water Article Stable Isotope Composition in Surface Water in the Upper Yellow River in Northwest China Mengyu Shi 1 , Shengjie Wang 1,2,* , Athanassios A. Argiriou 3 , Mingjun Zhang 1, Rong Guo 1, Rong Jiao 1, Jingjing Kong 1, Yaning Zhang 1, Xue Qiu 1 and Su’e Zhou 1 1 College of Geography and Environmental Science, Northwest Normal University, Lanzhou 730070, China; [email protected] (M.S.); [email protected] (M.Z.); [email protected] (R.G.); [email protected] (R.J.); [email protected] (J.K.); [email protected] (Y.Z.); [email protected] (X.Q.); [email protected] (S.Z.) 2 Key Laboratory for Ecology and Environment of River Wetlands of the Shaanxi Province, Weinan 714099, China 3 Laboratory of Atmospheric Physics, Physics Department, University of Patras, GR-265 00 Patras, Greece; [email protected] * Correspondence: [email protected] Received: 14 March 2019; Accepted: 6 May 2019; Published: 8 May 2019 Abstract: Although stable isotopes of hydrogen and oxygen in surface waters (especially in river waters) are useful tools to understand regional hydrological processes, relevant information at some upper reaches of large rivers in western China is still limited. During 2016–2017, we focused on the Liujiaxia Reservoir along the upper Yellow River, where we collected surface water samples at two locations, above and below the dam (identified as “lake water” and “river water”). The results show that the heavy isotopes in lake and river waters are enriched during the warm months, when the river discharge is large, and depleted during the cold months. The slopes of the water line (δ2H versus δ18O) for both the lake and river waters were lower than that of the global mean, due to evaporation. The different d values of the lake and river water reflect the regional evaporation and water sources. Keywords: stable isotope; surface water; river water; Yellow River 1. Introduction Stable isotopes of hydrogen and oxygen are important in understanding hydrological processes [1–5]. Surface runoff is a vital component of the terrestrial water cycle, and the interactions between surface water and other hydrological components (especially precipitation and groundwater) can be investigated using stable isotopes of water [6–10]. For example, simultaneous in-situ monitoring of the isotopic composition of surface water provides a basis to investigate the regional water budget [11–14], water interaction [15–17], and paleoclimate reconstruction [18,19]. In most cases, the stable isotope ratios of surface water reflect a seasonal variation, which is associated with the isotope signature of water from which it is derived (e.g., stream water, precipitation, and groundwater) and the climate [20–24]. According to the isotopic assessment of nationwide river water in the United States (U.S.), heavy isotopes of river water are more depleted in southeastern to northwestern (except for the East Coast of the U.S.) United States, due to evaporation and water vapor sources [25]. Generally, water isotopes experience kinetic fractionation depending on the climate conditions. Dansgaard [26] demonstrated that temperature and precipitation amount are important meteorological factors controlling stable isotope composition in precipitation, which is commonly known as temperature effect and precipitation effect, respectively. Light isotopes evaporate preferentially to heavy isotopes, so heavy isotopes of evaporated (condensed) water are more depleted Water 2019, 11, 967; doi:10.3390/w11050967 www.mdpi.com/journal/water WaterWater 20192019,, 1111,, 967x FOR PEER REVIEW 22 of of 10 depleted (enriched) than those of residual water. Hence, in some open water bodies (especially lakes), (enriched)the heavy isotopes than those were of residual more enriched, water. Hence, relative in to some precipitation, open water due bodies to stro (especiallyng evaporation lakes), [23,27,28]. the heavy isotopesThe wereYellow more River enriched, (Huanghe relative River), to precipitation,the second longest due to river strong in evaporationChina, originates [23,27 from,28]. the high- elevationThe YellowQinghai–Tibet River (Huanghe Plateau and River), flows theto the second Bohai longest Sea in eastern river in China. China, The originates isotopic signature from the high-elevationof river water at Qinghai–Tibet many sites along Plateau the and Yellow flows River to the has Bohai been Seaanalyzed in eastern and China.has provided The isotopic useful signatureinformation of riverto understand water at many the regional sites along water the Yellowcycle [29–33]. River has According been analyzed to these and studies, has provided in this usefuldrainage information basin, with to understanddifferent climate the regional conditions water and cycle hydrological [29–33]. According regimes, to the these isotope studies, values in this of drainagewater are basin, spatially with dependent different climate [29]. The conditions downstream and hydrological reaches are regimes, greatly theaffected isotope by values the East of water Asia aremonsoon. spatially This dependent may be [reflected29]. The by downstream the seasonal reaches variations