Development of an ammonia nitrogen transport model from surface soil to runoff via raindrop splashing

The loss of nitrogen with surface runoff not only reduces the utilization efficiency of nitrogen fertilizers in cultivated land but also causes environmental pollution of water bodies. In this study, a mathematical model was established and used to describe the process of ammonia nitrogen transfer from surface soil to runoff via raindrop splashing. In this model, the raindrop-induced water transfer rate (er) was treated as a time-dependent parameter that was not a constant value as was done in previous studies, and the model was evaluated with experiments of three raindrop sizes and three polyacrylamide (PAM) application rates. In addition, the runoff rates and sediment rates in runoff were analysed using the established runoff model based on the Kostiakov infiltration equation and the modified Rose model. The governing equations of the models were solved numerically, and the simulated results of the unit discharge of runoff and the sediment rate and ammonia nitrogen transport process agreed well with the observed data of our experiments. Model parameters of the initial values of er and the depth of the exchange layer (de) were larger in the larger raindrop size treatments but decreased by adding PAM to the soil surface. Therefore, PAM can be used as a soil ameliorant to decrease soil nutrient losses with runoff, thereby increasing land productivity and reducing surface water pollution. Moreover, the sensitivity of the model to various parameters showed that the raindrop splashing process has an important impact on the nitrogen transport from the surface soil to runoff.