Modeling Agricultural Water Use Efficiency in Tibet's Pengbo Irrigation District: An Application of the SWAT Hydrological Model

Abstract This study investigated the hydrological cycle dynamics under water management measures in the Pengbo irrigation area of Tibet, analyzing and evaluating the water balance and water productivity at the irrigation area scale. The findings provide a theoretical foundation for enhancing water use efficiency and promoting high crop yields in the alpine irrigation regions of Tibet. Utilizing the water balance method and SWAT hydrological model simulations, the water cycle processes in the Pengbo Irrigation District were elucidated, and the irrigation water productivity along with crop water productivity for highland barley, winter wheat, and rapeseed were calculated and analyzed. The results indicate that:a) The SWAT model, calibrated and vali-dated using runoff data from 2022 to 2023, shows a strong agreement between measured and simulated values. The calibration and validation results meet the accuracy criteria with R²≥0.82 and NSE ≥ 0.70.b) The spatial distribution of deep seepage and irrigation water in the study area exhibits similar patterns; regions with higher irrigation water volumes are prone to increased deep seepage. The deep seepage in the study area ranges from 197.2 mm to 314.5 mm, constitut-ing 29.3–36.7% of the total rainfall plus irrigation. The evapotranspiration during the crop growth period varies from 465.1 mm to 616.8 mm, representing 60.1–74.2% of the total rain-fall plus irrigation. c ) The spatial distribution of crop irrigation water productivity inversely correlates with irrigation water volume; areas with higher irrigation volumes exhibit lower crop irrigation water productivity. The average irrigation water productivity for highland barley, winter wheat, and rapeseed was 1.97 kg·m⁻³, 2.38 kg·m⁻³, and 0.80 kg·m⁻³, respectively. The average crop water productivity was 1.16 kg·m⁻³, 1.41 kg·m⁻³, and 0.47 kg·m⁻³. Winter wheat demonstrates significantly higher irrigation and crop water productivity compared to highland barley and rapeseed.The SWAT model simulations effectively characterize the water balance features of the Pengbo Irrigation District in Tibet and are suitable for simulating water cycle processes in similar alpine regions. Future agricultural de-velopment should focus on improving water use efficiency by optimizing crop irrigation systems, transitioning from flood irrigation to border irrigation, sprinkler irrigation, and micro-sprinkler irrigation. Additionally, considering crop yield and water productivity, it is advisable to increase the cultivation of winter wheat and highland barley while reducing the area dedicated to rapeseed.