Projecting the Impacts of Future Climate Change on Runoff in the Kashafrud River Catchment, Iran

ABSTRACT
Climate change directly impacts water resources, exacerbating risks such as floods and droughts. This study aimed to project the effects of climate change on runoff in the upper Kashafrud River basin. Downscaling was performed using the CMhyd model, and runoff projections were generated via the SWAT hydrological model. Outputs from selected CMIP6 models under the SSP1-2.6 and SSP5-8.5 scenarios were applied for the near-future (2025–2054) and mid-future (2055–2084) periods. After SWAT-CUP validation and calibration, the SWAT model was simulated under climate change conditions. The calibration yielded NSE, RMSE, and R² values of 0.73, 12.154, and 0.672 for the Dowlatabad station and 0.81, 13.25, and 0.72 for the Kartian station, respectively. Validation results showed values of 0.77, 14.205, and 0.684 for Dowlatabad and 0.76, 16.43, and 0.66 for Kartian. Analysis of climatic variables (precipitation and temperature) under climate change revealed increases in seasonal and annual maximum/minimum temperatures across all scenarios, as well as increased precipitation in all scenarios except SSP1-2.6 during the near-future period. Runoff under SSP1-2.6 is projected to decrease by 10.03% in the near future, while other periods and scenarios show increases ranging from 3.37% (SSP1-2.6, mid-future) to 15.56% (SSP5-8.5, mid-future). These shifts are primarily attributed to rising temperatures and intensified extreme precipitation events. The findings provide a critical management tool for water resource policymakers to adapt to climate change impacts.
Extended abstract
Introduction
Climate change is one of the most pressing challenges facing humanity today, directly impacting hydrological processes and water resources. It exacerbates risks such as floods and droughts, threatening both human life and ecosystems. The study area, the upper basin of the Kashafrud River, is a critical watershed in northeastern Iran and highly sensitive to climatic fluctuations. Understanding the implications of climate change on runoff and hydrological cycles in this region is essential for effective water resource management and planning. The primary objective of this research is to assess the impacts of climate change on runoff dynamics in the upper Kashafrud River basin under various future climate scenarios, using advanced hydrological and climate modelling tools. The effects of climate change on water resources are particularly pronounced in semi-arid regions like northeastern Iran, where water scarcity is already a pressing issue. The Kashafrud River basin is a vital water source for agricultural, domestic, and ecological needs in the region. As climatic variables such as temperature and precipitation shift, they directly alter the hydrological balance, influencing processes like evapotranspiration, infiltration, and runoff generation. These changes necessitate a deeper understanding of potential future scenarios to guide adaptive water management strategies. Recent studies highlight that urban and rural communities in semi-arid regions are especially vulnerable to the adverse impacts of climate change. For instance, urban heat islands exacerbate temperature rises, while rural areas face heightened risks of drought and resource depletion. The Kashafrud basin encompasses diverse topographical and climatic conditions, making it an ideal case study for understanding the broader implications of climate change in similar environments. This research not only examines projected changes in runoff patterns but also evaluates the cascading effects on local water security, agricultural productivity, and flood risks.
 
Methodology
To achieve the research objectives, state-of-the-art hydrological and climate modelling approaches were employed. The CMhyd model was used for downscaling and bias correction, while the SWAT (Soil and Water Assessment Tool) model simulated runoff. The study incorporated output data from the CMIP6 (Coupled Model Intercomparison Project Phase 6) ensemble, specifically focusing on SSP1-2.6 (optimistic scenario) and SSP5-8.5 (pessimistic scenario) for two future periods: 2025–2054 (near future) and 2055–2084 (mid-future). Meteorological data, including precipitation and maximum/minimum temperatures, were collected from historical datasets (1991–2020) provided by the Iran Meteorological Organisation (IMO). The CMhyd model applied statistical methods, including distribution mapping and linear scaling, to correct biases in precipitation and temperature data. Validation revealed higher accuracy in temperature simulations than in precipitation data, a critical step in ensuring reliable inputs for hydrological modelling under different scenarios. The SWAT model was calibrated and validated using SWAT-CUP (Calibration and Uncertainty Programs). Key performance metrics, including Nash-Sutcliffe Efficiency (NSE) and Root Mean Square Error (RMSE), confirmed the model’s reliability. After validation, the SWAT model simulated hydrological responses under future climate conditions, ensuring robust runoff projections that account for complex interactions between climate variables and hydrological processes.
 
