Spatiotemporal Variations of Suspended Sediment Transport in the Upstream and Midstream of the Yarlung Tsangpo River (The Upper Brahmaputra), China

EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2017) Copyright © 2017 John Wiley & Sons, Ltd. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.4258

Spatiotemporal variations of suspended sediment transport in the upstream and midstream of the Yarlung Tsangpo River (the upper Brahmaputra), China

Xiaonan Shi,1,2,3 Fan Zhang,1,2,3* Xixi Lu,4 Zhaoyin Wang,5 Tongliang Gong,6 Guanxing Wang1,2 and Hongbo Zhang1,2 1 Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China 2 Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China 3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China 4 Department of Geography, National University of Singapore, Arts Link 1, Singapore, 117570 5 State Key Laboratory of Hydro-Science and Engineering, Tsinghua University, Beijing 100084, China 6 The flood control and drought relief headquarters office of Tibet Autonomous Region, Lhasa, Tibet 850000, China

Received 15 June 2016; Revised 5 September 2017; Accepted 6 September 2017 *Correspondence to: Fan Zhang, Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China. E-mail: [email protected]

ABSTRACT: The Yarlung Tsangpo River, which flows from west to east across the southern part of the Tibetan Plateau, is the longest river on the plateau and an important center for human habitation in Tibet. Suspended sediment in the river can be used as an important proxy for evaluating regional soil erosion and ecological and environmental conditions. However, sediment transport in the river is rarely reported due to data scarcity. Results from this study based on a daily dataset of 3 years from four main stream gauging stations confirmed the existence of great spatiotemporal variability in suspended sediment transport in the Yarlung Tsangpo River, under interactions of monsoon climate and topographical variability. Temporally, sediment transport or deposition mainly occurred during the summer months from July to September, accounting for 79% to 93% of annual gross sediment load. This coincided with the rainy season from June to August that accounted for 51% to 80% of annual gross precipitation and the flood period from July to September that accounted for approximately 60% of annual gross discharge. The highest specific sediment yield of 177.6 t/km2/yr occurred in the upper midstream with the highest erosion intensity. The lower midstream was dominated by deposition, trapping approximately 40% of total sediment input from its upstream area. Sediment load transported to the midstream terminus was 10.43 Mt/yr with a basin average specific sediment yield of 54 t/km2/yr. Comparison with other plateau-originated rivers like the upper Yellow River, the upper Yangtze River, the upper Indus River, and the Mekong River indicated that sediment contribution from the studied area was very low. The results provided fundamental information for future studies on soil and water conservation and for the river basin management. Copyright © 2017 John Wiley & Sons, Ltd.

KEYWORDS: suspended sediment transport; soil erosion; Yarlung Tsangpo River; Tibetan plateau

