Multiscale phosphorus loss in farmland driven by precipitation: effects of farmland type

Precipitation is the primary driver of phosphorus loss from farmland. However, the multitemporal and spatial characteristics, as well as the mechanisms of phosphorus loss across different farmland types at the watershed scale, remain poorly understood. This study examines the multiscale impacts of different farmland types on phosphorus loss processes driven by long-term precipitation, focusing on two adjacent small watersheds, the Yulin River and the Dahong River, in the Sichuan Basin. First, based on remote sensing data from 2021 to 2022 (precipitation, temperature, land cover) and river water quality monitoring data (total phosphorus (TP), dissolved oxygen (DO), and permanganate index (COD)), the spatiotemporal variations in river water quality are analyzed. The results indicate that under similar climate conditions and pollution source distributions, the downstream TP concentration in the Dahong River (dominated by paddy fields) is significantly lower than that in the upstream, whereas in the Yulin River (dominated by dryland farming), the downstream TP concentration is higher than that in the upstream. Secondly, the study develops a multisource coupling analysis framework that integrates detrended cross-correlation analysis (DCCA), multifractal detrended cross-correlation analysis (MFDCCA), and remote sensing data to evaluate the impact of different farmland types on phosphorus loss behavior. The DCCA analysis results show that precipitation and TP exhibit a clear long-term correlation, with scaling exponents all exceeding 0.5. In the paddy field control area, precipitation and TP exhibit a positive correlation over long time scales. In contrast, in the dryland control area, a positive correlation is observed only over short time scales (< 60 days), while at longer time scales, the correlation turns negative and shows significant fluctuations. MFDCCA further reveals that the coupling relationship between TP and precipitation generally exhibits multifractal characteristics. The multifractal intensity is higher in the dryland-controlled fields (Δh = 1.47), indicating that TP is more sensitive to precipitation perturbations and less stable. The multifractal intensity is lower in the paddy fields (Δh = 1.20), indicating a more stable coupling relationship. Finally, by combining sliding window analysis with 3D convex hull volume calculations, the study quantitatively assesses the distribution differences of multifractal parameters. The results show that areas dominated by paddy fields exhibited stronger aggregation of multifractal parameters (convex hull volume difference of 0.69). In contrast, dryland areas showed more dispersed patterns (convex hull volume difference of -0.32). This study innovatively integrates fractal theory, remote sensing, and watershed observation data to establish a coupling analysis framework for identifying nonpoint source pollution. It elucidates the multiscale response patterns of phosphorus loss under different farmland types. It provides a new technical approach for quantitatively assessing the effects of farmland type on the spatial distribution and migration pathways of TP.