Spatiotemporal Characteristics and Response Mechanisms of Water Quantity and Quality in Rivers and Lakes Under Changes in Agricultural Farming Models: A Case Study of the Sihu Basin

To investigate the impact of changes in agricultural farming models on the water quantity and quality of rivers and lakes in plain water network areas, this study focuses on Four Lakes Main Channel (FLMC) and Honghu Lake in the Jianghan Plain's Sihu Basin. Based on remote sensing images and statistical yearbooks, the study identifies periods of large-scale adjustments in agricultural farming models within the basin. Using observational data from 2010 to 2023, the spatial and temporal characteristics of water quantity and quality in rivers and lakes before and after the large-scale adjustments in agricultural farming models are analyzed. The study also examines the proportion and trends of different types of nitrogen and phosphorus pollution loads over the past decade and employs correlation heatmaps to explore the response mechanisms of water quantity and quality to changes in agricultural farming models. The research results show:①From 2010 to 2023, significant changes occurred in the agricultural farming models in the Sihu Basin. Notably, since 2016, the proportion of paddy farming area increased by 14%, while the proportion of dryland crop area decreased by 11%, and freshwater aquaculture area decreased by 4%. ②Temporally, from 2017 to 2023, the outflow volume of FLMC and Honghu Lake increased by approximately 1.2 billion m³ and 0.5 billion m³, respectively, compared to the period from 2010 to 2016. The multi-year average nitrogen and phosphorus concentrations in FLMC decreased by about 2 mg/L and 0.04 mg/L, respectively, while those in Honghu Lake increased by approximately 0.46 mg/L and 0.06 mg/L. During the flood season, the cumulative flow of FLMC accounted for 65.7% of the annual total flow, and the cumulative nitrogen and phosphorus emissions accounted for 60.4% and 67.2% of the annual totals, respectively. ③Spatially, the nitrogen and phosphorus concentrations in FLMC were lower at the inlet and outlet but higher in the middle sections. In contrast, the nitrogen and phosphorus concentrations in Honghu Lake were higher at the inlet and lower at the outlet. ④Since the large-scale adjustment of agricultural farming models in 2016, the water quality of FLMC has improved slightly but not significantly. The turning point from quantitative to qualitative reduction in nitrogen and phosphorus entering the lake has not yet been reached. Due to factors such as climate, natural disasters, internal nutrient release, and changes in water environment carrying capacity, the nitrogen and phosphorus concentrations in Honghu Lake have not decreased despite the reduction in nitrogen and phosphorus entering the lake from FLMC. Therefore, in addition to continuing to optimize agricultural farming models, future efforts should focus on reducing nitrogen and phosphorus inputs into rivers and lakes, mitigating water environment risks caused by internal nutrient release and climate disasters, restoring aquatic vegetation, and enhancing the water environment carrying capacity.