Introduction Suetsugi, 2014). Suspended sediment is important for current erosion rate estimation (Jansson, 2002; Pelletier, 2012; Dean As an important process in hydrologic and ecological systems, et al., 2016), and plays an important role in the transfer of fluvial sediment has been identified as one of the most impor- most of the available nutrients and contaminants (Devesa- tant proxies in studies on soil erosion, channel evolution, hy- Rey et al., 2009). Over recent decades, there has been a per- draulic engineering, and aquatic ecology (Ritchie and sistent focus on the spatiotemporal patterns of sediment loads Schiebe, 2000; Walling, 2009; Hodge et al., 2011; Mueller in the global rivers, such as the Amazon River (Meade et al., and Pitlick, 2013, 2014; Bravard et al., 2014; Park and 1979; Richey et al., 1986), the Danube River (Mladenovic Latrubesse, 2014; Nearing et al., 2017). The sediment in a et al., 2013; Tóth and Bódis, 2015), the Yellow River (Fu, river system is composed of suspended and bed loads. 1989; Xu, 2004; Wang et al., 2007a), as well as cold-region Suspended sediment is predominant and commonly accounts rivers (Demildov et al., 1995; Glasser et al., 2004; Ollesch for approximately 90% of the total yields (Walling and Fang, et al., 2006; Singh et al., 2008; O’Farrell et al., 2009; Favaro 2003; Francke et al., 2008; Zhang et al., 2012; Heng and and Lamoureux, 2015), showing their dependence on factors X. SHI ET AL. such as climate change, geomorphologies, land use, and hu- the riverine sediment transport process in the region because of man activity. The Tibetan Plateau has one of the largest ice data scarcity, although it has been identified as the subject of masses on the Earth, referred to as the Asian ‘ice reservoir’ land surface hydrological processes of alpine region research (Qiu, 2013). It is the headwaters of many major Asian rivers, (Knight and Harrison, 2009). The gross sediment load output supporting billions of people in surrounding regions. These from the Yarlung Tsangpo River was roughly estimated as a rivers are also important carriers for sediment transported to small fraction (<10%) of the total load in the Ganges– the oceans. As estimated, around one-third of the global sed- Brahmaputra River (Wasson, 2003; Blöthe and Korup, 2013). iment loads to the oceans was generated from the large rivers Wang et al. (2015b) measured the depth of deposition layer originated from the Tibetan Plateau and its neighboring re- above bedrock at 29 cross-sections along the mainstream of gions (Milliman and Meade, 1983). Sediment transport re- the Yarlung Tsangpo River and estimated the total volume of gimes in the large rivers originated on the Tibetan plateau, sediment storage in the river valley as 518 billion m3, with de- such as the Yellow, the Yangtze, the Indus, and the Ganges– posits of mainly coarse gravel and sand. The present channel Brahmaputra Rivers, have received increasing attention be- was at an unstable state with continuous sediment deposits cause of the drastic changes (Lu, 2004). For example, the Yel- (Wang et al., 2015b). However, these arguments urgently need low River, once the river with the highest sediment load in the evidence in terms of riverine sediment data. Moreover, the spa- world (Shi et al., 2002), witnessed a continuous decrease of tial patterns and seasonal variation of suspended sediment sediment load since the 1950s by approximately 90% (Fu, transport are largely unknown in the Yarlung Tsangpo River. 1989; Xu, 2004; Wang et al., 2007a) as a result of regional cli- The primary objective of this study was to investigate spatial mate change and human activity in the Loess Plateau (Shi and temporal variations of suspended sediment in the Yarlung et al., 2002; Wang et al., 2010; Yu et al., 2013; Wang et al., Tsangpo River to understand sediment transport characteristics 2015a; Wang et al., 2016). The Yangtze River, ranked as the in terms of climatic and hydrological responses, sediment rat- 5th largest globally in terms of discharge and sediment load, ing curve analysis, and riverbed morphology and topography. also experienced a drastic reduction in sediment load (Yang The study was based on available data of daily discharge and et al., 2011; Dai and Lu, 2014). The upper river basin was re- suspended sediment concentration from four main stream ported to be the main sediment source for the Yangtze River gauging stations during the period 2007–2009. Results of the (Lu and Higgitt, 1999; Lu, 2004; Wang et al., 2007b), while study will aid in the identification of critical areas of erosion significant deposition occurred in the midstream and down- and determine features of sediment transport in the region, a stream (Chen et al., 2001; Yang et al., 2007a, 2007b; Wang foundation for future studies on soil and water conservation et al., 2007b). In the Mekong River, an extremely important in the Yarlung Tsangpo River basin. international river flowing across six Asian countries, the temporal variation of sediment load was not consistent with changes in the water discharge (Wang et al., 2011; Zhai Study Area et al., 2016), which was attributed to soil disturbance and sediment trapping due to dam construction (Wang et al., 2011). The Yarlung Tsangpo River originates from the GyimaYangzoin Wang et al. (2011) identified the largest sediment source area Glacier at an elevation of 5200 m a.s.l. (above sea level) on the above Chiang Saen and the largest sediment sink between northern slope of the Himalayan Mountains (Liu et al., 2014). It Luang Prabang and Nong Khai. The Indus River, as a result flows from west to east across the southern part of the Tibetan of the combined effect of alpine topography, meltwater sup- Plateau with a total length of 2057 km, an average gradient of ply, and the summer monsoon, transported a large volume 2.6‰, a mean elevation of 4621 m a.s.l and a basin area of of sediment (Nag and Phartiyal, 2015). In particular, the up- 240 480 km2 (from 80°120 E to 97°380E and from 27°26 N to stream of the Indus River in northern Pakistan had one of 28°540N) (Figure1) (Guan et al., 1984; Liu et al., 2007; Yao the highest rates of sediment transport reported (Meybeck, et al., 2010). The Yarlung Tsangpo River is the upper part of 1976; Ali and Boer, 2007). The Ganges–Brahmaputra River the Brahmaputra River which joins water from the Meghna was one of the most sediment-laden rivers in the world River before emptying into the Bay of Bengal. The river basin (Milliman and Meade, 1983), with annual sediment load of is located in a long and narrow valley with maximum width 1235 Mt (Abbas and Subramanian, 1984) to 1670 Mt of close to 300 km, bounded by the Himalayan Mountains in (Milliman and Meade, 1983), among which half was depos- the south and Kang Rinpoche (i.e. the Gangdise) and ited within the lower basin and the other half delivered to Nyenchen Tanglha Mountains in the north (Figure 1). There the ocean (Islam et al., 1999). The annual sediment load of are a total of 10 816 glaciers within the river basin, approxi- the Ganga River at Farakka was about 729 Mt, accounting mately 14 493 km2 in area and 1293 km3 in ice volume (Yao for 60% of the total sediment load in the Ganga-Brahmaputra et al., 2010). (Abbas and Subramanian, 1984). Quantification of sediment The study focused on the area in the upstream and midstream load and characterization of spatiotemporal patterns of these of the Yarlung Tsangpo River basin (Figure 1). Along the main rivers have provided important references for river basin man- stream of the Yarlung Tsangpo River, there are four gauging sta- agement and policy development. tions, i.e. the Lhaze gauging station at the terminus of the up- Among the major rivers originated from the Tibetan Plateau, stream, the Nugesha and Yangcun stations in the midstream, the Yarlung Tsangpo River, the upper reach of the Brahmaputra and the Nuxia station at the terminus of the midstream River, is the longest river in Tibet, with the steepest gradient and (Figure 1). Basic information related to each station is provided in the highest elevation in the world (Shi et al., 2011). It flows from Table I. Four major tributaries from upstream to downstream, the west to east across the southern region of the Tibetan Plateau, Duoxung Tsangpo, the Nianchu River, the Lhasa River, and the with its midstream serving as an important center for human Niyang River, are located with a drainage area of over 10 habitation in Tibet. Riverine sediment in the Yarlung Tsangpo 000 km2. Along three of the main tributaries there are gauging River is supposed to be highly variable over time and space stations, i.e. Shigatse station on the Nianchu River, Lhasa station given that such processes are influenced by the interplay of on the Lhasa River, and Baheqiao station on a tributary (Pasang monsoon climate, multiple hydrological components, and Qu) of the Niyang River (Figure 1). The study area was divided complex geomorphologies (Knight and Harrison, 2009; Lu into four stretches (Stretch 1, 2, 3, and 4) by the four gauging sta- et al., 2010). However, there have been few studies addressing tions along the upstream to downstream direction.

