Stable Isotope Composition in Surface Water in the Upper Yellow River in Northwest China

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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. The climate parameters (including air temperature, precipitation amount, wind speed, and relatively humidity) come from the National Meteorological Information Center, China (NMIC). The storage of the Bingling Lake during 2016–2017, based on the daily gauge-based records, was provided by the local water resources administration [36]. The water samples were stored in 50-mL narrow-mouth HDPE bottles with an inner cap and waterproof tape. All samples were analyzed by the Stable Isotope Laboratory, College of Geography and Environmental Science, Northwest Normal University, Lanzhou, China, using a liquid water isotope analyzer (DLT-100, Los Gatos Research, San Jose, CA, USA). Both the sample and isotopic standard were injected six times. In order to eliminate any memory eﬀect, we used the average value of the last four measurements as the ﬁnal result. The stable isotope ratio is expressed relatively to the Vienna Standard Mean Ocean Water (V-SMOW): δ = R /R 1 1000% (1) sample standard − × 18 16 2 1 where Rsample is the O/ O (or H/ H) ratio of the analyzed sample and Rstandard is the ratio of the V-SMOW. The analytic precision was better than 0.2% for the δ18O and 0.6% for the δ2H, ± ± respectively. This same protocol has also been applied in previous studies [4,37]. The characteristics of the surface water isotopes are shown in Supplementary Table S1.

3. Results and Discussion

3.1. Stable Isotopes in Surface Water The stable isotope ratios (δ18O and δ2H) in the lake and river waters are provided in Table1 and illustrated in Figure2. The maximum and minimum values of δ18O in the lake water were 9.49% − and 10.32%, and those of δ2H were 66.69% and 77.07%, respectively. For the river water, the − − − δ18O value ranges from 9.35% to 10.55%, while the δ2H ranges between 65.72% and 75.37%. − − − − The stable isotope composition of the lake and river waters exhibits a seasonal variability. In general, the isotopes values of surface water are enriched during the warm months (deﬁned in this study as the period from April to October) and depleted during the cold months (from November to March). The range of the isotope values of the river water is close to that of the lake water.

Table 1. Descriptive statistics of isotopes in the lake and river water from 2016 to 2017.

Sample δ Value Mean (%) SD (%) Max (%) Min (%) n Period δ18O 10.0 0.2 9.49 10.32 Lake water − − − 26 January 2016–April 2017 δ2H 73 3.0 66.69 77.07 − − − River δ18O 10.0 0.3 9.35 10.55 − − − 37 January 2016–August 2017 water δ2H 71 3.0 65.72 75.37 − − − SD: Standard deviation; n: Number of samples. Water 2019, 11, x FOR PEER REVIEW 4 of 10 Water 2019, 11, 967 4 of 10 Water 2019, 11, x FOR PEER REVIEW 4 of 10

Figure 2. Seasonal variations of δ2H and δ18O in (a) the lake water and (b) the river water from 2016 Figure 2. Seasonal variations of δ2H and δ18O in (a) the lake water and (b) the river water from 2016 toFigure 2017. 2. Seasonal variations of δ2H and δ18O in (a) the lake water and (b) the river water from 2016 to 2017. to 2017. Figure 3 shows the seasonal variation of inflow, storage, outflow, and the corresponding Figure3 shows the seasonal variation of inflow, storage, outflow, and the corresponding meteorologicalFigure 3 showsparameters. the seasonal The flow variation of water reachesof inflow, its maximumstorage, outflow, during the and warm the months,corresponding which meteorological parameters. The flow of water reaches its maximum during the warm months, correspondsmeteorological to parameters.the seasonality The of fl owprecipitation of water reaches and air its temperature. maximum during The local the warmprecipitation months, usually which which corresponds to the seasonality of precipitation and air temperature. The local precipitation concentratescorresponds into summer,the seasonality leading of to precipitation an increase ofan waterd air temperature.supply during The the local warm precipitation months. usually usually concentrates in summer, leading to an increase of water supply during the warm months. concentrates in summer, leading to an increase of water supply during the warm months.

