4.2 The land use of alluvial fan
Alluvial fan is an important land resource in LRB, even in Tibet region. There are mainly 4 types of landforms in LRB: mountains, valleys, river terraces and alluvial fans (Dai et al., 2018; Wei et al., 2012). The river terrace and alluvial fan land are easier utilized than mountain land, because of high altitude and severe ecological environment in LRB. Compared with alluvial fan land, the river terrace land has the priority in the utilization for the slow slope, fertile soil and near position of river. Therefore, the majority of towns and farmlands locate in the river terraces. The alluvial fans are formed by sediment exiting confined valley after the function of flooding (Stock et al., 2008), so that it has naturally slope. And the soil of some alluvial fans are less fertile due to the effect of repeated flooding and depositional processes (Bahrami & Ghahraman, 2019). However, the area of cropland and impervious area of alluvial fans account for 18.98% and 9.18% of their area in LRB (Table1). Moreover, although the ratio between grassland of alluvial fans and grassland of LRB is just 3.97%, the area of grassland of alluvial fans can reach 790.57 km2. Those characteristics reflects a fact that the land for living and producing is indeed shortage in LRB, and alluvial fans have been an important role in local. It is reported that there is a dramatically gap between the land requirement of local human and the area of available land in Lhasa area, especially near the major towns and the Lhasa valley (Chu et al., 2010). However, the available land, especially river terrace land is not unlimited in LRB facing the state that river terraces have been relatively adequate utilized. Therefore, the other type of easily available land like alluvial fans could be an important land resource reserve. The population and economy are quickly increasing in recent years (Wei et al., 2017), enhancing the importance of alluvial fan will be more in the future.