Objectives Employing models to predict the impacts of water and nitrogen management practices on vegetable yield, soil nitrogen leaching, and water and nitrogen use efficiency represents an effective approach for enhancing vegetable production management. The TOPSIS entropy weight method was utilized to evaluate the prediction accuracy of the coupled HYDRUS-AquaCrop model.

Methods Lettuce field experiments were conducted at the Shanghai Academy of Agricultural Sciences from April to May and from September to October in 2014. The experiment encompassed two irrigation levels: full irrigation at 100% (I1) and 60% (I2), which were combined with four nitrogen (N) application levels: 360 kg/hm² (N1), 300 kg/hm² (N2), 240 kg/hm² (N3), and 180 kg/hm² (N4), resulting in a total of eight treatments. At the 7-leaf, 21-leaf, and maturity stages of lettuce, soil samples were collected from the 0−40 cm layer in 10 cm increments to determine nitrate nitrogen content. Simultaneously, soil leachate was collected to measure leachate volume and nitrate nitrogen concentration. Leaf area was measured every seven days using a Li-3200 leaf area meter, and biomass was recorded at harvest. The data from the April season were used to calibrate the coupled HYDRUS-AquaCrop model, and the data from the September season were used for the model's validation. The TOPSIS entropy weight method was applied to calculate comprehensive scores for each treatment using nitrate nitrogen leaching (X1), economic yield (X2), water use efficiency (X3), and nitrogen use efficiency (X4). The optimal range of water and nitrogen management was proposed to serve as a refined irrigation and fertilization strategy for lettuce production in Shanghai.

Results 1) Moderate increases in irrigation and nitrogen application effectively enhanced lettuce canopy coverage and yield. However, when N application exceeded 300 kg/hm², nitrate nitrogen in deeper soil layers increased by 18.12%−21.12%, and nitrate nitrogen leaching loss increased by 9.25%−39.52%; meanwhile, lettuce canopy coverage and yield decreased by 19.39%−22.50% and 11.72%−12.05%, respectively. 2) The simulated values of soil water content, nitrate nitrogen content, canopy cover, and lettuce yield under different water and nitrogen management practices using the coupled HYDRUS-AquaCrop model had relative errors (Pe) ranging from −50.35% to 81.74%, coefficients of determination (R²) from 0.65 to 0.99, and normalized root mean square errors (NRMSE) from 8.50% to 34.72%. However, under the combined treatment of low irrigation (I2) and high nitrogen application (N1), the model accuracy declined, with NRMSE ranging from 11.51% to 20.13%. Under these conditions, the simulated soil nitrate nitrogen content was 6.71% to 29.82% lower than the measured values, and the simulated lettuce yield was 4.83%−59.08% higher than the measured values. 3) TOPSIS analysis results indicated that applying 220−240 kg/hm² of nitrogen fertilizer under 60%−70% full irrigation (96−112 mm) achieved comprehensive scores of 0.595−0.599. Compared with conventional water-nitrogen management, this strategy reduced nitrogen leaching losses by over 27%, maintained more than 89% of the lettuce yield, and achieved over 159% of the water use efficiency and over 113% of the nitrogen use efficiency.

Conclusions The coupled HYDRUS-AquaCrop model can effectively reveal the water and nitrogen utilization and leaching effects during lettuce growth under non-extreme management practices, providing a rapid prediction method for green and efficient intensive vegetable production.