2. Study area
The Zuli River is a first-order tributary of the Yellow River. The total length of this river is 224 kilometers, and the annual runoff is nearly about 1.19×108m3. The Zuli River Catchment occupies l0, 653 km2 of the western Loess Plateau. The Zuli river originates from the Huajialing Mountain area and is composed of the Zu and Li Rivers (Fig. 1). The main tributary of the Zuli River is the Guanchuan River (Fig. 1).
The Zuli River Catchment can be classified as an arid-semi arid climate zone. Temperatures in the whole catchment (Huining weather station in the midstream area; Fig.1) shows highly seasonal variations (Fig.2a). The climate is characterized by cold winters with a mean temperature of -6.3 ℃ and warm summers with a mean temperature of 18.9 ℃. The mean multi-annual precipitation recorded from 2005 to 2019 in the Huining weather station is about 387.1 mm (Fig.2b). Affected by the continental monsoon, the annual precipitation in the Zuli River catchment is mainly dominated by summer rainfall from May to August (Fig.2a). With respect to precipitation, the mean annual potential evaporation is about 1800mm and is nearly five times greater than the mean annual precipitation in the Zuli River catchment (Luo et al., 2010).
The Huajialing Mountain area is the source region of the Zuli River Catchment, which is also the watershed between the Zuli River and the Weihe River. Thick layers Silurian carbonate rocks are deposited in this area (Fig.3a), affected by large-scale tectonic activity, joints and fissures were developed in the strata of headwater. Additionally, karstic features have developed, and fissure water and bedrock fissure water are present in the fracture zone (Liu et al., 2019).
The Zuli River Catchment is a typical hilly area on the Loess Plateau. This river is located in the canyon area, with an altitude ranging from 1396 to 2377m (Fig. 3). The geological strata of the Zuli River Catchment can be divided into three main units. Simplified geology with major structural characteristics of the Zuli River catchment is displayed in Fig. 3. The basement is constituted by Silurian carbonate rocks, Cretaceous clastic rocks and Permian monzonitic granite (Fig. 3a). A Tertiary red clay layer and the Quaternary loess layer are deposited successively in the basement (Fig. 3b). A large amount of soluble minerals exists in the loess, such as halite, gypsum, etc (Tsunekawa, Liu, Yamanaka, & Du, 2014; Fan et al., 2016; Xiao, Zhang, & Jin, 2016).