HYDROLOGICAL PROCESSES Hydrol. Process. 22, 1996–2004 (2008) Published online 25 September 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.6784 Modelling the impact of afforestation on average annual streamflow in the Loess Plateau, China Xiao Ping Zhang, 1,2 Lu Zhang, 3 * Tim R. McVicar, 3 Tom G. Van Niel, 4 Ling Tao Li, 3 Rui Li, 1,2 QinKe Yang 1,2 and Liang Wei 1,2 1 Institute of Soil and Water Conservation, NorthwestAgriculture and Forest University, 3 Taicheng Road, Yangling, Shaanxi Province, 712100, China 2 Institute of Soil and Water Conservation, Chinese Academy of Sciences, 26 Xinong Road, Yangling, Shaanxi Province, 712100, China 3 CSIRO Land and Water, Christian Laboratory, Canberra, ACT, 2601, Australia 4 CSIRO Land and Water, Private Bag No. 5, Wembley, 6913, WA, Australia Abstract: To prevent severe soil erosion and to ensure sustainable development, the Chinese Central Government mandated in 1999 that forest cover would be significantly increased in the Loess Plateau, China. It is important to assess the subsequent impact on streamflow and its spatial distribution in the region. The water balance model of Zhang et al. (2001) was used in this study, and the results showed that the model was able to accurately simulate average annual evapotranspiration (ET), but not average annual streamflow when compared with streamflow measurements from 38 hydrologic stations in the coarse sandy hilly catchments (CSHC) in the Loess Plateau. The model was then calibrated using the measured streamflow, and the index of agreement increased from 0Ð21 to 0Ð63, the relative error and root mean square error decreased from 42Ð7 and 17Ð2 to 18Ð0% and 7Ð9 mm, respectively. Hierarchical cluster analysis and stepwise regression were employed to regionalize the optimized model parameter and relate it to the dryness index and forest cover. Once calibrated, the water balance model was used to assess the impacts of two plantation scenarios on streamflow. The two plantation scenarios assume nearly 5Ð8 and 10Ð1% of the study area can be planted with trees. It was predicted that streamflow from the region will decrease by 5Ð5 and 9Ð2% under the two plantation scenarios. The rate of streamflow reduction decreased from southeast to northwest mainly due to decreasing precipitation. Copyright  2007 John Wiley & Sons, Ltd. KEY WORDS regionalization; water balance model; average annual scale; afforestation effect; Loess Plateau Received 15 October 2006; Accepted 2 April 2007 INTRODUCTION The Loess Plateau (0Ð62 ð 10 6 km 2 ) is located in the middle reaches of the Yellow River basin (0Ð75 ð 10 6 km 2 ) in China (Figure 1(a)). Most areas of the Loess Plateau belong to arid and semi-arid climate with average annual precipitation ranging from 200 mm in the north- west to nearly 750 mm in the southeast. The rainy season from June to September accounts for 60–70% of the total precipitation, most of which is in the form of high- intensity rainstorms. The northwestern Loess Plateau is sandy, while in the southeast a series of complex geo- morphic landforms (i.e. plateaux, ridges, mounds and gullies) dominate the area, which is characterized by Loess-paleosol soils with an average depth in excess of 100 m that were deposited during the Quaternary period. Silty clay-loam textures are common with more sandy textures in the northwest and more clay in the southeast (Shi and Shao, 2000). From southeast to northwest, vege- tation is distributed as forest, forest-steppe, typical-steppe and desert-steppe zones (Wu and Yang, 1998). The high-intensity rainstorms, sparse vegetation and long agricultural history have caused severe soil erosion * Correspondence to: Lu Zhang, CSIRO Land and Water, Christian Laboratory, Canberra, ACT, 2601, Australia. E-mail: lu.zhang@csiro.au in this region. The average erosion rate ranges from 5000 to 10 000 tonnes km 2 year 1 , and in some tributaries it is as high as 20 000 to 30 000 tonnes km 2 year 1 (Shi and Shao, 2000; Xu et al., 2004). To control the severe soil erosion, a number of conservation measures have been implemented in the catchments of the Loess Plateau since the 1950s. Among them, afforestation is considered one of the most effective practices to improve environmental quality. An important part of the Chinese Central Government’s environmental policy towards sustainable development in this region is to significantly increase the area of forested land (The State Environmental Protection Administration of China, 2002). It is believed that forests use more water than grass, and hence, reduce average annual streamflow (Oyebande, 1988; Calder, 1999; Zhang et al., 2001). A number of experiments on plots or small catchments in the Loess Plateau agree in general with this finding (Zhu, 1960; Mi, 1982; Hou and Cao, 1990; Wu et al., 1998; Zhao and Wu, 2001; Huang et al., 2003). Liu and Zhong (1978) pointed out that increasing forest area within a catchment can reduce annual streamflow by altering the partitioning of precipitation into evapotranspiration (ET) and streamflow for catchments in the Loess Plateau; McVicar et al. Copyright  2007 John Wiley & Sons, Ltd.