Hydrological modeling of The Manavgat river basin using soil and water assessment tool (swat)

The Manavgat River Basin is located in the Mediterranean region and serves as a significant case study for understanding the hydrological dynamics of semi-arid regions affected by climate change and increasing water demand. In this study, the Soil and Water Assessment Tool (SWAT) was used to model hydrological processes within the basin and provide reliable information for sustainable water resources management. The study used daily meteorological data from the General Directorate of Meteorological Affairs (MGM) and hydrological data provided by the State Hydraulic Works (DSİ) Antalya 13th Regional Directorate. Additionally, high-resolution spatial data such as the NASA Digital Elevation Model (DEM) (30 x 30), FAO soil maps, and ESRI land use/cover maps (10 x 10) were integrated into the ArcSWAT model. The RSWAT package was used for automatic calibration and validation of the model, and strong model performance was achieved for the upper basin in Sinanhoca (R² ≥ 0.81, Pbias = 9.02%, p-Factor ≥ 0.65, r-factor ≤ 1.4). However, the model underestimated the flow rate at the main outlet of the lower basin (R² = 0.48, Pbias = -32 %, p-Factor = 0.45, r-factor = 1.12), revealing challenges in modeling complex karstic systems, large-scale reservoir effects, and groundwater flow from upper neighboring river basins. Land use/cover (LULC) change analysis shows a 32 % decrease in forest area and a 25 % increase in grassland area between 2017 and 2022. Although short-term hydrological effects are limited, the basin's complex topography and karstic formations play a significant role in the hydrological cycle and surface and groundwater interactions. Projections under RCP4.5 and RCP8.5 scenarios predict fluctuations in temperature and precipitation regimes. Precipitation is expected to decrease in spring and winter months (-0.55 mm) and increase in autumn (1.79 mm). Temperature is generally expected to increase, although a slight decrease may occur in autumn (0.04°C). Future flow predictions show an increase in flow in the winter season in both the upper and lower basins (14.394 m³/s and 21.079 m³/s, respectively) according to both scenarios; however, a significant decrease in flow may occur in spring and summer months under the RCP8.5 scenario. The study also examined the effects of recent forest fires, which affected 8.86% of the basin area, on hydrological processes. Although no significant changes were observed in the flow regime, slight differences were detected in groundwater dynamics. This comprehensive hydrological analysis of the Manavgat River Basin provides important information for the sustainable management of water resources under climate change and anthropogenic pressures. The study contributes to more effective water resources planning and management strategies in the Manavgat River Basin by revealing the capabilities and limitations of the SWAT model in simulating complex hydrological processes in karstic Mediterranean basins.