Use of oxygen-18 isotope to quantify flows in the upriver and middle reaches of the Heihe River, Northwestern China

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1 Environ Geol (2009) 58: DOI /s y ORIGINAL ARTICLE Use of oxygen-18 isotope to quantify flows in the upriver and middle reaches of the Heihe River, Northwestern China Y. H. Zhang Æ X. F. Song Æ Y. Q. Wu Received: 17 January 2008 / Accepted: 2 September 2008 / Published online: 20 September 2008 Ó Springer-Verlag 2008 Abstract In recharge areas, the Heihe River flow was separated into components of ice-snowmelt and precipitation according to 14 gauging stations and to monthly hydrograph using oxygen-18. As shown by the result of the two-component mixing model, on average, 19.8% of the runoff comes from ice-snowmelt. At three stations which are closer to glaciers and with headstream of ice-snow melted water, the ice-snowmelt runoff is larger than 28% of stream water. In addition, because most of the icesnowmelt infiltrates the groundwater, which later discharges into the river at mountain outlets, the ice-snowmelt percentage in runoff is lower than average at these stations with the elevation higher than 3,600 m. According to monthly hydrograph, the lowest percentage of ice-snowmelt in runoff is in July (6.46%), whereas during November it is the largest (26.1%). In the middle basin, the fraction of groundwater in runoff had a marked increase from 23.57% near Zhangye City to 60.28% near Gaotai City, and then a dramatic drop to 13.61% near Zhengyixia Station because of agricultural flood irrigation in Zhangye Basin. Keywords Runoff Heihe River Precipitation Isotope Y. H. Zhang (&) X. F. Song Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing, China yinghuazhang8@gmail.com; zhangyinghua@igsnrr.ac.cn Y. Q. Wu School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, China Introduction As one of the greatest inland rivers in northwest China, the Heihe River was confronted with water resources scarceness due to environmental changes, human activities, unreasonable development and irrational utilization under arid weather background. In recent years, there are increasing in the number of days with no continuous flow in the lower reaches of the Heihe River basin, which has undergone serious ecological challenges, including degradation of vegetation, decline of groundwater level, total evaporation of the West Juyanhai Lake and East Juyanhai Lake in the lower reaches and a heightened frequency of sand storms. All the above problems pose a serious threat to the livelihood of the local residents and the surrounding ecosystems and have attracted widespread attention of foreign and native scholars (Zhang et al. 2005). In the arid regions of western China, water resource comes mainly from mountain watersheds and disappears in the desert plain. Additionally, there is a frequency transformation relationship between surface and groundwater after flowing out of mountain outlets and into the middle basin. Therefore, the mountain area and the middle basin are the primary research areas in the Heihe River studies. An understanding of the hydrological and geochemical response of forested watersheds to precipitation and snowmelt in mountain watersheds is important for questions relating to water resource management and regional fluxes of mass and energy between the land, the atmosphere and the groundwater. This requires a comprehensive understanding of the mechanisms and pathways by which water travels through the system with significant influences on the transport of nutrients from the terrestrial to the aquatic system. In recent years, conservative isotopic tracing has been done in order to characterize various components

2 646 Environ Geol (2009) 58: contributing to runoff and the interaction between surface and groundwater. Because environmental isotopic tracers have the advantage of tracing, they are ideal for relating flow pathways and water residence time to river water than other common techniques used in classical hydrology. Dincer et al. (1970) pioneered the use of isotopes for hydrograph separation. Hinton et al. (1994) conjoined isotopic tracer of d 18 O together with a geochemical SiO 2 to show multiple contributions of runoff, soil water and groundwater to a streamflow, and the result appears to be satisfactory. Laudon et al. (2002) present a runoff corrected event water method that greatly improves the estimation of both spatial and temporal change in snowmelt isotopic signal and the temporary storage of meltwater in a catchment. Buttle (2005) elucidated the use of isotopic tracers to separate hydrograph in detail, along with the underlying