Results and Discussion
The findings indicate significant alterations in the hydrological cycle due to climate change. Seasonal and annual maximum and minimum temperatures are projected to increase across all scenarios. Precipitation trends vary, with increases observed in most scenarios except for SSP1-2.6 during the near future.
Runoff Dynamics: Runoff projections show contrasting patterns depending on the scenario and period. Under SSP1-2.6, runoff is expected to decrease by approximately 10.03% in the near future due to reduced precipitation intensity and frequency. In contrast, SSP5-8.5 projects significant increases in runoff, reaching up to 15.56% in the mid-future, driven by heightened precipitation extremes and elevated temperatures. These projections highlight the dual impacts of climate change—reduced runoff under optimistic scenarios versus intensified hydrological responses under pessimistic scenarios.
Spatial and Temporal Variability: Urban areas within the basin, such as Mashhad, are likely to experience intensified runoff due to urban heat island effects and increased impermeable surfaces. Elevated regions like Chakaneh show pronounced sensitivity to warming, with amplified runoff and reduced snowpack retention. These variations underscore the heterogeneous impacts of climate change across the basin, emphasising the need for localised adaptation measures tailored to different sub-regions. Water Balance Components: Hydrological component analysis reveals evaporation, infiltration, and base flow shifts. Evapotranspiration is projected to increase due to rising temperatures, further influencing soil moisture and groundwater recharge. Under SSP5-8.5, evapotranspiration values rise by approximately 4% compared to the baseline period, modifying the hydrological cycle. Reduced snowmelt under warming conditions is expected to affect water availability in mountainous regions, exacerbating challenges for downstream water users reliant on seasonal runoff.
Precipitation Patterns: The near-future period under SSP1-2.6 shows slight reductions in precipitation, adversely affecting runoff generation. Conversely, the mid-future period under SSP5-8.5 exhibits significant increases in precipitation intensity, potentially leading to more frequent and severe flood events. These changes are closely linked to atmospheric warming and shifts in regional weather patterns, which influence rainfall distribution and intensity.
Implications for Water Resource Management: Increased runoff in most scenarios highlights the potential for higher flood risks, necessitating adaptive strategies such as enhanced reservoir capacity and floodplain management. Conversely, reduced runoff under SSP1-2.6 underscores the need for water conservation measures to mitigate shortages. These findings are pivotal for designing Integrated Water Resource Management (IWRM) plans tailored to future climatic conditions. Policymakers must also consider broader implications, including impacts on agriculture, urban water supply, and ecosystem services.
 
Conclusion
This study underscores the profound impacts of climate change on the Kashafrud River basin’s hydrological regime. Rising temperatures and altered precipitation patterns will significantly influence runoff dynamics, posing challenges for sustainable water resource management. While increased precipitation in some scenarios may alleviate water shortages, it also escalates flood risks. Policymakers must prioritize adaptive measures to balance these dual challenges, integrating advanced climate modelling insights into regional planning.
Future research should focus on incorporating socioeconomic factors into hydrological models to better understand the interplay between human activities and climate change. Additionally, developing real-time monitoring systems and enhancing the resolution of climate projections can provide more accurate and actionable data for decision-makers. These efforts will be critical in building resilience against climate-induced hydrological risks in the Kashafrud River basin and similar regions.
Funding
There is no funding support.
 
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved thecontent of the manuscript and agreed on all aspects of the work declaration of competing interest none.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.