Figure 1. Map of the Yarlung Tsangpo River basin and hydrological station locations; four stretches of the river basin (1, 2, 3, and 4) were confined by the four main stream gauging stations, Lhaze, Nugesha, Yangcun, and Nuxia from upstream to downstream; three tributary gauging stations, i.e. Shigatse, Lhasa and Baheqiao stations on the Nianchu, Lhasa, and Niyang rivers, respectively. [Colour figure can be viewed at wileyonlinelibrary. com]

Climatic conditions River hydrology

The Yarlung Tsangpo River is part of the Indian Ocean hydro- The mean annual discharge of the Yarlung Tsangpo River logical system. The climate, affected by the Indian monsoon (Table I) gradually increases from 51.69 × 108, 158.05 × 108, as well as the plateau topography, produces significant spatial 305.77 × 108, to 576.37 × 108 m3 for the four stations from up- variations along the longitudinal direction. There is a gradual stream to downstream, respectively. A daily hydrograph with increasing trend in air temperature from the upstream to down- sediment concentration at the Yangcun station is shown as an stream direction, i.e. from west to east. Precipitation, which is example (Figure 2). The largest, the lowest and mean values mainly supplied by the Indian monsoon, is plentiful in the of daily discharge, and the largest and mean suspended sedi- downstream region but gradually decreases towards the head- ment flux and suspended sediment concentration for the four water region. Average annual precipitation ranges from main stream stations are listed in Table III. The largest river dis- 500 mm in the downstream section to less than 300 mm in charge is mainly driven by the summer precipitation. Under the the upstream section (Gao et al., 2012). The rainy season ar- condition of more frequent rainfall events in the downstream rives earlier and lasts longer in the downstream section. direction, river discharge and sediment concentration showed almost consistent fluctuations with more intensive peaks.

Land cover River morphology The main land-use types of the Yarlung Tsangpo River basin are grassland (74.5%) and barren or sparsely vegetated land Fluvial processes of the Yarlung Tsangpo River were affected by (15.2%) (Table II). Glacier area accounts for only 1.6% of the the lift of the Tibetan Plateau (Wang et al., 2015b). The uneven total area, of which 41.9% is distributed in the sub-basin of uplift of the Tibetan Plateau reshaped the river into its unique the Niyang River, according to ‘The Second Glacier Inventory morphology. The river consists of alternating sections of wide Dataset of China’ (Version 1.0) (Guo et al., 2014). Moreover, valleys and narrow gorges. The wide valleys have braided 80.5% of glacier area is located on the north bank of the river, and anastomosing channels, gentle bed gradients, and thick al- and 19.5% is situated on the south bank. 69.4% of permanent luvial deposits, while narrow gorge sections are single deeply- wetlands are in the upper stretch (Stretch 1) of the river. With incised channels with shallow layers of boulders that cover the midstream of the Yarlung Tsangpo River basin serving as bedrocks with large gradient (Guan et al., 1984; Li et al., an important center for human habitation in Tibet, over 90% 2006; Wang et al., 2015b). The longitudinal section of the main of total croplands, and urban and built-up areas are in Stretch stream of the river is shaped like a stairway, declining from east 2 and Stretch 3, while almost all of the forests with different tree to west. Wang et al. (2015b) showed that the bedrock surface in species are distributed in Stretch 4. gorge sections could be several hundred meters higher than

