Non-point source pollution modelling using Soil and Water Assessment Tool and its parameter sensitivity analysis in Xin’anjiang catchment, China Xiaoyan Zhai, 1 Yongyong Zhang, 2 * Xuelei Wang, 3 Jun Xia 1 and Tao Liang 2 1 State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China 2 Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China 3 Satellite Environment Center, Ministry of Environmental Protection (SEC, MEP), Beijing 100094, China Abstract: Non-point source pollution is a key issue in integrated river basin management around the world and has resulted in water contamination, aquatic ecology deterioration and eutrophication. Xin’anjiang catchment is the key drinking water source area for Hangzhou City, China. A promising model (Soil and Water Assessment Tool) was applied to assess the non-point source pollution and its effect on drinking water. Sensitivity analysis of model parameters was carried out using the Sequential Uncertainty Domain Parameter Fitting 2 sensitivity technique. Water discharge, sediment, total nitrogen and total phosphorus load processes from 2000 to 2010 were simulated, and the spatial distributions of non-point source pollutants were evaluated at the catchment and administrative country levels. The results show that the hydrological parameters of the Soil and Water Assessment Tool were dominantly sensitive for non-point source pollution simulation, including CN2, RCHRG_DP, ALPHA_BF, SOL_AWC, ESCO and SOL_K and the characteristic parameters of sub-basins (viz. HRU_SLP and SLSUBBSN). Also, water quality parameters (viz. CH_EROD, NPERCO, RSDCO and PPERCO, PHOSKD, etc.) have a significant effect on nutrients. The model performance was very satisfactory, especially for runoff, sediment and total phosphorus simulation. The non-point source pollutant load increased from 2001 to 2010 in the whole catchment. Total nitrogen load increased from 3428 tons (0.59 ton km 2 ) to 7315 tons (1.25 ton km 2 ), and total phosphorus load increased from 299 tons (0.05 ton km 2 ) to 867 tons (0.15 ton km 2 ). The contribution of rice land was the largest, accounting for nearly 95%, followed by tea garden (3.56%), winter wheat (1.37%), forest (0.07%) and grassland (0.02%). Moreover, She County and Xiuning County contributed more than half of the non-point source pollutants. This study was expected to provide a method and reference for non-point source pollution quantification and to support water quality management implementation in China. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS non-point source pollution; SWAT (Soil and Water Assessment Tool); SUFI-2; Xin’anjiang catchment Received 4 June 2012; Accepted 13 December 2012 INTRODUCTION Water pollution restricts global socioeconomic sustainable development, of which non-point source (NPS) pollution is the most uncontrollable issue. Water environment is gradually deteriorating because of nutrient losses (Zhu and Wen, 1994; Zhu et al., 2008), which may threaten the global environment along with climate change and biodiversity attenuation (Giles, 2005). NPS pollution contributed 73% of the chemical oxygen demand, 92% of suspended solids and 83% of bacteria to rivers in the USA and 89% of the nitrogen, which has caused serious eutrophication, in the Gulf of Mexico. About 37.2% of land has suffered soil erosion in China. In 2008, nitrogen and phosphorus losses from animal manure were 1.22 and 1.32 times higher than the fertilizer losses from intensive cultivation (Li, 2011). Furthermore, NPS pollution is the primary threat for water deterioration, resulting in many severe eco-environmental problems that have attracted public attention, such as the algal blooms in Chaohu Lake, Taihu Lake and Dianchi Lake in 2007. Currently, the Chinese government has put forward several related regulations and made heavy investments in NPS pollution prevention, but the achievements so far are few. The pollutant yield and migration mechanism of NPS pollut- ants at the catchment scale are still unclear. This difficulty has restricted the implementation of water pollution control and integrated river basin management in the river basins, especially in China (Xia et al., 2011). The NPS pollution models at various spatial and temporal scales have been increasingly developed and widely used globally. The lumped models included Revised Universal Soil Loss Equation; Chemicals, Runoff and Erosion from Agricultural Management Systems; Groundwater Loading Effects of Agricultural Manage- ment Systems and so on, and the distributed and integrated models included Water Erosion Prediction Project, Areal NPS Watershed Environment Response Simulation (ANSWERS), Annualized Agricultural NPS, *Correspondence to: Yongyong Zhang, Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China E-mail: zhangyy003@gmail.com HYDROLOGICAL PROCESSES Hydrol. Process. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9688 Copyright © 2012 John Wiley & Sons, Ltd.