**2. Study area**

Wenyu River watershed is a key area in Beijing (China), belongs to the water systems of the Beiyun River, which is the most intensive area of human activity in Hai River Basin (Figure 1). Wenyu River, the main stream is 47.5 km, which is originates from the south of Yan Mountain and flows from north to south though Haidian, Changping, Shunyi, Chaoyang and Tongzhou Districts, all of these districts are in the core area of Beijing City. Wenyu River is usually called "the mother river" of Beijing, because of all the main streams in Beijing City, it is the only river which originates in the border and never runs dry.

The total area of Wenyu River watershed is 2,478 km2 and the percentage of mountain and flatland area are 40.4% and 59.6%, respectively. The ground elevation in this area is in the region of 15-1000m. And the study area has the terrain characteristics with the high terrain in the northwest and low plain in the southeast. There are many tributaries in this watershed, with the Dongsha, Beisha, and Nansha Rivers in the upper reaches of Wenyu River, meeting in the Shahe Reservoir, and the Lingou, Qing, Ba and Xiaozhong Rivers flowing into the main stream of Wenyu River. The average annual temperature in this watershed is about 11.6 degree Centigrade (for the year 1959-2000). The predominant soil type is cinnamon (53.5%) of the total area. The average annual precipitation is 624.5mm (for the year 1959-2000), more than 80% of a year's total precipitation is concentrated in the flood season from June to September, the average annual water surface evaporation is 1,175mm, and about 42% of a year's evaporation is concentrated from April to June. The average annual runoff is 450 million cubic meters.

Fig. 1. Map of Wenyu River Watershed in Beijing (China).

As the main drainage canal in the Beijing City, the problems of water pollution and water ecosystems degradation in Wenyu River watershed have come along with the economic development in these years. Several documents estimated the pollution status (Wang and Song, 2008; Shi, 2008; You *et al.,* 2009; Hua *et al.*, 2010) of Wenyu River and pointed out that the water environment of this area was under sub-health; additionally, some other authors put forward the reasonable strategies to restore the ecological environment and improve the water quality in Wenyu River Watershed (Zheng *et al.,* 2007; Wang *et al.,* 2008; You *et al.,* 2009). Although there have many studies noted the water quality problems in Wenyu River Watershed, but the studies linking land use to water quality are limited.
