Improving fertigation efficiency by numerical modelling in a lettuce subsurface drip irrigation farm

Management strategies that lead to efficient fertigation are essential to minimize nitrate (N) leakage and environmental issues and to optimize crop yield. A helpful way for agricultural water and nutrient management is studying the water and solute movement by models. The objective of this study is to use the Hydrus-2D model to evaluate different nitrate fertigation strategies for improving fertigation management efficiency. The model was calibrated and validated for 2-years field unpublished data with Iceberg Lettuce (Lactuca sativa L.) for Subsurface Drip Irrigation (SDI) system under wastewater strategies. The validation of the water content model showed that the maximum and average of RMSE are equal to 1.2 and 0.7 cm³/cm³, respectively. Also, the Hydrus model performance was acceptable to simulate soil water content. The results of the soil Nitrate movement model showed the average RMSE is 2.77 mg/l with R² = 0.97. Different scenarios in fertigation intervals, fertigation time, replacement of drip line, nitrate concentration of wastewater, different nitrate concentrations, and times of fertigation through the growing season were defined. The results of these scenarios analyze to find maximum water uptake and minimum deep percolation, which help to achieve a drip irrigation design with minimal losses and the best efficiency. Scenario results showed that reducing fertigation time by 60% and halving the fertigation interval increase the uptake of water and nitrate by 3.89% and 11.55%, while the rate of deep percolation decreased by 27%. The results of dripline replacement scenarios showed that the depth of 12 cm, without moving in the horizontal direction, has created more favorable conditions. The result of increasing nitrate concentration shows that up to 24.2 mg/l nitrate concentration, increment the plant nitrate uptake by 63.25%, however increasing more than 36.6 mg/l of nitrate concentration had no significant effect on plant nitrate uptake. For finding the best time of fertigation through the growing season, 42 scenarios were defined. All applied nitrate injected into 1, 2, 3, 4, 6, and 11 irrigation events equally, form 6 fertigation strategies of F, B, T, Q, E, and C respectively. Based on which irrigation number, fertigation strategies start, indices 1–11 are assigned to these 6 fertigation strategies. Based on the results, the best fertigation strategies in this study, were F5, B4, T3, and Q3, which have minimum nitrate leaching losses of 17.79%, 19.9%, 20.8%, and 20.0%, respectively, while had high root nitrate uptake equal to 25.1%, 26.3%, 26.6%, and 26.6%, respectively. Therefore, fertigation of lettuce is most efficient at 22–30 days after planting, equal to 30–38 days before harvesting for the silty clay loam lettuce farm, under SDI at 12 cm depth conditions.