ESEARCH ARTICLE R ScienceAsia 44 (2018): 403–412 doi: 10.2306/scienceasia1513-1874.2018.44.403 Study on the stable isotopes in surface waters of the Naqu River basin, Tibetan Plateau a,b b b b, b b Guoqiang Dong , Baisha Weng , Tianling Qin , Denghua Yan ∗, Hao Wang , Boya Gong , Wuxia Bib,c, Jianwei Wangb a College of Environmental Science and Engineering, Donghua University, Shanghai 201620 China b State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038 China c College of Hydrology and Water Resources, Hohai University, Nanjing 210098 China ∗Corresponding author, e-mail: [email protected] Received 30 Jan 2018 Accepted 3 Nov 2018 ABSTRACT: To enhance our understanding of the regional hydroclimate in the Central Tibetan Plateau, different types of water samples were collected across the Naqu River basin in the summer (July, August) and winter (January, December) of 2017 for isotopic analysis. With Cuona Lake as the demarcation point, the δ18O values of the river water increased initially and then decreased from upstream to downstream along the river’s mainstream. In the Naqu River system, a general decrease of δ18O values in the trunk stream of the lower reaches (from the head of Cuona Lake) was revealed owing to the gradual dilution of increased isotopically-depleted tributary inflow. Lakes play an important role in regulating runoff and changes in the levels of stable isotopes in rivers or streams. Additionally, the decrease of δ18O is controlled by processes involved in the ‘isotopic altitude effect’. Larger contributions of winter precipitation in surface runoff at higher elevations would produce higher deuterium excess in stream water. On the regional scale, with Cuona Lake as the demarcation point, one of the clearest findings was that the river/stream’s deuterium excess values decreased first and then increased from the south to the north in the summer; but there was a positive linear increase in the winter. From geographical and climatological perspectives, the changes of deuterium excess could result from increasing effects of summertime, and the generation of continental/local recycled and monsoonal water vapor in the surface runoff northward. The study area is at the critical transition between the Indian monsoon system in the South and the Northern belt of the westerlies, as revealed by the intermediate deuterium excess values. KEYWORDS: δ18O and δD, hydrological cycle, water vapor sources, isotope-altitude relationship INTRODUCTION izing hydrological and ecological patterns and pro- cesses 4. Because of fractionation driven by various The Tibetan Plateau (TP) is often called the ‘Third hydrological and ecological processes, the composi- Pole’ or the ’Water Tower of Asia’ and is the source of tion of stable isotopes in environmental waters has many large Asian rivers, such as the Yangtze, Yellow, changed 5. At the forefront of isotopic hydrology 1 Lancang and Nujiang rivers . Hydrological changes research, much emphasis has been placed on spatio- on the TP are critical to the security of freshwater temporal variability of water isotopes in the Indus 6 resources in these rivers. As the only mid-latitude and the Ganges headwaters 7 to determine surface permafrost region, the TP is considered more sensi- runoff generation, moisture sources, and controls of tive to climatic warming than higher latitude Arctic environment and geography on precipitation. Yang 2 regions . et al found close hydrological relationships between At present, the integrated application of iso- rivers, thermokarst lakes and degrading permafrost topes and hydrochemical tracers has become an in the Beiliuhe River Basin of the TP by using stable effective method for studying complex hydrological water isotopes 8. By investigating the temporal and 3 processes of multiple spatio-temporal scales . The spatial variations of δ18O, δD and deuterium excess 18 application of water stable isotopes (δ O and δD) (d-excess) values of river water and their relation- is progressively valuable as natural tracers of the ships with climatic and geographic parameters, and hydrologic cycle in remote regions. Meanwhile, by inverse trajectory analysis of water vapor sources, these stable isotopes are important for character- www.scienceasia.org 404 ScienceAsia 44 (2018) the effects of altitude and climatic parameters on the the elevation of the watershed ranges from 4140 temporal and spatial variations of river water iso- to 5897 m a.s.l. (Fig. 1a). The upper Naqu River 9 tope values were clarified . The main hydrological flows approximately to Amdo City (Fig. 1b), and processes along the river were determined and the the tributaries primarily include the Lariqu, Sangqu, 10 isotope response to flow variation