Hydrological Response and Water Balance Assessment of the Kabul River Basin Using SWAT

Abstract

Human as well as the ecosystem is extremely vulnerable to address the demands of freshwater resources in the 21st century as per the changing climate. The future climate scenarios of 21st century show an overall intensification of the Water Resources of upper Kabul River Basin (KRB) with widespread changes expected in this region. To estimate the runoff of the water resources of KRB, this study aims at a research level due to the severe need of water resources, water and power development authority (WAPDA) and climate data from Pakistan meteorological department has been used. Soil and Water Assessment Tool (SWAT) is the main model used in this study, it is a semi distributed model. The model was calibrated and validated at Khairabad outlet gauge at Nowshera, with NSE values of 0.84 and 0.72, R² of 0.85 and 0.72, and PBIAS of 6.8% and 5.2% respectively, for the calibration and validation periods respectively, which shows that the model was satisfactory. Annual ET in 2000 was 159 m³/s, which is the same as in 2000, indicating an energy-limited regime, with mean annual streamflow dropping from 416 m³/s in 1993 to 159 m³/s by 2000 as ET/P increased from 26% to 45%, exhibiting progressive water stress during the drying period. The annual precipitation ranged between 541 and 1,089 mm with snowfall contributing between 26%–37% of the annual precipitation, which is very important in maintaining stream flow during spring and summer from the delayed snow melt in the Hindu Kush mountain range. As evident from the flow duration curve, the hydrology of KRB was highly flashy and very seasonal: Flows were high during the period of April to September (snowmelt and monsoonal), and low during the remaining months of the year. The findings give a quantitative hydrological baseline that is important for water resource management and flood preparedness in this strategically important transboundary basin.

Introduction

The Kabul River Basin (KRB) is increasingly vulnerable to climate change, experiencing more frequent floods, droughts, glacier and snow melt, and changing rainfall patterns. These changes significantly affect the basin's water balance, streamflow, groundwater recharge, and water availability for agriculture, hydropower, and domestic use. Rapid population growth, urbanization, and increasing groundwater extraction have further intensified water scarcity, causing noticeable groundwater depletion in several parts of the basin.

Understanding the rainfall-runoff relationship is essential for effective flood forecasting, reservoir operation, irrigation planning, and sustainable water resource management. However, hydrological modeling in the KRB faces challenges due to limited data availability, spatial variability, and uncertainties in model parameters, reducing confidence in simulation results.

To overcome these limitations, researchers recommend using the Soil and Water Assessment Tool (SWAT) combined with a multisite calibration approach, which accounts for the diverse climatic and physiographic conditions across different parts of the basin. SWAT integrates land use, soil, climate, topography, and hydrological processes to simulate runoff, evapotranspiration, groundwater recharge, and water balance under present and future climate conditions.

Previous studies indicate that under RCP 4.5 and RCP 8.5 climate scenarios, streamflow in most KRB rivers is projected to decrease by approximately 5–8% by 2030, mainly because of rising temperatures, accelerated snowmelt, and altered precipitation patterns. The Hindu Kush–Karakoram–Himalaya (HKKH) region, which supplies much of the basin's water, is particularly vulnerable to climate change.

The literature shows that SWAT has been successfully applied worldwide and performs well when properly calibrated. Nevertheless, heterogeneous basins like the KRB require careful calibration and integration of multiple climate datasets (e.g., GPCC and CHIRPS) to improve simulation accuracy. Other modeling approaches, including ARIMA, have also been used to forecast river discharge.

The Kabul River Basin is a transboundary river system shared by Afghanistan and Pakistan, covering approximately 91,297 km². It includes the Upper Kabul, Panjshir, and Lower Kabul sub-basins and receives water from major tributaries such as the Panjshir, Kunar, Swat, and Kalpani rivers. The basin contributes 10–12% of the annual flow to the Indus River system and supports millions of people through agriculture, hydropower generation, domestic water supply, and ecosystem services.

Conclusion

The SWAT model was satisfactorily calibrated and validated for its suitability to represent the water balance of the Kabul River Basin, having attained the NSE coefficient values of 0.84 and 0.72, respectively. The water balance analysis showed that the KRB is a precipitation-driven, snow-dominated system with the annual precipitation varying from 541 to 1,089 mm during the study period and the snowmelt being a significant component of spring and summer streamflow, and representing around 26–37% of the total annual precipitation on the study years. Despite large inter-annual precipitation variations, actual ET within the basin remained relatively stable (243–307 mm/year), supporting the concept that evapotranspiration in this basin is not water-limited, but rather energy-limited, and the increasing trend in PET in the final portion of the record suggests an increased demand for water by the atmosphere, probably related to warming. A decrease in mean annual streamflow from 416 m³/s in 1993 to 159 m³/s in 2000 (more than a 60% drop) followed the observed decrease in precipitation and snowfall, and the ratio of ET/P increased from 26% to nearly 45% during that period, reflecting increasing water stress as the basin dried. Therefore, the flow duration curve and the seasonal boxplots were used to confirm that the hydrology of KRB is highly variable and flashy, with most flow times during summer being dominated by high flows, whereas winter months flow times are essentially all low-flows, and many are close to zero, indicating a very seasonal regime that is almost entirely driven by the seasonal pulse of inputs from snowmelt and monsoons.Our research work correlation analysis showed that percolation and groundwater recharge are tightly coupled ( r = 0.95) and closely tied to snowmelt, while direct precipitation had surprisingly weak control on streamflow, consistent with the strong storagedelayed release dynamic characteristic of high-elevation snow-fed basins in the wider Hindu Kush region. Together, these results present a quantitative hydrological baseline for the KRB that is necessary for understanding the availability and vulnerability of water resources in the basin to climate variability as well as those hydrological processes that will need to be accurately represented in future assessments of climate change impact.

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Copyright

Copyright © 2026 M. Ishaq, A. Khan, H. A. Jan, M. Khan, M. Usman, U. Khan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.