Table I. Basic information of the mainstream hydrological gauging stations

Station Longitude Latitude Altitude Distance to river Drainage area Annual precipitation Annual discharge mouth (above the gauge station) m a.s.l km km2 mm 108 m3

Lhaze 87°350 29°070 4000 1376 47 832 273.10 51.69 Nugesha 89°450 29°180 3720 1178 106 060 378.23 158.05 Yangcun 91°490 29°160 3500 898 151 507 361.37 305.77 Nuxia 94°390 29°280 2780 530 191 235 479.97 576.37

Note: Annual precipitation and annual discharge are the mean values from 2007 to 2009.

Table II. Land cover in the upstream and midstream of the Yarlung Tsangpo River basin, the values denote area percentage of each land use of the total study area (adapted from the ‘1 km land cover map of China’ provided by the Lanzhou Cold and Arid Regions Science Data Center (http:// westdc.westgis.ac.cn))

Class Stretch 1 Stretch 2 Stretch 3 Stretch 4 Total of the class

Evergreen needle leaf forest 2.56% 2.56% Evergreen broadleaf forest 0.06% 0.06% Deciduous needle leaf forest 0.00% 0.00% Deciduous broadleaf forest 0.01% 0.01% 0.18% 0.20% Mixed forest 0.00% 0.28% 0.29% Closed shrub lands 0.17% 0.27% 0.63% 0.83% 1.90% Open shrub lands 0.00% 0.02% 0.86% 0.88% Woody savannas 0.01% 0.01% Grasslands 23.78% 24.30% 17.42% 9.03% 74.53% Permanent wetlands 0.03% 0.00% 0.01% 0.04% Croplands 0.01% 0.88% 0.56% 0.11% 1.56% Urban and built-up land 0.00% 0.01% 0.04% 0.00% 0.06% Snow and ice 0.36% 0.23% 0.31% 0.71% 1.61% Barren or sparsely vegetated land 2.31% 2.75% 4.54% 5.56% 15.16% Bodies of water 0.24% 0.39% 0.35% 0.15% 1.13% Total of the stretch(es) 26.9% 28.8% 23.9% 20.3% 100% that in the neighboring upstream wide valley sections. These ¼ ∑N ∑M = SL i¼1 j¼1SSC ij qij t N (2) kinds of gorge sections become knick points, before which there is usually a huge amount of sediment storage. where Q [V] and SL [M] are average annual or monthly discharge and sediment load that output at a given gauging station; qij [V/T] and SSCij [M/V] are daily discharge and suspended sediment concentration; t [T] is the time period; M Data and Methods is the number of days in one year or one month while N is This study mainly focused on the fluvial transport of suspended the number of years as 3 years with observation data. The sediment because of the paucity of bed load data. We obtained Nugesha station had limited sediment data from May to Octo- three years of data from the four national gauging stations lo- ber. Given that SL proportions for these months were dominant cated on the main stem of the river during the period 2007– over 95%, the annual gross at the Nugasha station was esti- 2009. The dataset for each station includes its coordinates (lat- mated by dividing an average ratio of the other three stations. As for each stretch of the river basin, runoff depth (R, [L]) and itude and longitude), drainage area, precipitation, flow velocity, 2 discharge, suspended sediment concentration, morphology of specific sediment yield (SSY, [M/L ]) were used as comparable cross-section, and composition of riverbed particles. In which, variables by dividing the relative stretch area, calculated by the precipitation, discharge, and suspended sediment concentration were at daily scale from 1 January 2007 to 31 December ¼ Qiþ1 Qi R (3) 2009, each with 1096 effective data points, except the Nugesha Aiþ1 Ai station, with only 552 effective sediment data points from every 1 May to 31 October. In addition, discharge and suspended SL þ SL sediment concentration data from the three main tributaries SSY ¼ i 1 i (4) (Figure 1) during the same period were available for developing Aiþ1 Ai a sediment budget. The discharge was measured using a velocity meter method following the national standard GB 50179, where R [L] and SSY [M/L2] are the runoff depth and specific while suspended sediment concentration was measured using sediment yield of a stretch between two gauging stations in a a sampling method with a strip sampler at 60% of water depth given time period, A [L2] is the drainage area above a given following the national standard GB 50159. gauging station, and i [] is the number of the gauging station. Annual and monthly discharge and sediment load were cal- The annual discharge at the lower Nuxia station in 2007, culated from daily observational data using the following 2008 and 2009 was about 98.9%, 122.5%, and 75.7% of the equations: historical mean from 1961 to 1999 (Zhang et al., 2013), respectively. And the average annual discharge during 2007–2009 ¼ ∑N ∑M = Q i¼1 j¼1qij t N (1) was about 99% of the historical mean. Therefore, the average

