Climate change impact on water quality in the integrated Mahabad Dam watershed-reservoir system

Climate change, besides global warming, is expected to intensify the hydrological cycle, which can impact watershed nutrient yields and affect water quality in the receiving water bodies. The Mahabad Dam Reservoir in northwest Iran is a eutrophic reservoir due to excessive watershed nutrient input, which could be exacerbated due to climate change. In this regard, a holistic approach was employed by linking a climate model (CanESM2), watershed-scale model (SWAT), and reservoir water quality model (CE-QUAL-W2). The triple model investigates the cumulative climate change effects on hydrological parameters, watershed yields, and the reservoir's water quality. The SDSM model downscaled the output of the climate model under moderate (RCP4.5) and extreme (RCP8.5) scenarios for the periods of 2021-2040 and 2041-2060. The impact of future climate conditions was investigated on the watershed runoff and total phosphorus (TP) load, and consequently, water quality status in the dam's reservoir. The results of comparing future conditions (2021-2060) with observed present values under moderate to extreme climate scenarios showed a 4-7% temperature increase and a 6-11% precipitation decrease. Moreover, the SWAT model showed a 9-16% decline in streamflow and a 12-18% decline in the watershed TP load for the same comparative period. Finally, CE-QUAL-W2 model results showed a 3-8% increase in the reservoir water temperature and a 10-16% increase in TP concentration. It indicates that climate change would intensify the thermal stratification and eutrophication level in the reservoir, especially during the year's warm months. This finding specifies an alarming condition that demands serious preventive and corrective measures.