Modeling impacts of sediment delivery ratio and land management on adsorbed non-point source nitrogen and phosphorus load in a mountainous basin of the Three Gorges reservoir area, China

Abstract

Agricultural nonpoint source (NPS) pollution at the Three Gorges reservoir area in China has been increasingly recognized as a threat to aquatic environment in recent years due to the serious eutrophication problem. Adsorbed NPS pollution is one of the major forms of NPS pollution in mountainous regions, the essential of the adsorbed NPS pollution is soil loss. Thus, simple, highly sensitive and continuous methods are required to simulate and quantify sediments yield at watershed scales. It is imperative to construct an integrated model to estimate the sediment yield and adsorbed NPS pollution load. According to the characteristics of climate, hydrology, topography, geology, geomorphology and land use types in Three Gorges reservoir area, a GIS-based dynamic-integrated-distributed model of annual adsorbed NPS load was presented in view of impacts of the rainfall intensity, sediment delivery ratio (SDR) and land management, where the temporally dynamic-continuous model of annual sediment yield was established by modifying the revised Universal Soil Loss Equation (RUSLE), and the spatially integrated-distributed model of annual adsorbed NPS load was then developed via the correlation between sediment yield and adsorbed NPS load. Furthermore, a case study of the Jialing River basin in China was applied to validate the integrated model, the dynamic-distributed coupling among GIS technology, sediment yield model, and adsorbed NPS load model was achieved successfully. The simulation results demonstrate the following: (1) runoff and sediment are influenced greatly by rainfall intensity, SDR and vegetation cover; rainfall and land management show high sensitivities to the integrated model; the average annual adsorbed TN and TP pollution loads from 2006 to 2010 decreased by 76 and 74 % compared with the previous treatment (1990), respectively. (2) Spatio-temporal variations of adsorbed NPS nitrogen and phosphorus load are mainly related to different land use types and the background level of nutriments in topsoil; different land use types have different contribution rates; the largest contribution rates of adsorbed total nitrogen (TN, 58.9 %) and total phosphorus (TP, 53 %) loads are both from the dryland cropland. (3) The identification of critical source areas can help to implement the prevention and control measures aiming at the reduction of water environmental pollution. These results will provide useful and valuable information for decision makers and planners to take sustainable land use management and soil conservation measures for the control of sediment pollution in the Three Gorges reservoir area. The application of this model in the catchment shows that the integrated model may be used as a major tool to assess sediment yield risks and adsorbed NPS pollution load at mountainous watersheds.