Application of the CMIP6 Approach for Determination of Climate Change Effects on Inflow to Sattarkhan Reservoir, Ahar, using the IHACRES model

Abstract
Background and Objectives
Climate change is one of the most important challenges of the present century. This phenomenon has numerous effects on water resources, ecosystems, and human societies. Reservoir dams, which play a crucial role in providing drinking water, flood control, hydropower generation, and irrigation, are among the systems most susceptible to climate change effects. The inflow into reservoir dams is highly sensitive to changes in climatic variables, making it essential to assess how future climate changes will impact this inflow. By predicting river flows and the runoff entering reservoirs, it is possible not only to manage the utilization of water resources but also to estimate and mitigate natural disasters such as floods and droughts. Examining and identifying the impacts of climate change is vital for planning adaptation strategies and ensuring the sustainability and resilience of reservoirs amidst a changing climate. This research was conducted to assess the impact of future climate change on the inflow to the reservoir of the Sattarkhan Dam in the Ahar Chai River basin, located in East Azerbaijan Province.
Methodology
The Ahar chai basin is a sub-basin of the Aras basin, located in the northwest of Iran, specifically in East Azerbaijan Province. The Sattarkhan Dam is constructed within this basin on the Ahar Chai River. The total annual precipitation and the average minimum and maximum temperatures in this basin are 276 mm, 1.53 °C, and 11.68 °C, respectively. According to the De Martonne classification, the region has a semi-arid climate. This study investigated the impact of future climate change on the inflow into the reservoir of the Sattarkhan Dam in the Ahar Chai basin. Climatic and hydrometric data from the period of 2008-2022 and the IHACRES rainfall-runoff model were used for estimate runoff. The output of the HadGEM3-GC31-LL model under the SSP1-2.6 and SSP5-8.5 scenarios from the CMIP6 was used for predict future climate.
Findings
The results indicated that precipitation amounts are expected to increase in most months under both SSP1-2.6 and SSP5-8.5 scenarios. Conversely, there will be an increase in both minimum and maximum temperatures during most months. Specifically, in July, the warmest month of the year, the maximum temperature is projected to rise by 1.45 °C, increasing from 15.84 °C in the base period (2008-2022) to 17.28 and 17.29 °C in the future (2025-2039) under the SSP1-2.6 and SSP5-8.5 scenarios, respectively. The highest maximum temperature recorded in the base period, which was 29.06 °C in August, is expected to reach 30.30 °C in July. The evaluation of the IHACRES model's performance revealed that, despite some errors in estimating minimum and maximum runoff, the model generally performed acceptably, with R², RMSE, and MAE values of 0.83, 1.17 m3.s-1, and 0.41 m3.s-1, respectively, during the calibration period, and R², RMSE, and MAE values of 0.51, 1.35 m3.s-1, and 0.90 m3.s1 during the validation period. A comparison of the runoff time series between the 14-year base period (2008-2021) and the future period (2025-2039) under the SSP1-2.6 and SSP5-8.5 scenarios indicated a significant decline in the inflow to the Sattarkhan Dam during the future period compared to the base period. Statistical and quantitative comparisons of observed runoff during the base period and predicted runoff for the future period showed that the average inflow to the Sattarkhan Dam reservoir was 1.39 m3.s-1 in the base period, but is expected to decrease by 7% and 19% to 1.30 and 1.13 m3.s-1 under the SSP1-2.6 and SSP5-8.5 scenarios, respectively. The maximum inflow is also projected to decrease significantly, dropping from 26.43 m3.s-1 in the base period to 12.28 m3.s-1 (54% reduction) and 10.75 m3.s-1 (59% reduction) under the SSP1-2.6 and SSP5-8.5 scenarios, respectively. On a monthly timescale, the most significant reduction is expected in April, where the average inflow is projected to decrease from 4.52 m3.s-1 in the base period to 3.13 and 2.82 m3.s-1 under the SSP1-2.6 and SSP5-8.5 scenarios, respectively. In June, July, August, and September, an increase in runoff is anticipated under both scenarios, while in February, an increase is expected only under the SSP1-2.6 scenario.
Conclusion
The results showed that, despite an increase in precipitation in most months, minimum and maximum temperatures will also rise. The IHACRES model performed well, with R², RMSE, and MAE values during the calibration period and the validation period. A comparison of runoff between the base period and the future period revealed a significant decrease in inflow to the Sattarkhan Dam. The reduction in inflow to the dam reservoir will affect its performance, posing challenges for drinking water supply, agriculture, and industry. This situation indicates concerning challenges regarding water resources in the region and the ability to meet water needs across various sectors. Addressing these challenges requires proactive management strategies and informed decision-making to mitigate future risks.