Introduction

Within the context of global water scarcity and climate change, land use and land cover change (LU/LCC) is recognized as a principal factor influencing hydrological processes at the watershed scale. Activities such as deforestation, agricultural expansion, and urban development alter the physical characteristics of the land surface, thereby transforming critical hydrological parameters including infiltration capacity, evapotranspiration, and runoff generation. These changes primarily function by reducing soil water infiltration and increasing surface runoff, which can exacerbate phenomena such as flooding, diminish groundwater recharge, and weaken base flow in rivers.

The Khorram Rud watershed in Lorestan Province, due to its hydropolitical role (as a headwater of the Karkheh River), the reliance of local communities on agriculture and livestock for their livelihoods, and its location in a water-stressed region, necessitates sustainable water resource management. Numerous studies in various parts of the world utilizing hydrological models such as SWAT have confirmed a significant relationship between land cover change and increased runoff. However, a quantitative assessment of this relationship in the Khorram Rud watershed, considering the changes over the past two decades, presents a scientific and practical necessity. The primary objective of this research is to simulate and quantify the impacts of land use changes from 2001 to 2020 on the hydrological response and surface runoff in the Khorram Rud watershed using the SWAT model. The findings of this study can provide a scientific basis for decision-makers to guide environmental planning and integrated water resource management in this watershed

Methodology

This study employed an applied-analytical approach through hydrological modeling. The primary modeling tool was the semi-distributed physical model SWAT (Soil and Water Assessment Tool), implemented within the ArcGIS 10.4 environment using the ArcSWAT extension. In this research, the required data were collected and prepared for modeling in four main categories: Topographic data based on a 30-meter resolution ASTER Digital Elevation Model (DEM) for watershed delineation, drainage network extraction, and subdivision into 21 sub-basins; a soil units map from the FAO global database; land use maps for the years 2001 and 2020, which were developed and validated through supervised classification of Landsat satellite imagery into five main classes and climatic and hydrological data, including daily precipitation and temperature data from the Khorramabad synoptic station and monthly discharge data from the Cham Anjir hydrometric station for a 28-year period (1993 to 2020).

During the implementation and analysis process, following the initial model setup and creation of Hydrological Response Units (HRUs), sensitivity analysis, calibration, and validation of the model parameters were performed using the SUFI-2 algorithm in the SWAT-CUP software, based on observed discharge data. The periods 1996-2012 and 2013-2020 were designated for calibration and validation, respectively. Model performance was evaluated using the statistical indices NSE, R², and PBIAS.

To assess the net effect of land use changes, two scenarios were defined and executed: a baseline scenario applying the 2001 land use map and a change scenario applying the 2020 land use map, while all other input data remained constant.



Results and discussion

The findings of this study reveal three key sets of results. First, a comparison of land use maps from 2001 and 2020 indicates extensive changes within the watershed. Specifically, the area of dense forest decreased by 69.99%, and sparse forest and rangeland decreased by 33.96%. In contrast, agricultural lands expanded by 72.57%, settlement areas by 61.68%, and grasslands by 106.53%. These changes, which were predominantly concentrated in the northern and eastern parts of the watershed, established the primary context for hydrological transformation.

Subsequently, the accuracy and performance of the SWAT model were evaluated. Following calibration and validation using the SUFI-2 algorithm, the model demonstrated good to very good performance. The Nash-Sutcliffe Efficiency (NSE) and coefficient of determination (R²) values were 0.66 for the calibration period and ranged from 0.80 to 0.85 for the validation period. Furthermore, the Percent Bias (PBIAS) index fell within an acceptable range (±10%), confirming the model's reliability for simulation purposes.

Finally, the quantitative impact of these land use changes on the runoff regime was investigated. A comparison of outputs from the baseline scenario (2001 land use) and the change scenario (2020 land use) revealed a significant influence of land cover alterations. Consequently, the mean annual streamflow increased by 0.16 units (equivalent to 1.8%). Furthermore, the slope of the increasing flow trend was steeper in the change scenario (0.164) compared to the baseline (0.161), indicating a gradual intensification of effects over the long term; a trend evident in 76% of the study period years. These findings, underscore the pivotal role of land use change in exacerbating surface runoff and increasing flood potential, particularly during intense rainfall events.



Conclusion

Key findings reveal that the basin has undergone extensive transformations, including a severe reduction in forest cover and a substantial expansion of agricultural and residential land. These changes have served as the primary driver of increased average surface runoff and altered the basin's hydrological response pattern; the underlying mechanism has operated chiefly through reduced soil infiltration capacity and an increase in the runoff coefficient. Furthermore, the decrease in time of concentration resulting from these changes has heightened the basin's sensitivity, particularly to intense rainfall events, thereby amplifying its flood potential. In summary, the results indicate that the Khorram Rud basin is transitioning from an infiltration-dominated hydrological system toward a runoff-dominated one. This shift poses a serious threat to water resource sustainability (by reducing aquifer recharge), settlement security (by increasing flood risk), and the basin's ecological resilience. Accordingly, it is recommended that the preservation and restoration of forest cover, especially in high-altitude and steep-slope areas, be prioritized in management planning; that hydrological considerations and the basin's carrying capacity be incorporated into agricultural and urban development planning; that watershed management and runoff control measures be implemented at the basin scale to partially offset the adverse effects; and finally, that future research investigate the combined effects of land use change and climate change on the water resources of this basin.