Figure 3. Seasonal variation of (a) inflow, (b) storage, (c) outflow, (d) air temperature, and (Figuree) precipitation 3. Seasonal amount variation from Januaryof (a) inflow, 2016 to (b August) storage, 2017. (c) outflow, (d) air temperature, and (e) precipitationFigure 3. Seasonal amount variation from January of (a )2016 inflow, to August (b) storage, 2017. (c) outflow, (d) air temperature, and (e) 2 18 Theprecipitation linear regression amount from between Januaryδ 2016H and to Augustδ O in 2017. surface waters reflects, to some extent, the local evaporationThe linear conditions regression [38 ].between The slope δ2H reflects and δ18 theO in relationship surface waters between reflects, fractionation to some extent, rates of the2H local and 18evaporationO, andThe thelinear interceptconditions regression represents [38]. between The the slope degreeδ2H reflects and of δ deviation18 theO in relationship surface of 2H waters from between the reflects, equilibrium. fractionation to some Figure extent, rates4 shows of the 2H local and the correlation18evaporationO, and the between interceptconditions the represents stable[38]. The hydrogen theslope degree reflects and oxygenof thedeviation relationship isotopes of 2H in from thebetween lake the water, equilibrium.fractionation the river Figurerates water, of 4 and 2showsH and the globalthe18O, correlation and meteoric the intercept between water linerepresents the (GMWL: stable the hydrogenδ2 degreeH = 8 δ 18of andO deviation+ oxyg10 [39en]). ofisotopes The2H from same in the timethe equilibrium. lake series water, of the theFigure lake river water 4 water,shows and riverandthe correlationthe water global samples meteoricbetween was waterthe selected stable line for hydrogen(GMWL: comparison δand2H = (Figureoxyg 8 δ18enO4 +isotopes). 10 The [39]). lake in The waterthe samelake and water,time river series the water river of samplesthe water, lake arewaterand almost the and global river distributed meteoric water samples on water the right-hand wasline selected(GMWL: lower for δ2 Hcomparison side = 8 ofδ18O the + GMWL,(Figure10 [39]). 4). andThe The thesame lake slopes timewater ofseries and the river lakeof the water lake linesampleswater (7.42) and are andriver almost river water distributed water samples line (7.29)wason the selected are right-hand much for lowercomparison lower than side that (Figure of ofthe the GMWL, 4). GMWL. The lake and Low water the slopes slopes and ofriver of lakethe water lake and riverwatersamples water line are (7.42) are almost commonly and distributed river associated water on line the with (7.29)right-hand low are humidity much lower lower andside with ofthan the evaporation thatGMWL, of the and GMWL. due the to slopes non-equilibrium Low of slopes the lake of evaporationlakewater and line river (7.42) [1 ,water40 and]. In are river western commonly water China, line associated where(7.29) precipitationare with much low lower humidity is limited, than thatand the withof strong the evaporation GMWL. evaporation Low due mayslopes to non- lead of equilibriumlake and river evaporation water are commonly [1,40]. In associated western withChina, low where humidity precipitation and with evaporationis limited, duethe tostrong non- equilibrium evaporation [1,40]. In western China, where precipitation is limited, the strong

Water 20192019,, 1111,, 967x FOR PEER REVIEW 55 of of 10 evaporation may lead to an enrichment of heavy isotopes in surface water [41,42]. In addition, the towater an enrichment in the Bingling of heavy Lake isotopes originates in surfacefrom higher water elevations, [41,42]. In where addition, it is the relatively water in cold, the Binglingand the value Lake originatesof the slope from of surface higher elevations,water close where to eight it is may relatively also cold,imply and an thealtitude value effect. of the However, slope of surface some waterriver closewater to samples eight may are distributed also imply anon altitudethe left side effect. of the However, GMWL, some showing river a waterweak evaporation, samples are distributedwhich may onbe associated the left side with of the the GMWL, flow and showing water source. a weak In evaporation, addition, the which river maywater be is associatedactually mixed with with the flow the andBingling water Lake source. water In addition,and the Taohe the river River water water, is actually which may mixed affect with the the relationship Bingling Lake of isotopes water and in the Taohesampled River water. water, which may affect the relationship of isotopes in the sampled water.

