Non-point-source nitrogen and phosphorus loadings from a small watershed in the Three Gorges Reservoir area

Non-point-source pollution has become a major threat to the water quality of the Three Gorges Reservoir (TGR); however, nutrient loadings from terrestrial sources are unclear due to a lack of in situ monitoring. A representative small watershed in the central part of the TGR area was selected to monitor the loss of nitrogen (N) and phosphorous (P) continuously along with the runoff from 2007 through 2009 to understand the exact sources and loadings. Results show that the non-point-source nitrogen and phosphorus comes mainly from the storm runoff from residential areas, citrus orchards and sloping croplands, which contributes up to 76% of the loadings in this watershed. Thus, a crucial measure for controlling non-point-source pollution is to intercept storm runoff from the three sources. Paddy fields provide a sink for non-point-source N and P by intercepting the runoff and sediment along with their different forms of nitrogen and phosphorus. The N and P removal efficiency by paddy fields from residential areas is within the range of 56% to 98%. Paddy fields are an important land-use option for reducing the non-point-source loading of N and P in the TGR area.