeling with the CE-QUAL-W2 model were combined with data mining techniques to develop a new method called "delta-normal stress" for identifying the critical temporal and spatial monitoring ranges. For this purpose, long-term variations of three quality parameters including nitrite-nitrate level, dissolved oxygen (DO) level, and water temperature near the outlet of the dam, which is the point of interest for reservoir exploitation, were analyzed. Based on this analysis, the time intervals and depth ranges with the highest frequency of significant variations in terms of each parameter were identified. The results showed that given the difference between the delta-normal stress trend of temperature and that of other parameters in Karkheh Dam Reservoir, temperature can be monitored at much lower sampling resolutions and using cheaper methods and equipment without sacrificing accuracy. Based on the frequency of occurrence of delta-normal stress of more than 20% above the total average, the key sampling times and locations for nitrite-nitrate and DO levels were determined to be the periods of January-February, February-March, and March-April, and depths of 60, 55, 50, and 5 m, respectively.

Applicability of water quality models around the world-a review

Water quality models are important tools used in the management of water resources. The models are usually developed for specific regions, with particular climates and physical characteristics. Thus, applying these models in regions other than those they were designed for can generate large simulation errors. With consideration to these discrepancies, the goal of this study is to identify the models employed in different countries and assist researchers in the selection of the most appropriate models for management purposes. Published studies from the last 21 years (1997-2017) that discuss the application of water quality models were selected from three engineering databases: SpringerLink, Web of Science, and Scopus. Seven models for water quality simulations have been widely applied around the world: AQUATOX, CE-QUAL-W2, EFDC, QUALs, SWAT, SPARROW, and WASP. The countries most frequently applying water quality models are the USA, followed by China, and South Korea. SWAT was the most used model, followed by the QUAL group and CE-QUAL-W2. This study provides the opportunity for researchers, who wish to study countries with fewer cases of applied water quality models, to easily identify the work from that region. Furthermore, this work collated central themes of interest and the most simulated parameters for the seven countries that most frequently employed the water quality models.

Effects of temperature-control curtain on algae biomass and dissolved oxygen in a large stratified reservoir: Sanbanxi Reservoir case study

A temperature-control curtain (TCC) is a new technique of selective withdrawal for controlling the outflow temperature of a reservoir. A TCC can significantly affect the reservoir hydrodynamic and thermal structure, but its effects on water quality and ecology remain unknown. In this study, we developed and calibrated a hydro-thermal-water quality model to numerically analyze how a TCC located 1 km from a dam affected algal biomass and water quality in a reservoir. According to our results, when a TCC was used, the mean annual chlorophyll a (Chl-a) concentrations in the reservoir decreased. Chl-a concentrations remained constant during the heating period until normal water levels were reached, and increased during the cooling period and decreased until year-end drawdown levels were reached. The dissolved oxygen (DO) concentrations decreased and the anoxic proportions increased throughout the year. The yearly mean Chl-a and DO concentrations in the reservoir declined continuously as the water-retaining proportion (P_r) of the TCC increased from 0 to 87.5%, while the anoxic proportion (DO < 2 mg/L) first increased and then decreased, peaking at a P_r of 62.5%. The change patterns of the anoxic proportion were consistent with those of thermal stability, demonstrating the applicability of thermal stability in predicting reservoir hypoxia. Moreover, the environmental impact of TCCs will increase under global warming, and TCCs can mitigate the increased algal biomass and further decrease DO in warmer climate conditions. Under a medium-high climate scenario, Representative Concentration Pathway 6.0, and a TCC having 75% P_r, the yearly mean Chl-a, DO concentrations, and anoxic proportions of Sanbanxi Reservoir are predicted to reach 10.7 μg/L, 4.2 mg/L, and 39.6%, respectively, by 2046-2065. Thus, changes in the water environment and ecology (particularly the likely deterioration of water quality because of selective withdrawal under global warming) should be considered as a component of water management practices.