was evaluated . Chengqu, Mugequ, Gongqu and Luoqu. One large In contrast, there were only few studies re- outflow lake, Cuona Lake, is located immediately ported in the Nujiang River headwater region with north of the Naqu River (Fig. 1b). The mean eleva- stable isotopic tracers even though the region is tion of this region is more than 4500 m a.s.l. Since geographically significant in hydrology, regional ge- the Himalayas obstruct water vapor transport from ology and climatology. To date, the systematic the Indian Ocean to the south, and the Karakoram changes of stable isotopes in surface waters and pre- and Pamir mountains obstruct water vapor transport cipitation in the Himalayas and on the TP have been from the Mediterranean and the Atlantic Ocean to specifically s tudied t o d educe t he t ransport range the west, a semi-arid climate is mainly found in this 11 of Indian summer monsoon water vapor flux . region. Data from the Himalayan front showed an obvious The study area experience cold and dry conti- reverse correlation with altitude, and its lapse rate nental alpine climate with a mean annual air tem- 18 (0.29 /100 m for δ O) was nearly equal to the perature of 0.6 °C and annual mean precipitation 12 globalh calibration of 0.28 /100 m . In this re- of 477.8 mm.− The average summer and winter search, a variety of waterh samples (such as those temperatures are 9.7 °C and 10.7 °C, respectively. from river, stream, lake and reservoir waters) were The peak temperatures occur− in July and August, collected in the Nujiang River headstream region and the monthly mean air temperature is above (Naqu River basin) in January, July, August and 0 °C from May–September. Precipitation events December 2017. The results of this seasonal stable that occur from June–September account for 80% isotope hydrology study were used to determine: of the total annual precipitation, and the highest (1) the patterns of isotopic variation in these waters precipitation occurs in July and August. During the and the corresponding dominations, and (2) the hy- freezing season from November to April, the total drological processes upstream of the Nujiang River precipitation is normally less than 5 mm, and the and the regional water vapor sources. Isotopic data river channel usually contains no liquid water for of stream/river waters in this area were collected to sampling. propose a baseline of isotopic systematics with sea- sonal variations of water vapor sources and isotope- altitude correlations of stream/river waters. SAMPLING AND ANALYTICAL METHODS GEOGRAPHY AND CLIMATE A variety of water samples were collected during a synoptic investigation in January, July, August The Nujiang River flows f rom t he c entral TP and December 2017 across the headwater region (Fig. 1a), extending 2000 km and covering a water- of the Nujiang River (Naqu River basin). Three shed area of 137800 km2 in China. The headstreams lake water samples were obtained at Cuona Lake of the Naqu River are located in the hinterland (Fig. 1b). We collected 83 river water samples from of the TP and are situated between the Tanggula the Naqu River and its tributaries (river stream) Mountains to the north and the Nyenchen Tanglha / from Amdo to Nagqu along National Roads 109 and Mountains to the south (Fig. 1a). The Tanggula 317, conducting a roughly north-to-south vertical Mountains, which range from east to west along the section (covering approximately 3 degrees of lati- latitude of 32–33°N, making the study site a tran- tude; Fig. 1b). sitional zone in the precipitation system between All water samples were collected in 50 ml high- the north and south of the TP 13. This river finally density polyethylene bottles and stored at cool tem- derives from the glacier on the southern slope of the peratures until analysis. The δ18O and δD val- Tanggula Mountains (Fig. 1a), which has a range of ues in all water samples were determined by cav- altitude between 5000 and 5800 m above sea level ity ring down laser spectroscopy (CRDS) using an (a.s.l.). L2130-i Picarro water isotope analyzer. For δ18O This study was conducted in the Naqu River and δD measurements, the analytical precision was basin, which is the source of the Nujiang River, 0.025 and 0.1 , respectively. The isotopic located in the hinterland of the TP (Fig. 1a). The values± of water± were reported as per mil ( ) area of the total watershed covers 16967.4 km2, and h h unit relative to the standard Vienna Standard Meanh www.scienceasia.org ScienceAsia 44 (2018) 405 80°E 85°E 90°E 95°E 100°E 90°30'E 91°E 91°30'E 92°E 92°30'E （ ） (b) a 40°N #D 1 # Lariqu2 # 3# 32°30'N .! Sangqu 35°N Xining ! #Amdo 4 # # 32°N .! Nyainrong Tibetan Plateau Yangtze River Yellow River #D 35°N Cuona Lake