Distribution of Water and Nitrogen and Prediction Model of Wetting Front Under Vertical Pipe Surface Drip Irrigation of Biogas Slurry in Sandy Soil

This study analyzed the distribution of water and nitrogen in the vertical pipe surface drip irrigation system under the condition of sandy loam soil, and established a prediction model for the wetting front transport distance. A constant-head infiltration test established a critical stable infiltration model, matching dripper flow rate (Q) with pipe diameter (D), pipe burial depth (B), and biogas slurry concentration (C). Indoor soil box experiments with Q, C, B, and D were conducted. Results show that a stable infiltration rate has a power function relationship with C, B, and D (R2 = 0.96). Wetting front transport distances in three directions are proportional to Q and inversely proportional to C, B, and D. The empirical model is reliable (RMSE < 1.18 cm, NSE > 0.95). Soil moisture and total nitrogen accumulate near drippers. Q and C significantly affect wetting front transport distance. Water plays a limited role in nitrogen transport within the soil profile. The model accurately predicts wetting front distance, filling knowledge gaps, and providing theoretical support for system optimization.