Figure 2. The hydrograph with sediment concentrations at Yangcun from 1 January 2007 to 31 December 2009. [Colour figure can be viewed at wileyonlinelibrary.com] Copyright © 2017 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2017) SPATIOTEMPORAL VARIATIONS OF SEDIMENT IN YARLUNG TSANGPO RIVER

Table III. Information on daily discharge and sediment for the four main stream stations of the Yarlung Tsangpo River

Station Largest Lowest Mean Largest daily Mean daily Largest daily Mean daily daily daily daily suspended sediment suspended sediment suspended sediment suspended sediment discharge discharge discharge flux flux concentration concentration m3/s m3/s m3/s kg/s kg/s kg/m3 kg/m3

Lhaze 838 40 164 1260 60 2.1 0.14 Nugesha 3100 121 502 8500 746 4.1 0.48 Yangcun 5910 246 970 12400 551 2.3 0.22 Nuxia 8940 409 1829 6060 331 1.0 0.10 values from the three years of measurements are representative equations was the highest for Lhaze, followed by Nugesha and used to analyze suspended sediment characteristics in the and Yangcun, and the lowest for Nuxia, which showed a de- Yarlung Tsangpo River. Moreover, the monthly mean precipita- creasing tendency from the upper station to the lower station. tion, discharge, and sediment concentration at the middle The SSC-Q hysteresis for the upper three stations consistently Nugesha station during the three years were compared with followed clockwise hysteretic loops, whereas SSCs were sys- the historical mean from 1961 to 1999. Their seasonal distribu- tematically higher for the same level of discharge during the tions are consistent while the relative errors in term of propor- water rising stage than the water falling stage. For the lower tion of the highest three months accounting for annual gross Nuxia station, sediment hysteresis followed a pattern of a single values are 12.9%, 0.5%, and 1.4% for precipitation, dis- curve, whereas the rising stage and falling stage overlapped, charge, and sediment concentration, respectively. This indi- which illustrated a high correlation between discharge and sed- cates that the seasonal patterns in the three years are iment concentration. representative. Sediment rating curves were used to describe discharge and SSC relationships for different locations and temporal scales. A Seasonal variation of discharge and sediment loads power function, a commonly used regression form (Leopold and Maddock, 1953; Walling, 1974 and 1977), was used to Figure 4 shows average monthly precipitation, discharge, and fit the discharge (Q, [V/T]) and SSC [M/V] relationships: sediment load from the four main stream stations in the upstream and midstream of the Yarlung Tsangpo River, and Figure 5 shows average monthly runoff depth and specific sed- ¼ b SSC aQ (5) iment yields (SSY) within the four stretches between neighboring stations. Runoff depth and SSY were calculated by where a and b are regression coefficients. Coefficient a gener- dividing the corresponding stretch area in order to remove the ally represented soil erodibility and sediment source availabil- drainage area effect. ity while coefficient b was used as an index reflecting At the upper Lhaze station, summer rainfall accounted for transport capacity of water flow (Walling, 1974). approximately 90% of annual precipitation. This percentage A sediment budget was developed to show the changing pat- was approximately 80% for the Nugesha and Yangcun stations. tern of SL along the upstream and midstream of the Yarlung The Nuxia station had a more uniform precipitation distribution Tsangpo River with contributions from the four main tributaries throughout the year, for which 51% occurred in summer and (Figure 1). Among the four tributaries, SL were calculated from 43% occurred in April, May, September, and October. The the daily data of Q and SSC by Equation (2) for the Nianchu rainy season arrived earlier and lasted longer in the down- River (at the Shigatse station) and the Lhasa River (at the Lhasa stream section in comparison with the upper part. Precipitation stations), respectively. Owing to the lack of observational data, peaks occurred in July when the monsoon was strongest. The SL for the Niyang River was estimated approximately using its discharges were concentrated in July, August, and September, drainage area multiplied by SSY of a smaller tributary (Pasang accounting for about 60% of the annual water flow for all the Qu) calculated from the daily data Q and SSC using Equa- four stations. Peak flood occurred in August. From December tions (2) and (4) at the Baheqiao tributary station (Figure 1), to the following April, discharges were mostly at the base flow whereas SL for the Duoxung Tsangpo River was estimated ap- level. proximately using its drainage area multiplied by the average The high values of sediment loads were also concentrated in SSY of Stretch 1. July, August, and September, accounting for greater than 90% of annual gross sediment loads in the upper three stations and approximately 80% in the lower Nuxia station. Peak sediment Results loads occurred in August. This indicated that serious soil erosion mainly occurred during these three months. Stretch 4 ex- Sediment rating curves and hysteresis hibited negative SSY during the period from July to September with absolute values higher than the other three stretches. This The details of sediment rating curves assessment and refine- suggested that the area between Yangcun and Nuxia experi- ment are beyond the scope of this study. Here we briefly report enced high deposition that mainly occurred in July, August, the sediment rating curves developed based on daily data and and September. sediment hysteresis of monthly mean data for the four main stream stations (Figure 3). A power function was used for fitting the sediment rating Spatial variations of discharge and sediment loads curves for the four main stream stations (Figure 3). The curves are all well fitted with R2 higher than 0.46. The a constants of Spatially, the sediment load generated in each stretch as well as regression equations were higher at Nugesha and Yangcun than discharge was compared in Table IV. Annual discharge from at Lhaze and Nuxia stations. The exponent b of regression each stretch showed an increasing trend from upstream to