assumptions, and concluded its advantages and challenges in future research, particularly in conjunction with hydrometric techniques. As a first approximation, the isotopic input (precipitation) and its output (e.g., fountain) curves can be described by sinusoidal functions, and with several mathematical flow models such as the exponential model (mathematically equivalent to the good-mixing model) and the dispersive model (Eriksson 1958; Nir 1964; Maloszewski and Zuber 1982), the mean residence time for the water in the subsurface reservoir can be estimated, in addition to other watershed hydrological characteristics (Herrmann and Stichler 1981; Burgman et al. 1987). McGuire et al. (2002) evaluated mean residence time in subsurface waters in the mid-appalachians with dispersion and exponential models, and found that the exponential model provided better simulation for stream baseflow 18 O; however, the dispersion model was better for estimating soil water 18 O. In addition, while a distinct isotope signature is always different between surface and groundwater, the signature can be effective in characterizing the interaction and mixing between them (Katz et al. 1997; McCarthy et al. 1992; Fórizs et al. 2005; Bradley et al. 2006). To utilize water resources effectively and predict the change in groundwater environments, it is essential to analyze the percentage of ice-snowmelt and precipitation in runoff, to evaluate the groundwater age in the mountain area and to quantify interactions between surface and groundwater in the middle reaches of the Heihe River. With the use of the above reviewed isotopic methodologies, the specific objectives of this study were threefold. One is to determine the spatial and temporal proportions of icesnowmelt and precipitation in runoff in the upriver areas of the Heihe River basin, another is to evaluate the mean water residence time for the groundwater in the mountain area, and the other is to analyze the transformation relationship between the surface and the groundwater in the middle basin. The runoff compositions and interactions between different water types were determined by using a mixing model with the influences on them analyzed, and the age of the subsurface water was estimated using an exponential model. Study site This work was carried out at the Heihe River basin located at coordinates N and W, in northwestern China (Fig. 1). The whole basin area is km 2 with 59 km 2 area of snow or ice cover. The Heihe River with 821 km in length originates from the Qilian Mountains in Qinghai Province, flows through the middle basin called the Hexi Corridor in Gansu Province, reaches the lower reaches known as the Ejina Basin in Inner Mongolia Autonomous Region, and disappears into West Juyanhai Lake and East Juyanhai Lake of Ejina Basin. West Juyanhai Lake dried up in 1961 and East Juyanhai Lake dried up in 1992 due to overexploitation of water resources in the upper and middle watersheds. The whole basin spans four different geomorphologic units, namely, the ice-snow permafrost zone in the high mountains, the forest zone in the mid-mountains, the oasis zone at the piedmonts, and the desert zone in the lower reaches (Wu et al. 2004). In the mountain areas above the Yingluoxia station with an altitude higher than 1,700 m, annual precipitation ranges from 200 to 700 mm, and annual evaporation is about 700 mm. In the middle reaches of the river with the elevation range from 1,280 to 1,700 m, annual precipitation ranges from 50 to 200 mm, and annual evaporation ranges from 2,000 to 3,000 mm. In the lower reaches of the river down from the Zhengyixia station, annual rainfall is only 42 mm, but annual evaporation is 3,755 mm. Owing to being in the hinterland of the Asian continent, the region has an obvious characteristic of a continental climate that is extremely hot in summer and severely cold in winter and the major part of the rainfall (about 60 70%) occurs during July September. The elevation in the studied watershed, i.e., the upper and middle reaches of the Heihe River, ranges from 1,280 to 5,120 m. Heihe River stream flow comes mainly from ice-snowmelt and precipitation in the Qilian mountain areas. The Qilian Mountain outlet runoff component to the Heihe River is about m 3 /a. From the mountain outlet (Yingluoxia station) to the end of the middle reaches of the Heihe River (Zhengyixia station), the main river is 185 km in length with an average slope of 2%, and the exchange between the surface and groundwater is frequent. Human activities, such as pumping groundwater, irrigating land, and constructing dams, make the exchange between surface and groundwater more complex.

