Raindrop-induced ejection at soil-water interface contributes substantially to nutrient runoff losses from rice paddies

Abstract Nitrogen (N) and phosphorous (P) ejections driven by raindrops promote the solute concentrations in the surface runoff and thereby the runoff losses from croplands. Compared to a considerable number of studies on upland fields, little evidence for the importance of raindrop-induced ejection has been generated for rice paddies primarily due to the lack of field observations and appropriate modeling system. Our extensive rainfall simulation experiments, along with the update of a physical-based solute transport model, shed new light onto the contribution made by raindrop-induced ejection to the total runoff losses from 6 typical rice paddies across China. Raindrop-induced ejection accounted for 22–59 % to 27–63 % of the total runoff losses of N and P across China, respectively, regardless of rice-growing stages. The rainfall intensity and initial concentrations in ponded water explained most (43 % for N and 53 % for P) of the variance of raindrop-induced ejection. N and P runoff losses were further underestimated by 23 % and 29 %, respectively, if ignoring the raindrop-induced ejection at soil-water interface in the model. Progress toward mitigating the runoff losses from rice paddies and associated water pollution will be difficult without reduction in the raindrop-induced ejection particularly in the rice paddies with higher extreme precipitation and more fertilizer application in future.