Coupling model for moisture and nitrogen transport on unsaturated slope and its application

The major methods to describe the nitrogen loss caused by slope runoff were based on the analytical method, which failed to describe the difference in moisture and nitrogen transport processes at different points of the slope. Moreover, the previous researchers kept the focus on the nitrogen loss by the slope runoff and focused less on the nitrogen along with the infiltration. To overcome the disadvantages, a slope scale numerical model was developed to describe the slope rainfall-runoff-infiltration processes and the nitrogen transport processes in this study. For the water movement module, the one-dimensional kinematic wave equation described the slope runoff process, while the Green-Ampt equation described the infiltration process. The Preissmann weighted implicit four-point scheme solved the water movement governing equations numerically. For the solute transport module, the study region was divided into a series of sub-regions and the mass balance equations developed for each sub-region based on the mixing layer theory. Both the mass balance equations for NH+ 4-N and NO- 3-N was formed and the nitration process, denitrification process, loss caused by slope runoff and infiltration was considered for NH+ 4-N, while only the nitration process, loss caused by slope runoff and infiltration was considered for NO- 3-N. Two published experiments were used to evaluate the performance of the developed model. The results demonstrated that the developed model had a satisfactory calculation accuracy and kept an excellent mass balance budget. Moreover, two indoor soil tank modeling experiments with the unsaturated initial conditions were carried out. The two indoor soil tanks had different slope angles, one was 8.85° and the other was 7.55°. Both the experimental results and the numerical calculation results analyzed the water movement and the nitrogen transport processes. The RMSE value and ARE value of the slope runoff for the first soil tank were 5.3×10-7 m2/s and 11.6%, while the RMSE value and ARE value of the slope runoff for the second soil tank were 8.6×10-7 m2/s and 15.0%. The mass balance error of the water balance module for the first soil tank and the second soil tank were 1.87×10-2 % and 7.22×10-3 %, respectively. The results indicated that the satisfactory performance of the water movement module. The RMSE value and ARE value of the concentration of NH+ 4-N and NO- 3-N in runoff for the first soil tank were 0.016 mg/L and 5.5%, and 1.41 mg/L and 31.9%, while the RMSE value and ARE value of the concentration of NH+ 4-N and NO- 3-N in runoff for the second soil tank were 0.010 mg/L and 3.5%, and 0.23 mg/L and 10.6%. The mass balance error of the NH+ 4-N and NO- 3-N for the first soil tank were 4.99×10-4 % and 4.53×10-4%, while the mass balance error of the NH+ 4-N and NO- 3-N for the second soil tank were 4.55×10-4 % and 1.25×10-3%. The results indicated that the nitrogen transport module also had satisfactory performance. Besides, the unsaturated hydraulic conductivity had a great influence on the infiltration quantity. Moreover, the NH+ 4-N had a strong adsorption capacity, which led to an increase of the concentration of NH+ 4-N in the mixing layer, while the NO- 3-N had a weak adsorption capacity and the concentration of NO- 3-N in the mixing layer decreased obviously under the effect of runoff and infiltration processes.