Coupled Water–Nitrogen Transport and Multivariate Prediction Models for Muddy Water Film Hole Irrigation

Against the backdrop of global water scarcity, utilizing sediment-laden river water for agricultural irrigation is a critical strategy for ensuring food security. However, the associated water and nitrogen transport processes are influenced by the coupled effects of multiple factors, and the governing mechanisms are not yet fully understood. To investigate the coupled effects of muddy water sediment concentration (ρ), physical clay content (d0.01), applied nitrogen concentration (N), and pressure head (H) on infiltration characteristics during film hole irrigation, this study conducted an indoor soil-box experiment using an orthogonal design to analyze soil water and nitrogen transport dynamics. Results indicated that sediment properties were the dominant factors governing infiltration, with their relative influence on cumulative infiltration following the order ρ > d0.01 > H > N. ρ and d0.01 strongly inhibited infiltration; for instance, an increase in ρ from 3% to 9% reduced the initial infiltration rate by as much as 49.3%. Conversely, H and N exhibited a slight promoting effect. High muddy water sediment concentration and physical clay content significantly restricted water and nitrogen transport, causing substantial amounts of ammonium nitrogen (NH4+-N) to be retained within the surface soil layer adjacent to the irrigation hole. Paradoxically, the same factors that reduced infiltration (ρ and d0.01) led to a significant increase in the average change in volumetric water content (Δθ) within the wetted soil volume. Based on these findings, multivariate power function models were developed to predict key parameters. The models demonstrated high predictive accuracy, with coefficients of determination (R2) of 0.9715 for cumulative infiltration, 0.94 for wetting front migration, and 0.9758 for Δθ, and validation errors were within acceptable limits. In conclusion, the film hole irrigation process is predominantly governed by physical clogging from sediment particles, a mechanism that decisively controls the spatial distribution of water and nitrogen. Furthermore, the slight enhancement of infiltration by nitrogen fertilizer suggests a potential physicochemical mechanism, possibly involving ion-induced flocculation of clay particles. The models developed in this study provide a quantitative basis for precision fertigation management in China’s Yellow River irrigation district and other regions with similar conditions.