Figure 3. Relationships between daily (left) and monthly mean (right) SSCs and discharge from 2007 to 2009 for the four main stream stations investigated. For the daily data (left), hollow circles denote rising limb while solid circles denote falling limb. [Colour figure can be viewed at wileyonlinelibrary.com] downstream direction, with a relative ratio of 1:2.1:2.9:5.2. De- significant amount of suspended sediment into the Yarlung spite the effect of drainage area, annual runoff depth also Tsangpo River, while Stretch 1 contributed comparatively less showed a significant increasing trend with a relative ratio of sediment, and Stretch 4 had a large amount deposition. 1:1.7:3.0:6.3. Results indicated that lower stretches in the A sediment budget was developed for the entire upstream Yarlung Tsangpo River basin received a larger amount of water and midstream of the Yarlung Tsangpo River (Figure 6). Given supply from precipitation and meltwater from snow and the limited gauging stations available and short-term observa- glaciers. tions, the budget is only a schematic representation of mean Stretch 2 between Lhaze and Nugesha contributed the annual SL, which showed a general pattern of SL contribution highest amount of sediment load, followed by Stretch 3, while from each stretch and tributary. In order to avoid inconsistent Stretch 1 contributed relatively less suspended sediment to tendencies in sediment budget between wet or dry years as re- downstream. The SL in Stretch 4 was negative, which indicated ported by Chalov et al. (2015), the sediment budgets were sep- that the area between the Yangcun and Nuxia stations was arately checked in 2007 (normal year), 2008 (wet year), and dominated by deposition. The SSY indicated a very high aver- 2009 (dry year) and the result showed a consistent tendency aged deposition rate in Stretch 4 (174.3 t/km2/yr). The absolute in spatial pattern. The SL in the river significantly increased value was nearly the same as the highest SSY value in Stretch 2. from 1.89 Mt at the upper Lhaze station to 17.35 Mt at the It was thus concluded that Stretch 2 and Stretch 3 supplied a mid-Yangcun station but decreased to 10.43 Mt at the lower

Figure 4. Mean monthly precipitation, discharge, and sediment loads (SL) from 2007 to 2009 for the four main stream stations in the upstream and midstream of the Yarlung Tsangpo River. [Colour figure can be viewed at wileyonlinelibrary.com]

Nuxia station. It reached a maximum at the Yangcun station, Discussion and thus Stretch 2 and Stretch 3 were the main sources contrib- uting a large amount of the sediment to the Yarlung Tsangpo Environmental controls on sediment loads River. According to a previous study by Wen et al. (2002), grav- ity erosion including landslide and debris flow accounted for The spatial distribution of sediment loads and sediment hyster- 83.7% and 80.9% of the sediment source, and sheet erosion esis patterns were mainly attributed to climate, hydrological re- over low coverage grassland accounted for 16.3% and 19.1% sponse to regional riverbed morphology, and sediment supply of the sediment source in Stretch 2 and Stretch 3, respectively. from major tributaries. In Stretch 4, the SL showed a great decrease (6.92 Mt) from the The a constants of regression equations at Nugesha and Yangcun to the Nuxia station, although it received the highest Yangcun were relatively higher than those at Lhaze and Nuxia, precipitation and had the highest runoff depth. The decrease while the exponents b decreased from upper station to lower in the SL illustrated that the area between Yangcun and Nuxia station. The constant a represents soil erodibility or sediment was mainly dominated by deposition, trapping about 40% of source availability and a higher a-value means higher soil erod- the SL inputs from Yangcun. Among the four tributaries, the ibility or more sediment source available (Walling, 1974; Nianchu River contributed the largest amount of sediment to Asselman, 2000). The exponent b was used as an index the Yarlung Tsangpo River, although it has the smallest drainage reflecting sediment transport capacity of water flow and a area, followed by Niyang River, Lhasa River, and Dongxung higher b-value could mean higher sediment transport capacity Tsangpo River. Sediment loads from these three tributaries were (Walling, 1974; Asselman, 2000). Therefore, the relatively almost the same. Lhasa River contributed less than 20% of the higher a-values at Nugesha and Yangcun could indicate that SL in Stretch 3, although it supplied over 70% of regional flow Stretch 2 and Stretch 3 contribute more sediment supply to discharges. the main river, relative to the other two stretches. This partly