2 18 Figure 4.4. The relationship between δ 2H and δ18OO in in ( (aa)) lake lake water and ( b) river water from JanuaryJanuary 2016 to March 2017. The global meteoric waterwater line (GMWL) is based on Craig [[38].38]. 3.2. Deuterium-Excess in Surface Water 3.2. Deuterium-Excess in Surface Water Deuterium excess (d, deﬁned as d = δ2H – 8 δ18O) values of water can be used to identify vapor Deuterium excess (d, defined as d = δ2H – 8 δ18O) values of water can be used to identify vapor source regions and below-cloud evaporation [23,27,41,43–46]. The d value is the intercept value of source regions and below-cloud evaporation [23,27,41,43–46]. The d value is the intercept value of δ2H while maintaining a slope equal to eight. D-excess is usually associated with climate parameters δ2H while maintaining a slope equal to eight. D-excess is usually associated with climate parameters such as air temperature, wind speed, and relative humidity in the water vapor source area [25,45–49]. such as air temperature, wind speed, and relative humidity in the water vapor source area [25,45– The d value in precipitation is usually high in western China (d = 8 – 12) and low in eastern China 49]. The d value in precipitation is usually high in western China (d = 8 – 12) and low in eastern China (d = 4 – 12)[47]. In the study area, the actual evaporation is high, which corresponds to a high (d = 4 – 12) [47]. In the study area, the actual evaporation is high, which corresponds to a high temperature and low precipitation amount [50]. Consequently, the d value in the surface water is temperature and low precipitation amount [50]. Consequently, the d value in the surface water is inﬂuenced by the local climate condition. Figure5 shows that d value weakly correlates with δ18O in influenced by the local climate condition. Figure 5 shows that d value weakly correlates with δ18O in the river water (r = 0.28), indicating that the high δ18O usually corresponds to a low d value. the river water (r = −0.28), indicating that the high δ18O usually corresponds to a low d value.

Water 2019, 11, x FOR PEER REVIEW 5 of 10 evaporation may lead to an enrichment of heavy isotopes in surface water [41,42]. In addition, the water in the Bingling Lake originates from higher elevations, where it is relatively cold, and the value of the slope of surface water close to eight may also imply an altitude effect. However, some river water samples are distributed on the left side of the GMWL, showing a weak evaporation, which may be associated with the flow and water source. In addition, the river water is actually mixed with the Bingling Lake water and the Taohe River water, which may affect the relationship of isotopes in the sampled water.

Figure 4. The relationship between δ2H and δ18O in (a) lake water and (b) river water from January 2016 to March 2017. The global meteoric water line (GMWL) is based on Craig [38].

3.2. Deuterium-Excess in Surface Water

Deuterium excess (d, defined as d = δ2H – 8 δ18O) values of water can be used to identify vapor source regions and below-cloud evaporation [23,27,41,43–46]. The d value is the intercept value of δ2H while maintaining a slope equal to eight. D-excess is usually associated with climate parameters such as air temperature, wind speed, and relative humidity in the water vapor source area [25,45– 49]. The d value in precipitation is usually high in western China (d = 8 – 12) and low in eastern China (d = 4 – 12) [47]. In the study area, the actual evaporation is high, which corresponds to a high temperature and low precipitation amount [50]. Consequently, the d value in the surface water is 18 Waterinfluenced2019, 11 ,by 967 the local climate condition. Figure 5 shows that d value weakly correlates with δ 6O of in 10 the river water (r = −0.28), indicating that the high δ18O usually corresponds to a low d value.

Water 2019, 11, x FOR PEER REVIEW 6 of 10

Figure 5. Correlation between D-excess and δ18O in the lake and river waters. The dashed line denotes the linear fit of lake water and the solid line denotes the linear fit of the river water. Figure 5. Correlation between D-excess and δ18O in the lake and river waters. The dashed line denotes the linear fit of lake water and the solid line denotes the linear fit of the river water. During the study period, d values of the lake water varied from 4.15‰ to 11.25‰ and from 3.89‰During to 13.39‰ the study for the period, riverd watervalues (Figure of the lake6). The water d values varied of from the lake 4.15% water to 11.25% were lower and from 3.89%March to August 13.39% (exceptfor the for river June) water in 2016 (Figure and6 were). The higherd values from of September the lake water 2016 to were March lower 2017. from The March d values to Augustof the river (except water for were June) lower in 2016 from and April were to higher September from September (except for 2016 August) to March in 2016 2017. and The wered values higher of thefrom river October water 2016 were to lower February from April2017. to Generally, September th (excepte warm for months, August) with in 2016 high and solar were radiation, higher from air Octobertemperature 2016 toand February flow, correspond 2017. Generally, to relatively the warm strong months, surface with evaporation. high solar radiation, However, air temperaturecontrary to and2016, flow, the correspondd values of toriver relatively water strongwere higher surface from evaporation. May to August However, in contrary2017. This to indicates 2016, the dthatvalues the ofevaporation river water condition were higher is not from the May only to factor August influencing in 2017. This D-excess, indicates and that other the evaporation factors such condition as river isrecharge not the sources only factor and influencing river inputs D-excess, may also and play other an important factors such role as [51]. river Moreover, recharge sources it looks and like river the inputslake water may d also values play may animportant have a long-term role [51 ].positive Moreover, linear it lookstrend like that the may lake need water to bed values removed may before have athe long-term values can positive be compared linear trend between thatmay the needriver toand be lake removed water. before According the values to the can detrended be compared plot between(Figure S1 the inriver the Supplementary and lake water. Material), According the to low the detrendedvalues of d plot in spring (Figure and S1 insummer the Supplementary can be more Material),easily seen. the low values of d in spring and summer can be more easily seen.

