2.1 Study area
The study area is Lhasa River basin (LRB) with 32 471km², located in the
south of Tibet Autonomous Region, the southwest of China, lying from
29°20′N–31°15′N 90°05′E–93°20′E (Figure 1). This area is consisted of
Chenguan, Tolung Dechen, Taktse, Medro Gongkar, Lhundup, Dumshung and a
part of Seni, Chali and Chushur county, as shown in Figure 1. The
altitude of study area is varied from 3500m to 7162m (Zhang et al.,
2010). The majority altitude of this area is over 4000m, while the
middle and downstream of Lhasa river valley are less 4000m (Wei et al.,
2012). It belongs to plateau temperate semi-arid monsoon climate region
and the average temperature is about -1.7-9.7℃(Zhang et al., 2010). The
annual precipitation is about 340-600mm and concentrated form June to
September (Wei et al., 2012). The vegetations are characterized by
alpine steppe, alpine meadows and alpine shrub shrubs, cushion
vegetation and so on (Lin et al., 2008). There are seven soil types of
this region: alluvial soil, meadow soil, subalpine steppe soil,
subalpine meadow soil, alpine steppe soil, alpine meadow soil and alpine
frozen soil. Those soil types are corresponded with Fluvaquents,
Haplumbrepts, Calcic cryoboroll, Typic cryaquept, Cryuborolls, Cryaquoll
and Aridisols in the USA taxonomy. The first former types of soil are
arable land soil that can be planted with winter barley (Hordeum
vulgare L ), winter wheat (Triticum aestivum L ), and maize
(Zea mays L ) (Wei et al., 2012).
The terrain of northwest is higher than the southeast, and the landform
is combined with mountains, valleys, river terraces, alluvial fans and
so on (Dai et al., 2018; Wei et al., 2012). Terraces and alluvial fans
are lower than mountains, and most of the arable land is located in
these lower areas (Wei et al., 2012). Besides, the investigation in 2018
(Ma et al., 2018) is showed that: 1) alluvial fan is a common landform
in the valleys of LRB; 2) many villages and farmlands are located in
alluvial fans; 3) the land-use scale of alluvial fan is lower than river
terrace that has been sufficiently used to develop towns and cities.