Figure 5. Mean monthly runoff depth and specific sediment yields (SSY) from 2007 to 2009 within the four Stretches between the neighboring stations. [Colour figure can be viewed at wileyonlinelibrary.com]

Table IV. Annual mean discharge and sediment for the four stretches of the Yarlung Tsangpo River

Items Stretch 1 Stretch 2 Stretch 3 Stretch 4 Above Lhaze Lhaze-Nugesha Nugesha-Yangcun Yangcun-Nuxia

Drainage area (km2) 47832 58228 45447 39728 Discharge (108 m3) 51.7 106.4 147.7 270.6 Runoff depth (mm) 108.1 182.7 325 681.1 SL (Mt/yr) 1.89 10.34 5.12 6.93 SSY (t/km2/yr) 39.4 177.6 112.8 174.3 explained the dominant sediment contribution from the two (Fondriest Environmental, Inc, 2014). Sediment composition stretches, although precipitation and runoff depth in both at the surface layer of the riverbed was provided at each station stretches were not the largest. The reducing b-values could in- (Table V). The lowest Nuxia station had fine sand and sandy dicate that sediment transport capacity gradually decrease from pebble, which was different from the upper three stations dom- upstream station to downstream station. Because of discharge inated by coarse sand and pebble. Finer bed sediment sug- increasing from upstream to downstream, the decreased trans- gested that suspended sediments were finer when they arrived port capacity was mainly due to fluvial morphology and flow at the lowest Nuxia station (Southard, 2006), indicating that velocity. The information of river morphology such as longitu- coarse sediments had experienced intensive deposition as river dinal profiles, width of cross-sections and flow velocity at the water surged through Stretch 4. four stations were collected (Table V). From upstream to down- The river channel in Stretch 4 consisted of an alternating stream, the stream gradient from ~30 km above each station to fluvial landscape with three wide valleys and two gorges (Wang each station, generally presented a decreasing trend. The width et al., 2015b). Deposition occurred in wide valley areas where of cross-sections at the four stations significantly increased. As flow velocity slowed down. The largest modern deposition oc- affected by both slope gradient and width of cross-section, curred in the lowest wide valley near the confluence of the mean flow velocities of 1.90, 1.87, 1.52, and 1.35 m/s at the Niyang River into the Yarlung Tsangpo River, about 30 km above four stations in the summer months from July to September Nuxia station (Wang et al., 2015b). Although the river segment showed a declining trend in the downstream direction. Both in Stretch 4 with the longest river line had the highest modern the low gradient and the low velocity yielded decreasing sedi- deposition, the total historical deposition in the stretch was less ment transport capacity downstream. than those in Stretch 2 and Stretch 3 (Wang et al., 2015b). An im- The patterns of sediment hysteresis showed clockwise hyster- portant reason is that Stretch 4 lies within a large forest and etic loops for the upper three stations, but a single curve for the grassland area that is well protected from regional soil erosion. lowest Nuxia station. The main feature of the clockwise hyster- Although Stretch 4 had the largest precipitation and runoff depth etic loops was that SSC were systematically higher during the values (Table IV), the SSC and erosion intensity were lower. water rising stage compared the water falling stage for the same Stretch 2 and Stretch 3 with similar climate conditions and level of discharge, indicating that the sediment wave preceded land use types are the most important areas of human habita- the water wave to give higher concentrations on the rising than tion in Tibet, with relative flat topography. There were no signif- the falling limb. For the lower Nuxia station, sediment hystere- icant differences in riverbed morphology between both sis followed a single curve pattern, whereby the rising stage and stretches; each comprised of a shorter gorge and a longer wide the falling stage overlapped. This pattern indicated high corre- valley with braided and anastomosis channels (Wang et al., lation between discharge and sediment concentration. A linear 2015b). Stretch 3 had higher precipitation and runoff depth rel- pattern was reported as a reflection of a lower entrainment ative to Stretch 2, but only half the SL of Stretch 2 with a lower threshold (Martin et al., 2014), which was supported by lower SSY (by a factor of 0.6). Average SSC at Yangcun was only one- flow velocity and lower transport capacity at the station. third of SSC at Nugesha. Landscape patterns of the Nianchu As a result, Stretch 4 had high potential for sediment deposi- River in Stretch 2 and the Lhasa River in Stretch 3 were signifi- tion. Deposition is generally dominated by coarser particles cantly different. It was reported that the tributary Nianchu River

Figure 6. Schematic map of mean annual sediment load (SL) spatial distributions. Black circles are data points, including the known SLs at the four main stream stations and estimated SL of four main tributaries. Among these tributaries, SL from Nianchu River and Lhasa River were obtained based on synchronous observational data, SL from the Niyang River was estimated approximately using observed SSY from its tributary (Pasang Qu) multiplied by its drainage area, whereas SL from Duoxung Tsangpo was estimated approximately using the average SSY of Stretch 1 multiplied by the drainage area due to the lack of observational data. Solid lines denote values determined from direct observation while dashed lines denote indirect approximate estimation.

