The total phosphorus (TP) concentration, as the primary limiting eutrophication factor in the Mahabad Dam reservoir in Iran, was studied, considering the combined impacts of climate change, as well as the scenarios on changes in upstream TP loadings and downstream dam water allocations. Downscaled daily projected climate data were obtained from the Beijing Normal University Earth System Model (BNU-ESM) under moderate (RCP4.5) and extreme (RCP8.5) scenarios. These data were used as inputs of a calibrated Soil and Water Assessment Tool (SWAT) model of the watershed in order to determine the effects of climate change on runoff yields in the watershed from 2020 to 2050. The SWAT model was calibrated/validated using the SUFI-2 algorithm in the SWAT Calibration Uncertainties Program (SWAT-CUP). Moreover, to model TP concentration in the reservoir and to investigate the effects of upstream/downstream scenarios, along with forecasted climate-induced changes in streamflow and evaporation rates, the System Dynamics (SD) model was implemented. The scenarios covered a combination of changes in population, agricultural and livestock farming activities, industrialization, water conservation, and pollution control. Relative to the year 2011 in which the water quality data were available, the SD results showed the highest TP concentrations in the reservoir under scenarios in which the inflow to the reservoir had decreased, while the upstream TP loadings and downstream dam water allocations had increased (+29.9%). On the other hand, the lowest TP concentration was observed under scenarios in which upstream TP loadings and dam water allocations had decreased (−18.5%).

The purpose of this study was to investigate thermal stratification and eutrophication in the Zayandeh Roud Dam reservoir located in the central part of Iran using the CE-QUAL-W2 two-dimensional model. The model was used to simulate the temperature and water quality parameters of the reservoir from 2013 to 2015. The selected water quality parameters, which are effective in eutrophication, included nitrate (${\mathrm{NO}}_3$ ), phosphate (${\mathrm{PO}}_4$ ), dissolved oxygen (DO), and total phosphorus (TP). Model calibration and validation were done from October 2013 to January 2015, and the results showed an appropriate convergence between the simulated and observed data. Based on the simulated data, summer stratification in the Zayandeh Roud reservoir caused changes in water quality and anaerobic conditions of the hypolimnion. The reservoir eutrophication condition was investigated based on Carlson’s index, which indicated that the reservoir was in the eutrophic condition in most months of the year. In addition, two management scenarios, reduction of nutrients loads and reaeration of the hypolimnion, were evaluated to mitigate the eutrophication. The former can produce a mesotrophic condition while the latter needs excessive reaeration in order to decrease trophic levels and phosphorus concentration in the hypolimnion.