Assessing the response of non-point source nitrogen pollution to land use change based on SWAT model

In recent years, with the growth of the population and the continuous expansion of agricultural land, non-point source (NPS) pollution has gradually become the primary cause of deteriorating water quality in the aquatic environment. Compared to point source pollution, NPS pollution is more diffuse, complex in its mechanisms, and challenging to pinpoint its sources. This study utilized the SWAT-Land Use Update Tool (SWAT-LUT) to dynamically update multi-year land use and land cover (LULC) data into the SWAT model to investigate the differences in nitrogen pollution sources in Jincheng City under different LULC scenarios. Two models were constructed in this study: SWAT-UNI, which utilized static 1997 LULC data, and SWAT-MULTI, which incorporated dynamic LULC data from 1997 to 2022. During the calibration period, SWAT achieved R2 and NSE values exceeding 0.82 for the daily streamflow simulation results, and these values remained above 0.76 during the validation period. Additionally, the Patch-generating Land Use Simulation (PLUS) model was employed to forecast the land use evolution in Jincheng City from 2022 to 2032 to explore the future response of NPS nitrogen pollution. From 1997 to 2022, significant changes were observed in agricultural land, forested land, and grassland areas within Jincheng City. Agricultural land and forested land increased by 3.29% and 4.71% of the total area of Jincheng City, respectively, while grassland decreased by 10.4%. In the prediction of land use evolution from 2022 to 2032, the evolutionary trends remained similar to previous patterns, albeit with a slightly decelerated pace. Simulation results indicated that the top three sources of nitrogen pollution in Jincheng City's water bodies in 1997 were atmospheric deposition (39.8%), nitrogen fertilizer application (29.8%), and soil nitrogen reservoirs (21.4%). With the continuous expansion of agricultural land, nitrogen pollution from nitrogen fertilizer application accounted for 35.6% of the TN (Total Nitrogen) load in water bodies in 2022, surpassing atmospheric deposition to become the dominant factor. The contribution of soil nitrogen reservoirs to nitrogen pollution in water bodies within Jincheng City showed a continuous upward trend over the twenty-five years, resulting in a total nitrogen load of 565.1 tons in 2022, ranking second and becoming a crucial aspect in pollution control efforts. Regarding seasonal distribution, the crop growing season (March to September) was identified as the critical period for controlling nitrogen pollution from nitrogen fertilizer application, while the autumn and winter seasons were crucial for controlling nitrogen pollution from atmospheric deposition and soil nitrogen reservoirs. The predictive results for future NPS nitrogen pollution indicate a continual increase in annual TN inflow into the river from nitrogen fertilizer application and soil nitrogen reservoirs, reaching 1841.6 tons in 2032, accounting for 65.2% of the total inflow. This research contributes to supporting decision-making for NPS pollution control measures in Jincheng City.