Effects of water level fluctuation on thermal stratification in a typical tributary bay of Three Gorges Reservoir, China

Xiangxi River is a typical tributary of Three Gorges Reservoir (TGR) in China. Based on field observations in 2010, thermal stratification was significant in most months of the year. Through field data analysis and numerical simulations, the seasonal and spatial variation of thermal stratification as related to the impact of the operation of TGR were investigated. Thermal stratification was most pronounced from April to September in the Xiangxi River tributary. Air temperature (AT) and water level (WL) were the two dominant variables impacting thermal stratification. AT affected the surface water temperature promoting the formation of thermal stratification, and high WLs in TGR deepened the thermocline depth and thermocline bottom depth. These results provide a preliminary description of the seasonal variation and spatial distribution of thermal stratification, which is important for better understanding how thermal stratification affects algae blooms in Xiangxi River.

Impact of intra-annual runoff uniformity and global warming on the thermal regime of a large reservoir

Thermal stratification is common in reservoirs and greatly influences the aquatic environment. Changes in the uniformity of intra-annual runoff have been detected in several basins, but few studies have focused on the impacts that these changes have on thermal regimes. Using runoff data for Sanbanxi Reservoir, China, during 1950-2015, the long-term trends of intra-annual runoff uniformity were statistically analyzed and extrapolated for the 2050s and 2090s, and the relationship between these trends and the thermal regime of the reservoir were investigated. Moreover, the thermal regime was evaluated for future climate scenarios accounting for global warming. This study shows the following: 1) for South China, the concentration degree (C_d) for the distribution of intra-annual runoff in natural basins such as Sanbanxi Reservoir tended to be higher, but for rivers significantly impacted by human activities, C_d tended to be lower. 2) a higher C_d was associated with an increased reservoir temperature and released water temperature, and decreased thermal stability. For Sanbanxi Reservoir, a 10% increase in C_d corresponded to a change in annual average temperature, thermal stability, and released water temperature of 0.036 °C, -48.4 J m^-2, and 0.153 °C, respectively. These changes were larger in summer than in other seasons; 3) global warming is predicted to increase reservoir temperature, released water temperature, and thermal stability, having a more significant influence on these parameters than intra-annual runoff uniformity; 4) future changes in thermal regimes will intensify oxygen stratification and hypolimnetic anoxia, promoting algal blooms, and delaying fish spawning. Effects of two methods aimed at controlling the thermal regime were also analyzed, including changing the operation level and intake elevation of the reservoir. This study investigated the response of the thermal regime of Sanbanxi Reservoir to climate change, and provides theoretical support for the management of water temperature and the reservoir's aquatic environment.

Modeling the effect of temperature-control curtain on the thermal structure in a deep stratified reservoir

Temperature-control curtain (TCC) is an effective facility of selective withdrawal. Previous research has estimated the influence of TCC on the outflow temperature, but its effect on the thermal structure of a reservoir area is unknown, which is crucial to the reservoir ecology. For this purpose, taking the Sanbanxi Reservoir as a case study, a 2-D hydrodynamic and temperature model covering the whole reservoir was built and calibrated to simulate the flow and temperature fields under different TCC scenarios, and the change rules of thermal stability and outflow temperature are obtained. When the water-retaining proportion (P_r) of bottom-TCC increases, the temperature difference between inflow and outflow monotonously decreases, while the thermal stability first increases and later decreases. The maximum thermal stability exists at P_r = 62.5%; it goes against water quality improvement and should be avoided in practice. A bottom-TCC with P_r > 80% is practical for deep reservoirs such as Sanbanxi Reservoir to decrease the temperature difference between inflow and outflow without the increase of thermal stability. In terms of top-TCC, as P_r increases, the temperature difference between inflow and outflow monotonously increases and thermal stability decreases. The top-TCCs are recommended when a smaller thermal stability is more preferentially considered than outflow temperature, or a cool outflow in the summer is required for downstream coldwater fishes. In addition, the TCC cannot decrease or increase the outflow temperature all of the time throughout the whole year, and it primarily changes the phase and variation range of the outflow temperature. This study quantitatively estimates the potential effect of TCCs on the thermal structure and water environment management and provides a theoretical basis for the application of TCC.

