Modeling and assessing regional agro-hydrological processes and water-nitrogen use in arid and shallow groundwater areas

In arid irrigation areas with shallow groundwater, accurately assessing agro-hydrological processes and water–nitrogen use efficiency is crucial for agricultural management. However, simulating regional water-salt-nitrogen-plant interactions in such regions remains challenging. To address this gap, this study enhanced the mechanistic, distributed hydrological model (SurfMOD) to simulate soil nitrogen dynamics and solute transport in surface water flow. The improved SurfMOD was tested in the Jiyuan Irrigation System (Jiyuan), located in the upper Yellow River basin, using multi-source observation datasets collected during 2021–2022. Calibration and validation results demonstrated that the model accurately simulated layered soil water, salt, and nitrogen dynamics, groundwater fluctuations, as well as crop growth. The temporal variations in drainage flow and solute loads were also reasonably reproduced. Moreover, the model exhibited high numerical stability and computational efficiency. The study further revealed water, salt, and nitrogen cycling in salinity-affected areas. It also demonstrated that neglecting salinity in modeling caused systematic biases of 6%, 23%, and 2% in crop yield, field beneficial water use fraction (BWUF field ), and field nitrogen use efficiency (NUE field ), respectively. The BWUF field ranged widely from 0.42 to 0.98, with a regional BWUF of 0.76; meanwhile, 82.1% of the farmland achieved appropriate NUE field , and the regional NUE reached 0.80. Although some fields exhibited relatively low BWUF field and NUE field , the region as a whole maintained high water-nitrogen use efficiency. Results indicated that in arid areas with shallow groundwater, border-flood irrigation led to considerable deep percolation in some local irrigated farmlands. However, lateral groundwater flow and capillary rise effectively redistributed and reused water and nitrogen, thereby sustaining high water-nitrogen use efficiency, crop productivity, and crop diversity on a regional scale. Overall, this study provides a reliable tool for simulating coupled agro-hydrological processes involving water-salt-nitrogen-plant interactions and offers a valuable case study for regional water-nitrogen management.