3 Environ Geol (2009) 58: Fig. 1 Location of sampling sites in the study area From the hydrogeological setting, the southern part of Zhangye Basin is the bedrock of the Qilian Mountain. There is a large fault zone along northeast side of the mountain and down-faulted bedrock extends as the bed level of aquifer in the basin. The geological scenario is shown as a cross-section in Fig. 2. With the Qilian Mountain uplifting, several thousand meters of Quaternary alluvium and fluvial deposits accumulated and forms main aquifers with abundant groundwater in the north-south trending basin. In the southern part of the basin near the mountain, the aquifer consists of thicker gravel with a coarse sand layer, but in the middle part of the basin, there are three aquifers composed of middle-fine sands as shown in Fig. 2.

4 648 Environ Geol (2009) 58: Fig. 2 Hydrogeological proximate cross section of the middle Heihe River basin Methods In the recharge areas of the Heihe River basin, ice-snowmelt plays a key role in the Heihe River, so it is important to determine the temporal and spatial distribution of icesnowmelt percentage in runoff in order to rationally use water resources. In most previous research work with isotopic tracers, runoff is usually separated into different water types, such as two-component of event and pre-event water sources or three runoff components of event water, pre-event water, and the full mixture of precipitation from previous events (Saito 2000; Laudon et al. 2002; McNamara et al. 1997). In Heihe River mountain watersheds, the river water is composed of ice-snowmelt, local precipitation, and groundwater runoff, however, the groundwater comes only from ice-snowmelt and local precipitation without external groundwater flow into this area, so the runoff can be separated into two-component of ice-snowmelt and the full precipitation mixture. Therefore, based on the mass balance of water and a tracer mass balance, a twocomponent separation model is used as follows (Pearce et al. 1986): Q 1 ¼ Q 2 þ Q 3 ð1þ Q 2 ¼½ðC 1 C 3 Þ=ðC 2 C 3 ÞŠ Q 1 ð2þ where Q 1, Q 2 and Q 3 represent three different water components and C 1, C 2 and C 3 are the corresponding tracer concentrations. In order to evaluate the mean groundwater residence time in the studied mountain watersheds, the exponential model is used, all that solutions were summarized and their physical meaning explained by Maloszewski and Zuber (1982). In the general case of a steady-state groundwater system, the input and output stable isotopic tracer concentrations can be approximated given by sinusoidal functions with a period of one year. Therefore, in the case of stable isotopes for the exponential model, the mean transit time T can be determined using the amplitude damping by the following equation (Herrmann and Stichler 1981; Maloszewski and Zuber 1982; Leopoldo et al. 1992): T ¼ x 1 ðf 2 1Þ 1=2 ð3þ Where x equals to 2p per 1 year, and f is the amplitude damping given by the ratio of A out /A in. A is the difference between the maximum absolute value and the mean of the tracer concentration. Suffixes out and in are the recharge and the discharge, respectively, which correspond to precipitation and spring water. The amplitude damping f will be close to 1 if there is not much water mixing, with the water keeping its identity. The smaller the mixing, the less attenuated the amplitude, with an increase in the f value. There are 16 hydrological gauging stations with a rain gauge in upper and middle reaches of the Heihe River, among which 14 stations locate in mountain recharge areas (Fig. 1). From August 2000 to August 2001, every precipitation event was collected at the 14 stations with a polyethylene plastic rain collector, river water was sampled every month by hand at the same station, groundwater samples were taken straight from the spring every month at the Fengle gauging station mountain outlet, and every sampled water filled a 100 ml polyethylene bottle with double lids. From the contour of the groundwater levels in the middle reaches of the Heihe River (Fig. 3), river water recharges to groundwater from Yingluoxia to Zhangye City, and below Zhangye City, the groundwater flow direction is toward the Heihe River and groundwater discharges into the river. Therefore, the sampling sites were selected along the Heihe River and listed from A to E as shown in Fig. 3. Groundwater sampling from wells in contiguous aquifers near riverbank were carried out and surface water samples were collected 30 cm below river level for enough mixed water. All samples were stored cold and dark until analyzed. The stable oxygen-18 isotope of

