Figure 7 Annual sediment load (brown bars), water discharge (gray-blue
bars), precipitation (blue line), and temperature (red line) in Ming
Yong catchment across the three hydrological years.
Our observations show that there are large interannual variations of
suspended sediment flux among the three hydrological years (2013, 2015,
and 2016). The highest sediment yield occurred in 2015 (2283
t/km2/year)– more than doubled the sediment yield in
2013 (1104 t/km2/year). These interannual variations
seem to be mainly driven by the differences in annual precipitation,
rather than the mean annual temperature (Figure 6). Specifically, the
highest annual precipitation was recorded in 2015, accompanied by a
doubling of the number of extreme precipitation days (defined as daily
precipitation over 20 mm) in Deqin meteorological station. During a
wetter hydrological year, the overall increased rainfall can increase
the sediment transport capacity and evacuate more deposited sediments
from supra/sub-glacial debris and glacier valleys, causing a higher
annual sediment yield (Delaney et al., 2018; Li 2020; Micheletti &
Lane, 2016). Concurrently, more frequent rainstorms enhance stream power
and erosivity, thus increasing river channel erosion that can
(re-)mobilize deposited sediments. (Li et al. 2021; Lugon & Stoffel,
2010; Wulf et al., 2010). These extreme sediment events contribute
disproportionally to the annual sediment yield and amplify the temporal
variability in sediment transport (Lloyd et al., 2016; Wulf et al.,
2012).
Simultaneously, temperature-sensitive glacier dynamics may also affect
interannual variability in sediment yield (Costa et al., 2018; Stott &
Mount, 2007). For example, the rate of glacier bedrock erosion or
subglacial/proglacial sediment transport can be enhanced by the
increased glacier melt flow due to higher temperatures (Herman et al.,
2021; Singh et al., 2020; Chakrapani & Saini, 2009; Stott & Mount,
2007). However, the temperature variations detected within our study
period pales in comparison to precipitation changes. Therefore, in the
absence of extreme melting events that could shadow the impact of
precipitation, we argue that the interannual variations in the sediment
yield in Ming Yong catchment are largely determined mostly by
precipitation
5.2 Comparisons
with other proglacial catchments
The sediment yield for Ming Yong glacial catchment is relatively high as
compared to the those recorded from other glacierized basins on the
Tibetan Plateau (Figure 8). In the Tibetan Plateau, sediment yields from
glacierized basins generally decreases with increasing basin area
(Figure 8a), but increases with larger glacier coverage (Figure 8b), in
line with what is observed at a global scale (Milliman & Farnsworth,
2011). The high sediment yield of Ming Yong glacial catchment can be
explained by its small but heavily glacierized (68%) basin area and the
short distance between the sampling station and glacier snout (Hallet et
al., 1996; Wulf et al., 2012). The observed inverse relationship between
sediment load and basin area might be because larger basins have poorer
sediment delivery ratio – due to increased sediment storage and
weakened sediment connectivity (Walling, 1983; Wohl et al., 2019).
Furthermore, small glacierized basins generate disproportionally high
sediment yield, because a high transport efficacy can be sustained by
short sediment transport distance, steep valley gradients or high stream
power of glacier melt flow (Gurnell et al., 1996; Wulf et al., 2012). In
the Ming Yong glacial catchment, the sampling station is only 3 km
downstream of the glacier snout and the slope ranges from 9 to 71%
(Figure S1a). Apart from its physical setting, the relatively high
precipitation in Ming Yong catchment (over 600 mm/year) also contributes
to its high sediment mobilization and yield (Table 2). For example, the
summer rainfall can flush the supraglacial debris cover and proglacial
sediment storage in large volumes downstream (Riihimaki, 2005;
Srivastava et al., 2014). Increased snowmelt and rainfall during the
onset of the thaw season can also increase water infiltration from
surface to base and enhance the subglacial drainage system, thereby
facilitating the export of subglacial sediment (Alley et al., 1997;
Delaney & Adhikari, 2020).