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).