5 Environ Geol (2009) 58: water samples was measured by a CO 2 H 2 O equilibrium method using a MAT-251 mass spectrometer at National Research Center for Geoanalysis, Chinese Academy of Geological Science (Socki et al. 1992). All d 18 O values are expressed relative to Vienna-standard mean ocean water (VSMOW) and the standard error of the analysis was ±0.2%. Fig. 3 Water table contours of the middle Heihe River Basin aquifer system Results and discussion Characteristics of stream flow components Water resources in the Heihe River basin are precipitation and ice-snowmelt. Ice-snowmelt plays a key role in the runoff. In order to rationally utilize scarce water resources, the mechanism of mountain outlets runoff must be understood along with climate warming and drying which results Qinlian Mountain glacier retreating. In order to delineate the system of spatial and temporal characteristics, study of ice-snowmelt percent of the Heihe River stream flow was done by a two-runoff component model. The icesnowmelt percent, using only oxygen isotopes, is described as follows on the basis of Eq. 1 and 2: Per ¼½ðC r C p Þ=ðC is C p ÞŠ 100% ð4þ where Q r, Q is and Q p represent runoff, ice-snowmelt and rainfall volumes, respectively, C r, C is and C p are the corresponding tracer concentrations, and Per is the percentage of ice-snowmelt in the Heihe River runoff. The natural tracer technique requires that the tracer signatures be conservative and the signatures of different water components must also be distinct. As shown in following Tables 1 and 2, the significant difference between the oxygen isotopes of different components is good enough to use the above tracer model. Ren (1999) had sampled 15 ice-snowmelt in Qilian Mountains to analyze the d 18 O, in which two samples were highly enriched 18 O Table 1 Spatial characteristics of ice-snowmelt in Heihe River streamflow Stations C p (%) C r (%) C is (%) Per (%) Stations C p (%) C r (%) C is (%) Per (%) Binggou a b b b b a Zamashike a a Fengle a Xindi a b b Liqiao a Sunan a b b Shuangshuisi a Wafangcheng a b b Qilian a Yingluoxia a b b Tuole a Yinggezui a 6.46 b 16.3 b Yeniugou a Yuanyangchi a Mean of the ice-snowmelt percentage (%) a Mean of the 15 sampling data b Mean of the 13 sampling data (apart from two singular values) b b a b

6 650 Environ Geol (2009) 58: Table 2 Temporal characteristics of ice-snowmelt in Heihe River streamflow Month C p (%) C r (%) C is (%) Per (%) Month C p (%) C r (%) C is (%) Per (%) January a b b b 6.46 b a July a 7.28 a February a August a b b March a September a b b April a October a b b May a November a b 26.1 b June a December a Mean of the ice-snowmelt percentage (%) a Mean of the 15 sampling data b Mean of the 13 sampling data(apart from two singular values) b b a b with singular values of and -7.89%. The two singular values may be caused by the isotopic fractionation along with ice-snow melting. Therefore, the d 18 O of icesnowmelt C is in two tables are referred to the mean value of the original 15 sampling data and the mean value of the 13 sampling data, which are apart from the two singular values of the 15 orignal numbers. Because of the altitude effect on isotope in this study area (Zhang and Wu 2007a, b), d 18 O values in precipitation at stations with altitude not lower than this station weighted by rainfall were averaged to get C p in Table 1. All d 18 O values in precipitation weighted by rainfall were averaged to get C p in Table 2, i.e., the d 18 O of the full mixture of precipitation from previous events. d 18 O values in river water were averaged by station and by month, respectively to get C r in Tables 1 and 2. Results of the runoff separation according different hydrological gauging stations are shown in Table 1. According to the table, the mean percentage of the icesnowmelt in runoff is evaluated 22.35% using the average of 15 ice-snowmelt s d 18 O, and that is only 19.83% with use of the average apart from two enriched isotopic singular values. This large variation with 2.52% was caused by icesnowmelt isotopic fractionation during melting impeding an accurate separation of stream flow, and the error is large when ice-snowmelt contributes a dominant fraction of the streamflow (Taylor et al. 2002; Laudon et al. 2002). The ice-snowmelt percentages are less than the mean percentage at the stations with higher elevation than 3,600 m such as Tuole, Yeniugou, Zamashike and Qilian, because in these higher areas large ice-snowmelt infiltrating into