Figure 6. Variation of the D-excess value in (a) lake water from January 2016 to March 2017 and (b) river waterFigure from 6. Variation January of 2016 the to D-excess August 2017.value in (a) lake water from January 2016 to March 2017 and (b) river water from January 2016 to August 2017. The mean d value in the lake water (7.08%) was found to be lower than that of the river water (9.19%The). mean If the d lake value and in riverthe lake water water have (7.08‰) the exactly was found same waterto be lower source, than this that diﬀ oference the river in isotope water signature(9.19‰). If is the usually lake consideredand river water to reﬂect have the the evaporation exactly same of lakewater water source, being this higher difference than thatin isotope of the signature is usually considered to reflect the evaporation of lake water being higher than that of the river water. Because of the interception of the dam, the lake water in this study may be mixed in a relatively long residence time. However, another river stream, the Taohe River, contributes to the lake above the dam and does influence this hydrological process. The river water sampled was collected below the dam, which was mixed with the lake water in the study, as well as the Taohe River water. In this study, the surface water of the Taohe River was not directly collected, which may impact the explanations of the relationship of the stable isotope signature in the sampled river and lake waters. Although the proportional contribution of the Taohe River to the stream of the Yellow River is not very large [35], these integrated effects make the stable isotope signature of the river water more complex. The regression equations between d value and air temperature are d = −0.08 T + 7.53 (r = −0.37) for the lake water and d = −0.002 T + 9.31 (r = −0.01) for the river water, respectively (Figure 7a,d). There is no correlation between d and temperature in the river water. The mixture process, considering the Taohe River, may weaken the temperature effect of surface water. There are negative correlations between d value and wind speed, and the regression lines were d = −2.43 M + 9.71 (r = −0.26) for lake water and d = −1.86 M+ 11.54 (r = −0.20) for river water, respectively (Figure 7b,e). In addition, the correlation between d value and relative humidity is very weak. The regression

Water 2019, 11, 967 7 of 10 river water. Because of the interception of the dam, the lake water in this study may be mixed in a relatively long residence time. However, another river stream, the Taohe River, contributes to the lake above the dam and does influence this hydrological process. The river water sampled was collected below the dam, which was mixed with the lake water in the study, as well as the Taohe River water. In this study, the surface water of the Taohe River was not directly collected, which may impact the explanations of the relationship of the stable isotope signature in the sampled river and lake waters. Although the proportional contribution of the Taohe River to the stream of the Yellow River is not very large [35], these integrated effects make the stable isotope signature of the river water more complex. The regression equations between d value and air temperature are d = 0.08 T + 7.53 (r = 0.37) − − for the lake water and d = 0.002 T + 9.31 (r = 0.01) for the river water, respectively (Figure7a,d). − − There is no correlation between d and temperature in the river water. The mixture process, considering the Taohe River, may weaken the temperature effect of surface water. There are negative correlations between d value and wind speed, and the regression lines were d = 2.43 M + 9.71 (r = 0.26) for − − lake water and d = 1.86 M+ 11.54 (r = 0.20) for river water, respectively (Figure7b,e). In addition, Water 2019, 11, x FOR PEER− REVIEW − 7 of 10 the correlation between d value and relative humidity is very weak. The regression equations are dequations= 0.03 φ +are5.23 d = ( r0.03= 0.12) ϕ + 5.23for the(r = lake0.12) waterfor the and laked water= 0.02 andφ +d =8.32 0.02( ϕr =+ 8.320.08) (rfor = 0.08) the for river the water, river respectivelywater, respectively (Figure (Figure7c,f). The7c,f). above-mentioned The above-mentione correlationd correlation coe coefficientsfficients for for the the lake lake water water werewere generallygenerally largerlarger thanthan thosethose forfor thethe riverriver water.water. FurtherFurther conclusionsconclusions requirerequire anan improvedimproved samplingsampling network,network, includingincluding moremore sitessites alongalong thisthis river,river, atat leastleast atat thethe TaoheTaohe River River mouth. mouth.

