River discharge, land use change, and surface water quality in the Xiangjiang River, China Zhao Zhang, 1 * Yi Chen, 1 Pin Wang, 1 Jiabing Shuai, 1 Fulu Tao 2 and Peijun Shi 1 1 State Key Laboratory of Earth Surface Processes and Resource Ecology, Academy of Disaster Reduction and Emergency Management, Beijing Normal University, Beijing 100875, China 2 Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Abstract: To compare the impacts of river discharge on the surface water quality of the Xiangjiang River in China, 12 surface water quality parameters recorded at 31 sampling sites from January 1998 to December 2008 along the river and its main tributaries were analyzed. Significantly higher concentrations of total nitrogen, ammoniacal nitrogen, and total phosphorus, and biochemical oxygen demand were observed during low-flow periods than during high-flow periods, implying a higher risk to local residents drinking untreated water during low-flow periods. Pollution indexes, including the inorganic pollution index and integrated pollution index (IPI), were negatively related to impervious surface area (ISA) and cropland area (CLA) when ISA (CLA) was less than 160 (3000) km 2 . However, the relationship was positive when ISA (CLA) was larger than 160 (3000) km 2 , which provided a reasonable explanation for the observed spatial patterns of water quality. Distinct increasing temporal trends for two kinds of pollution indexes were also found. The annual ISA was significantly related to the rapid degradation of water quality from 1998 to 2008, with correlation coefficient (r) values of 0.816 (p = 0.002) and 0.711 (p = 0.014) for the organic pollution index (OPI) and IPI, respectively. However, annual rainfall was negatively correlated with the two indexes with r values of 0.785 (p = 0.002) and 0.448 (p = 0.093) for OPI and IPI, respectively. Our study highlights that decision makers should be more aware of recent increases in the pollution of the Xiangjiang River, especially at downriver sites and during low-flow periods. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS surface water quality; river discharge; land use change; annual rainfall; Xiangjiang watershed Received 19 July 2012; Accepted 10 June 2013 INTRODUCTION There is a general consensus that the surface water quality of a river changes with changes in discharge due to rainfall or snowmelt. Field data have indicated distinct effects of discharge on water quality variables, including dramatic increases in the concentrations of nitrate and dissolved organic carbon (Zhang et al., 2007), fecal outbreaks (Rose et al., 2000), the flush effect of total suspended solids in urban and forested watersheds (Inamdar et al., 2006; Li et al., 2007; Zhang et al., 2010), and a diluent effect on some pollutants (Cherkauer, 2007). Most studies have related the changes in water quality to rainfall or snowmelt, which drive alterations in pollutant sources and transport through different hydrological processes such as surface flow and subsurface flow, and the mixing dynamics of water bodies. In addition, many studies have related changes in water quality, such as changes in water transparency (Hampton et al., 2008) and oxygen content (Jankowski et al., 2006) and the occurrence of harmful algal blooms (Paerl and Huisman, 2008), to increasing air temperature through field observations or climate change projections. Interna- tional studies on water quality variability have shed some light on the seasonal influence of climate change (such as rainfall or snowmelt, and air temperature) on some hydro- chemical parameters. Results from modeling are able to represent general trends of water quality changes on a long-term scale. However, short-term changes, such as those occurring on a monthly or seasonal scale, do not agree well with observational data (Volk et al., 2009; Marcé et al., 2010). Rivers are systems that carry a significant load of materials from both natural and anthropogenic sources in one direction. The water quality of a river is influenced by many factors, including atmospheric chemistry, the underlying geology, climate change, and anthropogenic activities (Feller, 2007; Hussain et al ., 2008; Prathumratana et al., 2008; Raymond et al., 2008). *Correspondence to: Zhao Zhang, State Key Laboratory of Earth Surface Processes and Resource Ecology, Academy of Disaster Reduction and Emergency Management, Beijing Normal University, Beijing 100875, China. E-mail: zhangzhao@bnu.edu.cn HYDROLOGICAL PROCESSES Hydrol. Process. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9938 Copyright © 2013 John Wiley & Sons, Ltd.