ground as lateral groundwater flow mostly discharges into river at mountain outlets (Zhang and Wu 2007a, b). The western and eastern parts of the studied mountain areas are distributing glaciers, whose ice-snowmelt is the river s headstream, and these three Fengle, Xindi and Liqiao stations are closer to the glaciers areas, therefore, their icesnowmelt percentages larger than 28% in the stream flow are far higher than the mean. Using the average of the 13 ice-snowmelt data without singular value, the mean percentage of ice-snowmelt in stream flow is 19.83% implying its key role in runoff, and accordingly the mean percentage of the mixed former precipitation events to stream flow is 80.17%, so under warming and drying climate background more attentions must be paid to the ice-snowmelt in order to rationally utilize the scarce water resources in the Heihe River basin. As shown in Table 2, there are large diversities of icesnowmelt percentage among months resulting from uneven distribution of monthly rainfall. Ice-snowmelt percentage is less than the average corresponding to more precipitation from June to August, and is larger with little rainfall from November to next year January. Because in November the rainfall is small and in the previous months with high air temperature more ice-snowmelt infiltrated into groundwater as lateral flow delaying its discharge into the river, the percentage of ice-snowmelt is the largest with 26.1% amounting to runoff in November. Conversely, in July and its before month June the precipitation is very abundant and ice-snowmelt is small in the previous months with low air temperature, so the least percentage, i.e., 6.46% of runoff is recharged by ice-snowmelt in July.

7 Environ Geol (2009) 58: Table 3 Annual and monthly means of the d 18 O values in precipitation and spring water at Fengle hydrological gauging station Month d 18 O in rainwater (%) d 18 O in spring water (%) Month d 18 O in rainwater (%) d 18 O in spring water (%) January July February August March September April October May November June December Annual mean Estimations of the water residence time In this study, d 18 O in different water types, as input or output isotopic signal, is seasonal variation, similar with sinusoidal curve, and can be approximately described by sine functions (Zhang and Wu 2007a, b), so the approach using 18 O as presented in Leopoldo et al. (1992) was employed to estimate subsurface water residence time. The distributions of annual and monthly d 18 O mean values in precipitation and spring water were shown in Table 3. These water samples were collected at the Fengle hydrological gauging station during the period of the experiment. As can be seen in the table, the means of the rainwater and spring water d 18 O are and -8.8%, respectively, with the variation in rainwater individual values ranging from to -19.8% and in spring water ranging from -8.6 to -9%. The largest d 18 O amplitudes of precipitation and spring water are respectively 8.968% (A in ) and 0.2% (A out ), so amplitude damping values (f) equals to ; according to Eq. 3 the resulting mean residence time of the precipitation in the subsurface reservoirs of mountain catchment areas at Fengle stations are about 7 years. The low variety of d 18 O in spring water accounting for the low amplitude and the long residence time indicates the large mixing of former precipitation, which is in accord with the low porosity and low permeability in mountain areas (Fig. 2), resulting slow infiltration of the rain water. Analysis of surface water-groundwater interaction The fraction of groundwater discharged to surface water, using only oxygen isotopes, can be calculated as follows deduced by Eqs. 1 and 2: Per ¼½ðC r C ur Þ=ðC g C ur ÞŠ 100% ð5þ where C g and C ur represents tracer concentration in groundwater and in upriver water, respectively, and Per is the percentage of groundwater in the Heihe River runoff. However, it can be seen from Fig. 1 that there are two confluent streams flowing together to B site, so it would rather consider both branches water than only one. A three-component tracer model was used to induce an equation as follows to compute the fraction of groundwater in runoff: Per ¼ ðc r C ur 0ÞðCur 0 C0 ur 0Þ ðc0 r C0 ur 0ÞðC ur C ur 0Þ ðc g C ur 0ÞðCur 0 C0 ur 0Þ ðc0 g C0 ur ÞðC ur C ur 0Þ 100% ð6þ where C ur 0 represents tracer concentration in another branch water, C ur, C r and C g are mentioned in above equations, Cur 0 0; C0 ur ; C0 r and C0 g stands for another tracer concentration, respectively corresponding to C ur 