Sediment plume model-a comparison between use of measured turbidity data and satellite images for model calibration

In this study, we built a two-dimensional sediment transport model of Lake Diefenbaker, Saskatchewan, Canada. It was calibrated by using measured turbidity data from stations along the reservoir and satellite images based on a flood event in 2013. In June 2013, there was heavy rainfall for two consecutive days on the frozen and snow-covered ground in the higher elevations of western Alberta, Canada. The runoff from the rainfall and the melted snow caused one of the largest recorded inflows to the headwaters of the South Saskatchewan River and Lake Diefenbaker downstream. An estimated discharge peak of over 5200 m³/s arrived at the reservoir inlet with a thick sediment front within a few days. The sediment plume moved quickly through the entire reservoir and remained visible from satellite images for over 2 weeks along most of the reservoir, leading to concerns regarding water quality. The aims of this study are to compare, quantitatively and qualitatively, the efficacy of using turbidity data and satellite images for sediment transport model calibration and to determine how accurately a sediment transport model can simulate sediment transport based on each of them. Both turbidity data and satellite images were very useful for calibrating the sediment transport model quantitatively and qualitatively. Model predictions and turbidity measurements show that the flood water and suspended sediments entered upstream fairly well mixed and moved downstream as overflow with a sharp gradient at the plume front. The model results suggest that the settling and resuspension rates of sediment are directly proportional to flow characteristics and that the use of constant coefficients leads to model underestimation or overestimation unless more data on sediment formation become available. Hence, this study reiterates the significance of the availability of data on sediment distribution and characteristics for building a robust and reliable sediment transport model.

Oxycline formation induced by Fe(II) oxidation in a water reservoir affected by acid mine drainage modeled using a 2D hydrodynamic and water quality model - CE-QUAL-W2

The Sancho reservoir is an acid mine drainage (AMD)-contaminated reservoir located in the Huelva province (SW Spain) with a pH close to 3.5. The water is only used for a refrigeration system of a paper mill. The Sancho reservoir is holomictic with one mixing period per year in the winter. During this mixing period, oxygenated water reaches the sediment, while under stratified conditions (the rest of the year) hypoxic conditions develop at the hypolimnion. A CE-QUAL-W2 model was calibrated for the Sancho Reservoir to predict the thermocline and oxycline formation, as well as the salinity, ammonium, nitrate, phosphorous, algal, chlorophyll-a, and iron concentrations. The version 3.7 of the model does not allow simulating the oxidation of Fe(II) in the water column, which limits the oxygen consumption of the organic matter oxidation. However, to evaluate the impact of Fe(II) oxidation on the oxycline formation, Fe(II) has been introduced into the model based on its relationship with labile dissolved organic matter (LDOM). The results show that Fe oxidation is the main factor responsible for the oxygen depletion in the hypolimnion of the Sancho Reservoir. The limiting factors for green algal growth have also been studied. The model predicted that ammonium, nitrate, and phosphate were not limiting factors for green algal growth. Light appeared to be one of the limiting factors for algal growth, while chlorophyll-a and dissolved oxygen concentrations could not be fully described. We hypothesize that dissolved CO2 is one of the limiting nutrients due to losses by the high acidity of the water column. The sensitivity tests carried out support this hypothesis. Two different remediation scenarios have been tested with the calibrated model: 1) an AMD passive treatment plant installed at the river, which removes completely Fe, and 2) different depth water extractions. If no Fe was introduced into the reservoir, water quality would significantly improve in only two years. Deeper extractions (3m above the bottom) would also improve the water quality by decreasing the hypoxic zone. However, extractions at the epilimnion would increase the amount of hypoxic water in the reservoir.

Selective withdrawal optimization in river-reservoir systems; trade-offs between maximum allowable receiving waste load and water quality criteria enhancement

In this paper, a new systematic approach is designed to maximize the demand coverage and receiving waste load by river-reservoir systems while enhancing water quality criteria. The approach intends to control the reservoir eutrophication while developing a trade-off between the maximum receiving load and shortage on demand coverage. To simulate the system, a hybrid process-based and data-driven model is tailored. Initially, the two-dimensional hydrodynamics and water quality simulation model (CE-QUAL-W2) is linked with an effective single and/or multiple optimization algorithms (PSO) to evaluate the proposed scenarios. To increase the computational efficiencies, the simulation model is substituted with a surrogate model (ANN) in an adaptive-dynamically refined routine. The proposed method is illustrated by a case study in Iran, namely, Karkheh River Reservoir, for 180-monthly periods. The results showed the applicability of the methodology especially to solve high-dimensional multi-period complex water resource optimization problems. Also, the results demonstrated that eutrophication could be reduced under the optimal inflow phosphate control and reservoir operation, regulating the total phosphorous concentration in the reservoir.