are greatly of stable affected isotope by thecomposition. East Asia monsoon.Water in Thisthe middle may be reaches reflected is byenriched the seasonal in heavy variations isotopesof due stable to evaporation isotope composition. and complex Water moisture in the sources middle reachesat the monsoon is enriched margin. in heavy The isotopes heavy due isotope to evaporation values of andriver complex water moistureare lower sources in the atupper the monsoon reaches margin.because Thethe isotopes heavy isotope of precipitation values of river and water surface are water lower are in themore upper depleted reaches in becausehigh elevation the isotopes inland of precipitationareas, although and the surface arid background water are more may depletedalso affect in the high signature elevation of isotopes inland areas, [29]. Consequently, although the arid the backgroundisotopic composition may also aofff ectriver the water signature can ofbe isotopes applied [ 29to]. understand Consequently, the theclimatic isotopic and composition hydrological of riverconditions. water can be applied to understand the climatic and hydrological conditions. Among these studies studies [29–33], [29–33], very very few few were were conduc conductedted in inthe the high-altitude high-altitude upper upper reaches reaches of the of theYellow Yellow River River [33], [33 especially], especially at the at theeastern eastern Tibeta Tibetann Plateau Plateau and and the thewestern western Loess Loess Plateau, Plateau, where where the theclimate climate is mostly is mostly arid arid and andsemi-arid, semi-arid, and andsurface surface water water is for is agriculture for agriculture and anddomestic domestic use [34]. use [The34]. TheLiujiaxia Liujiaxia Reservoir Reservoir (Figure (Figure 1), 1a), famous a famous reservoir reservoir in in western western China, China, is is located located at at the upper YellowYellow River. TheThe water water level level of theof widethe baywide behind bay thebehind dam the (also dam called (also the Binglingcalled the Lake) Bingling is approximately Lake) is 2 atapproximately 1700 m above at sea1700 level. m above The lakesea level. occupies The anlake area occupies of approximately an area of approximately 130 km , with 130 a capacity km2, with of 8 3 39 10 m [35]. The8 annual3 average air temperature in the Yongjing Meteorological Station (35 58 N, a capacity× of 39 × 10 m [35]. The annual average air temperature in the Yongjing Meteorological◦ 0 103Station◦180 E)(35°58 from′ N, 1 January 103°18′ 2016E) from to 31 1 DecemberJanuary 2016 2017 to was 31 December 10.1 ◦C, and 2017 the was annual 10.1 average °C, and precipitation the annual amountaverage wasprecipitation 280.7 mm. amount This reservoir was 280.7 plays mm. a very This important reservoir roleplays in a irrigation very important and flood role control in irrigation across theand region flood andcontrol will across serve asthe the region main and water will source serve of as the the nearby main Lanzhou water source city (the of the capital nearby city ofLanzhou Gansu Province)city (the capital in future city years. of Gansu During Province) 2016–2017, in future the monthly years. During average 2016–2017, storage capacity the monthly of the Binglingaverage 8 3 Lake was 35.68 10 m . However, the existing knowledge8 3 of the isotopic signature in surface water storage capacity× of the Bingling Lake was 35.68 × 10 m . However, the existing knowledge of the aroundisotopic the signature dam is stillin surface limited, water even around though isotopethe dam techniques is still limited, have beeneven widelythough applied isotope to techniques detect the hydrologicalhave been widely process applied worldwide. to detect the hydrological process worldwide. Figure 1.1. MapMap showingshowing the the sampling sampling sites sites of of lake lake water water and and river river water water in this in study.this study. The Yellow The Yellow River, theRiver, Daxia the River,Daxia andRiver, the and Taohe the River Taohe flow River into flow the Binglinginto the LakeBingling and Lake flow outand through flow out the through dam. the dam. Water 2019, 11, 967 3 of 10 During the period of 2016–2017, we established monitoring sites near the Liujiaxia Reservoir and collected surface water samples at two sites, above and below the dam. The purpose of the present study is to improve the knowledge of stable isotopes at the upper reaches of the Yellow River and to investigate the isotope signature of surface water in this region. Moreover, the influence of the dam on the isotopic signature in the surface water will be considered. The sampling network around the Liujiaxia Reservoir could be useful to understand the hydrogen and oxygen isotopes in the regional water cycle. 2. Data and Methods Surface water samples were collected twice a month at a depth of 20 cm from the river downstream the Liujiaxia dam (35◦570 N, 103◦170 E, 1646 m; hereafter referred to as river water), from January 2016 to August 2017, and the Bingling Lake (35◦530 N, 103◦180 E, 1731 m; hereafter termed to as lake water), from January 2016 to March 2017.