0; C ur, C r, C g. Another tracer such as Cl - here can be chosen as well because it always behaves conservatively (Clark and Fritz 1997; Turner et al. 1992). In the middle reaches of the Hiehe River, with Eqs. 5 and 6 the calculated results about percentage of groundwater in runoff were shown in Table 4. It can be seen from the table that there was a marked increase for the fraction of groundwater discharged into runoff from 23.57% at A site near Zhangye city to 60.28% at C site near Gaotai city, and then a dramatic drop to 13.61% at E site near Zhengyixia station. The above distribution results from agricultural flood irrigation in Zhangye basin including regions of Zhangye, Linze and Gaotai. Irrigation water was evaporated with isotope fractionation effect Table 4 The concentration of tracers in different water types and the fraction of groundwater discharged to surface Sites C g (%) C r (%) C ur (%) C ur (%) Per (%) A (A 0 ) B (B 0 ) (A) a a 8.75 a (B 0 ) 10 a (A) C (C 0 ) D (C) E (D) a The Cl - tracer concentration in different runoff components with mg/l units

8 652 Environ Geol (2009) 58: under arid climate before recharging groundwater, so all d 18 O of groundwater were more enriched than that of river water. In these areas with densely irrigated fields, irrigation area is about ha, m 3 /a river water was extracted through diversion canal and m 3 /a groundwater was pumped for irrigation. The most river water along the Heihe River was continuously pumped out into anti-permeable canal for irrigation, which resulted in the decrease of infiltration from river water into groundwater and even transformed the interaction between surface water and groundwater. Deep groundwater extracted out for irrigation and its replenishment sources reduce had caused some environmental problems such as exhausted groundwater resources, decline of groundwater level, ground settlement and vegetation degeneration. Conclusions Using two-component mixing model based on mass balance, stream flow was isotopically separated into ice-snowmelt and the full mixture of precipitation from previous events over a period of approximately 1 year. The separation was done well since these d 18 O signatures of different water bodies were distinct. A comparison of the isotopic results among different stations and among different months made it possible to analyze the reasons resulting in the diverse percentage of ice-snowmelt in stream flow. The small percentage of ice-snowmelt at some stations was caused by its most infiltration into groundwater in higher areas then discharging into river at mountain outlets, conversely the large percentage of that occurs only at three stations closer to mountain glacier areas. As a whole, the ice-snowmelt proportion of stream flow is small in summer with large rainfall, whereas is great in winter with small rainfall. As an example of station in mountain areas, at Fengle hydrological gauging station the relative large attenuation of the spring water d 18 O signatures was illustrated by the results of the amplitude damping, showing a well mixing of the former precipitation events during their discharge into the river. The residence time of precipitation in the subsurface reservoirs of mountain catchment areas was estimated about 7 years. In oases of the middle Heihe River basin, flood irrigation on vast stretches of farmland made a great impact on the transform between groundwater and surface water. The percentage of groundwater discharging into runoff is up to the largest near Gaotai City. The irrational use of water resources and irrigation causes some problems such as environmental deterioration and land subsidence. With more geohydrological data, the volume of the groundwater system in mountain areas and in oases of the middle Heihe River basin can be evaluated from both the mean residence time and the runoff components. The importance of ice-snowmelt in runoff has implications that a warming and drying climate may impose significant changes in the hydrology of the Heihe River basin, which may then influence the reasonable utilization of short water resource. Acknowledgments This work was supported by a grant from the Innovation Project of CAS (O7V70020SZ) and the National Natural Science Foundation of China ( ). We would like to appreciate Jan Schwarzbauer for his helpful discussions, valuable advice and review. We also wish to thank Kobi Anker and the anonymous reviewers for their reading of the manuscript, and for their suggestions and critical comments. 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