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.