Table V. Basic information on morphology, flow velocity, and sediment composition at the four gauging stations

Stations Width of Width of Mean velocity Max depth of Stream gradient, Sediment composition cross-section valley during July and sediment 30 km above at the surface (m) (m) September deposits each station layer (m/s) (m)

Lhaze 98.5 - 1.90 - 0.66% coarse sand and pebble Nugesha 229 1040 1.87 452 0.11% coarse sand and pebble Yangcun 471 2320 1.52 431 0.21% coarse sand and pebble Nuxia 527 3291 1.35 408 0.17% fine sand and sandy pebble located at the south bank of the Yarlung Tsangpo River was the Comparison with other rivers on the Tibetan most critical area in terms of soil erosion and water loss in the plateau midstream (Qian, 2002). The Shigatse gauging station at the junction of the Nianchu River (Figure 1) had SSY similar to We conducted a preliminary evaluation of riverine sediment the stretch average SSY, but only half of the stretch average run- conditions between the upper and lower parts of some large off depth. As a result, the Nianchu River contributed much Asian rivers originated from the Tibetan Plateau (Table VI). more sediment to the main stream of the Yarlung Tsangpo River Gross SL output from the upstream and midstream of the relative to the average sediment production of the stretch. In Yarlung Tsangpo River was 10.43 Mt. When normalized by contrast, water flow from the Lhasa River, located at the north the drainage area, the average SSY for the whole study area bank of the Yarlung Tsangpo River, was very clean and had was 54 t/km2/yr, which was rather low compared with lower SSC. The Lhasa River contributed more than 70% of flow 1126 t/km2/yr in the whole Brahmaputra River basin. The study discharge, but less than 20% of SL in Stretch 3, according to area accounted for 30% area of the whole basin of the data of the Lhasa gauging station near the junction of the Lhasa Brahmaputra River. However, it contributed only 1.5% sedi- River (Figure 1). Therefore, it can be concluded that although ment load and 9.5% water discharge to the Brahmaputra River Stretch 2 and Stretch 3 were generally similar in erosional con- (at Bahadurabad station). An additional comparison was conditions, the difference in contributions from turbid Nianchu ducted among the headwater basins of five major rivers in the River in Stretch 2 and clear Lhasa River in Stretch 3 was the Tibetan Plateau (Figure 7 and Table VI). It was found that the main cause for the differences in SSC and SSY between these Yarlung Tsangpo River has the higher runoff depth but the lowest two stretches. SSY value. The contribution from the Yarlung Tsangpo River in

Table VI. Suspended sediment loads comparison of the headwater basins of major rivers in the Tibetan Plateau

River Gauging Drainage area (km2) Water discharge Sediment load Data series station length Area percentage* Discharge percentage Runoff SY percentage SSY /104 km2 /108m3 depth /mm (Mt/yr) (t/km2/yr)

Yurlung Tsangpo Nuxia 19.1 29.9% 576 9.5% 301 10.43 1.4% 54 2007–2009 Upper Indus River Besham 21.2 23.3% 790 37.4% 372 176 40.7% 830 1969–1998 Upper Yellow River Tangnaihai 12.2 16.2% 200 66.4% 164 13 1.7% 103 1956–2005 Upper Yangtze River Zhimenda 13.8 8.1% 122 1.4% 89 9 2.2% 68 1957–1999 Upper Mekong River Changdu 5.4 38.2% 152 26.7% 280 19 12.7% 210 1959–1979

*Percentage denotes the percentage of upper part to the whole river basins. The data sources are from Islam et al. (1999) for the Brahmaputra River, from Ali and Boer (2008) and Meybeck (1976) for the Indus River, from He (1995) and Liu et al. (2013) for the Mekong River, and from the gauging stations for the Yellow River and the Yangtze River.

Figure 7. Topography, rivers (blue lines), boundaries of five investigated basins (black lines) and location of the gauging stations. UYE, UYA, UM, UB and UI represent the upper parts of the Yellow, Yangtze, Mekong, Brahmaputra, and Indus rivers, respectively. [Colour figure can be viewed at wileyonlinelibrary.com]

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