Figure 7. Correlation between meteorological parameters (surface air temperature, wind speed, and Figure 7. Correlation between meteorological parameters (surface air temperature, wind speed, and relative humidity) and D-excess in the lake (a, b and c) and the river water (d, e and f). relative humidity) and D-excess in the lake (a, b and c) and the river water (d, e and f). 4. Conclusion 4. Conclusion In this work, we provide recent measurements of the stable isotope composition of lake water and riverIn water this atwork, the upper we provide reaches recent of the measurements Yellow River. Theof the isotopic stable valuesisotope of composition the lake and of river lake waters water exhibitand river a seasonal water at variation. the upper The reaches heavy of isotopes the Yellow of surface River. water The wereisotopic more values enriched of the during lake theand warm river monthswaters exhibit and depleted a seasonal during variation. the cold The months. heavy Moreover, isotopes of a highsurface precipitation water were amount more enriched would increase during thethe waterwarm supply,months resulting and depleted in an increaseduring the in rivercold months. ﬂow along Moreover, the ﬂow a pathways high precipitation during the amount warm months.would increase The slopes the ofwater the lakesupply, water resulting and river in wateran increase lines are in lowerriver flow than thatalong of the the flow GMWL pathways due to evaporation.during the warm Low months.d values The correspond slopes of to the positive lake wateδ18Or and values river in water the lake lines and are river lower waters. than Moreover,that of the theGMWLd values due ofto theevaporation. lake water Low are higher d values than correspond those of the to riverpositive water. δ18O The values river in water the lake samples and wereriver waters. Moreover, the d values of the lake water are higher than those of the river water. The river water samples were collected at the lower part of the dam, where the lake water and Taohe River water is mixed. Thus, the isotopic signature of the collected river water is a result of the integrated effects.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: The variation of original and detrended D-excess in (a) the lake water and (b) the river water, Table S1: Inventory of surface water isotopes in the Liujiaxia Reservoir from 2016 to 2017.

Author Contributions: Conceptualization, S.W. and M.Z.; Software, M.S.; Validation, M.S.; Formal analysis, M.S.; Investigation, M.S.; Methodology, S.W., R.J., and Y.Z.; Resources, R.G. and S.Z.; Data curation, J.K.; Writing—original draft preparation, M.S.; Writing—review and editing, M.S., S.W., and A.A.A.; Supervision, X.Q.; Funding acquisition, S.W. and M.Z.

Funding: The study was supported by the National Natural Science Foundation of China Nos. 41701028 and 41771035, the Scientific Research Program of Higher Education Institutions of Gansu Province Nos. 2016B-019 and 2018C-02, and the Foundation of Key Laboratory for Ecology and Environment of River Wetlands of the Shaanxi Province No. SXSD201703.

Acknowledgments: The authors greatly thank the colleagues in the Northwest Normal University for their help in the laboratory analysis. Thanks also to J. E. Saylor (University of Houston) for the constructive suggestions.

Water 2019, 11, 967 8 of 10 collected at the lower part of the dam, where the lake water and Taohe River water is mixed. Thus, the isotopic signature of the collected river water is a result of the integrated eﬀects.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/11/5/967/s1, Figure S1: The variation of original and detrended D-excess in (a) the lake water and (b) the river water, Table S1: Inventory of surface water isotopes in the Liujiaxia Reservoir from 2016 to 2017. Author Contributions: Conceptualization, S.W. and M.Z.; Software, M.S.; Validation, M.S.; Formal analysis, M.S.; Investigation, M.S.; Methodology, S.W., R.J., and Y.Z.; Resources, R.G. and S.Z.; Data curation, J.K.; Writing—original draft preparation, M.S.; Writing—review and editing, M.S., S.W., and A.A.A.; Supervision, X.Q.; Funding acquisition, S.W. and M.Z. Funding: The study was supported by the National Natural Science Foundation of China Nos. 41701028 and 41771035, the Scientific Research Program of Higher Education Institutions of Gansu Province Nos. 2016B-019 and 2018C-02, and the Foundation of Key Laboratory for Ecology and Environment of River Wetlands of the Shaanxi Province No. SXSD201703. Acknowledgments: The authors greatly thank the colleagues in the Northwest Normal University for their help in the laboratory analysis. Thanks also to J. E. Saylor (University of Houston) for the constructive suggestions. Conflicts of Interest: The authors have declared no conflict of interest. The work described here has not been submitted elsewhere for publication, in whole or in part, and all the authors listed have approved the manuscript that is enclosed.

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