برآورد فرسایش خاک در حوزه رودخانه زیانشای با استفاده از روش سزیوم ۱۳۷ Estimation of soil erosion in the Xihanshui River Basin by using 137Cs technique

۱ Introduction The 137Cs technique has been used in soil erosion studies since the 1960s. 137Cs is an artificial radionuclide with a half-life of 30.17yr. It was released into the stratosphere and globally distributed following thermonuclear weapon tests in the late 1950s and early 1960s. As a by-product of nuclear fission, 137Cs fell on the earth with rainfall and was strongly fixed by surface soil and sediment. Since the subsequent redistribution in the soil profile is mainly caused by soil detachment and deposition or soil tillage, the migration characteristics of 137Cs can be used as an effective sediment tracer that can provide a basis for estimating soil erosion and deposition rates (Estrany et al., 2010; Philip et al., 1996). In most parts of China, 137Cs deposition mainly occurred in the 1950-1970s. Although the Chernobyl nuclear accident of April 1986 in the Soviet Union caused the first settlement peak of 137Cs in most regions of the globe, it contributed to little fall-out in China which territory is located in East Asia, far away from the site of the accident. Europe and West Asia were very severely affected (Qi et al., 2006). The 137Cs technique has been used in many regions for estimating soil erosion (Li et al., 2003; Kosmas et al., 2001; Zhang et al., 2006; Taijiro et al., 2005; Zhang et al., 2008; Saito-Kokubu et al., 2008; Yang et al., 2006). Research conducted in many worldwide environments has demonstrated that this technique is accurate in estimating soil loss and sediment deposition rates. Moreover, this technique presents many advantages over conventional monitoring techniques (Loughran, 1989), including the potential for deriving retrospective estimates of erosion and deposition rates based on a single site visit, and the potential for assembling distributed information for individual points in the landscape that can be used to study spatial patterns of soil redistribution. Thus, in watersheds situations where there is a lack of sediment data over a long term, the 137Cs technique can be used to quickly and accurately quantify soil erosion and redistribution rates. The focus of this study is to use the 137Cs technique for estimating soil erosion in the Xihanshui River Basin, a tributary to the Upper Yangtze River Basin and one of its major sediment sources. This is of special interest. Sediment production due to soil erosion in the Xihanshui River Basin directly affects the service life-span and normal operations of the Three Gorges Dam, the biggest hydro-power project in the world. The sediment sources are from three representative land uses that include cultivated land, vegetated land, and revegetated land. These are discussed in this study, and general characteristics of typical soil erosion were obtained for the Xihanshui River Basin. ۲ Study area The Xihanshui River Basin is located in the south-eastern part of Gansu Province, China, and covers an area of 1.02×۱۰۴ km2 . The Xihanshui River, an upstream tributary of the Jialing River, has its source in the Qishou Mountains near Tianshui City, Gansu. This river crosses the Longnan regions, that include Lixian, Xihe, Chengxian, and Kangxian, then enters Shaanxi Province and joins the Jialing River at Lueyang (Fig. 1). The area north of the Xihanshui River drains the southwestern edge of the Loess Plateau. The Xihanshui watershed borders the Yellow River Basin in the north. The basin has an annual precipitation ranging from 400 to 800 mm, and rainstorms occur mostly from July to September. Therefore, these rainstorms readily cause serious erosion in the loess region. For example, the basin north of the Daqiao Hydrological Station covers an area of 6,491 km2 . It has an annual sediment yield of 21.2 million tons and an annual sediment transport rate of 3,260 t km-2 yr-1. Therefore, this basin has been one of the largest producing sediment yield areas of China. The sources of soil loss in the Xihanshui River Basin are gully erosion, soil mass movement, debris flow, freeze-thaw erosion, wind erosion, and erosion caused by gold mining. Knowledge of these soil losses is essential for determining sediment budgets which would allow the distribution of sediment in different parts of a river basin (Wang et al., 2007). This study discusses the fundamental aspects of sediment budgeting as applied to the Xihanshui River Basin, using the 137Cs technique.