Effects of rainfall intensities on sediment loss and phosphorus enrichment ratio from typical land use type in Taihu Basin, China.

Soil erosion induced by rainfall has become one of the major environmental disasters. It can lead to degradation of land productivity and water pollution, which limit human survival and development. Therefore, understanding the effects of rainfall intensities on soil nutrient and sediment transportation from farmland via surface runoff is important. This study simulated three types of rainfall intensities (low intensity 0.83 mm min-1; medium intensity 1.17 mm min-1 and 1.67 mm min-1; and high intensity 2.50 mm min-1). Field plots were established to evaluate the effects of rainfall intensities on sediment yield and phosphorus enrichment ratio (ERP) in runoff sediment. Each plot was 2 m long and 1 m wide. The rainfall lasted for 30 min after effective runoff generation. Runoff and sediment, as well as the phosphorus (P) concentration of sediment, were measured. Results showed that the low rainfall intensity had a longer time to begin runoff than high rainfall intensity. The runoff volume increased with the rainfall intensity; however, no obvious differences were observed in the runoff rate under three rainfall intensities, except for 2.50 mm min-1. The curve value of sediment concentration increased after 5-10 min of effective runoff decreased slightly, and then relatively became stable after 20 min of effective runoff. Although no obvious differences were observed in the dynamic changes of sediment concentration and rainfall intensity, a power function existed between rainfall intensity and sediment loss. The similar curves of ERP value showed that it was higher in the initial stage, decreased gradually, and reached a stable level after 25 min of effective runoff, which varied around 1. During the rainfall-runoff event, no significant correlation was noted between ERP and sediment yield. Whether any relationship existed between them needs to be further studied by setting up different field plots under large-scale conditions. The findings of this study provide valuable aid to farmers on how to manage farmland effectively to prevent soil nutrient loss and reduce water eutrophication.