Distribution of stable isotopes in water from an alpine river in China

Being referred to as the ‘water towers’ for the foot-zones and adjoining lowland areas, mountains are important sources of water, often stored as snow and ice (Wiesmann et al. 2000; Singhal & Jain 2002; Verbunt et al. 2003; Viviroli & Weingartner 2004; Viviroli et al. 2007; Salzmann et al. 2014). For many mountain regions, the seasonal character of runoff is closely related to hydrologic processes in the cryosphere, and ice and snow are the crucial components of the hydrological cycle. About 40% of the global population lives in the basins of rivers originating in different mountain areas (Beniston 2003). As glaciers and snow cover is lost rapidly under warmer climatic conditions, they will provide short-term increases in meltwater contribution to river flows, but as they diminish, river runoff will decline while the variability of river flows will increase (Immerzeel et al. 2010). The understanding of the role of glaciers and snow melt in controlling mountain hydrology remains limited due to the complicated modelling conditions and scanty observation data (Barnett et al. 2005; Cyranoski 2005; Immerzeel et al. 2010; Luce et al. 2013).

Characterized by high accumulation and ablation, monsoon temperate glaciers are more active than continental (cold) glaciers, leading to considerable local hydrological variability with regional climate change (Kaser et al. 2006; Radić & Hock 2014). Accounting for more than 22% of China's total glaciated area, monsoon temperate glaciers occur mainly in the south-eastern part of the Tibetan Plateau. As the highest mountain on the eastern margin of the Tibetan Plateau, Mount Gongga is in the transitional belt between that plateau and the Sichuan Basin. The hydrological regimes in this mountainous area and its lowland regions have been considerably affected by variations in glaciation and snow cover. A number of scientific investigations have been conducted around Mount Gongga due to the distinct features of this alpine region, including its diverse climate, geology, topography, etc. However, previous studies on monsoon temperate glaciers have focused mainly on glacier mass changes and their measurement (e.g. Heim 1936; Zheng & Ma 1994; Shen et al. 2004), with little consideration of the hydrological role of snow and glaciers in Mount Gongga. Located on the eastern slope of this mountain, Hailuogou (HLG) watershed is for investigating the distinctive hydrologic regimes of the proglacial zone.

On a range of spatial and temporal scales, analysing the stable isotopic composition of water has become an effective means of investigating complicated hydrologic regimes. For example, many researchers use stable isotopes to investigate rainout processes and moisture sources, water movement to and from rivers and lakes, surface-ground- water interactions, and the enhanced hydrological cycles in alpine regions (e.g. Craig 1961; Dansgaard 1964; Rozanski et al. 1982; Araguas et al. 2000; Jeelani et al. 2010; Mcgrane et al. 2014). These studies demonstrate the applicability and usefulness of stable isotopes (¹⁸O and ²H) as conservative tracers for investigating complex hydrologic systems.

Large volumes of water are discharged from the glaciers in the HLG watershed, which have been under negative mass balance conditions in recent years. The water collected from Huangbengliu (HBL) Gulley, a tributary of HLG River in its upper valley, tended to be more depleted in ¹⁸O and ²H than either rainwater or groundwater, but more enriched than meltwater (Meng et al. 2014).

Previous studies by the authors have shown that meltwater is a very important water source in HLG River's upper reaches (Meng & Liu 2016). The spatial variation pattern of δ¹⁸O and δ²H in the HLG watershed, from the upper basin downward, is not well known. Isotope hydrology data were generated in the study area in 2011.

The objectives of the current research were to: (1) evaluate the spatial variations in δ¹⁸O and δ²H in the HLG River system; (2) examine the relationship between δ¹⁸O and δ²H in the surface water; (3) present a δ¹⁸O-based hydrologic model to evaluate the relative contribution of various water components and estimate the hydrologic processes shaping stream hydrology; and (4) assess the role of ice-snow melt- and tributary- water inputs in controlling river hydrology in the proglacial zone.

Mount Gongga is on the south-eastern edge of the Tibetan Plateau. Its highest peak stands at 7,556 m asl (above sea level). Climatically it is in the transition belt between the Tibetan Plateau with a dry-cold climate and the warm-wet monsoon climate of the Sichuan Basin. As the largest area of modern glaciation in the Hengduan Mountains, Mount Gongga is one of the principal glaciated regions controlled by the monsoonal climate in south-west China.

The HLG watershed, on the eastern slope of Mount Gongga, itself a highly glaciated area, covers 94.75 km², of which 34.67 km² (36.6%) is glaciated and the remaining non-glaciated area is mainly forested (Shen et al. 2004). This region, affected by the south-east and south-west monsoons in summer, and the westerly circulation in winter, belongs to the wet monsoon climate.

There are seven glaciers in the HLG watershed, of which HLG glacier is the largest, with a total area of about 25.7 km² and a length of 13.1 km. Most of its ablation area stretches into the forest zone.

At 30.21 km long and with a mean slope of 6.93%, the HLG River is typically glacial. Its main stem receives meltwaters from the HLG glacier in the headwater region and runoff from its tributaries, which are mostly non-glacial streams, in the lower valley. The Yanzigou (YZG) River, on the east slope of Mount Gongga, with YZG glacier (about 30.1 km²) covering its upper reach, is also typically glacial. The two rivers – HLG and YZG – combine to form the Moxi River at Moxi town, and finally flow into the Dada River (Figure 1).

Water samples were collected from 27 locations along the HLG River and its tributaries for stable isotope analyses in July 2011. Prior to sampling, brand new sample bottles were washed at least three times in-situ. Water samples were passed through 0.45 μm pre-cleaned membrane filters and were collected in 500 ml high-density polyethylene bottles with tight-fitting screw caps. Evaporation through leaky bottle caps or partially filled bottles can cause problems. In order to minimize post-sampling changes in the water's isotopic composition, the bottles were filled slowly until completely full. Surface water samples were dipped along the river shore at water depths of 10 to 30 cm, where relatively clean water was flowing. Paired samples were taken at every location to minimise the chances of loss during transport.

Two meteorological stations, HLG (at 3,000 m asl) and Moxi (MX) (1,640 m asl), and a hydrological station (2,920 m asl), approximately 1 km from the HLG glacier terminus, were established and maintained by the Gongga Mountain Alpine Ecosystem Station of the Chinese Academy of Sciences on the east slope of the mountain.

The study area is affected by the south-east monsoon, being humid (the observed average annual relative humidity exceeds 90% in the headwater region) and rainy, and is in the wet monsoon climatic zone of the subtropical mountain. An important feature of the regional climate is the evident vertical zoning; the air temperature and precipitation change clearly with changes in altitude. The study area has an integrated vertical zone from the valley's sub-tropical belt to the permanent alpine snow-belt.

Twenty-two years (1988–2009) of meteorological data were collected at the two meteorological stations, and the wide range of temporal and spatial temperature and precipitation variations in the study area. January is the coldest month and July the warmest.

The data from the climatic stations show that, in 2009, the monthly mean air temperature ranged between −4.1 °C and 12.9 °C at HLG and between 3.9 °C and 20.6 °C at MX. Between 1988 and 2009, the annual mean air temperature in MX Station exceeded 13 °C, whereas at HLG Station it was only about 4.3 °C. The fact that the air temperatures at HLG station is higher than that at MX, demonstrates the decrease of temperature with altitude in this region.

Under the influence of the monsoon and topography, rainfall on the study area is abundant. Generally, the highest precipitation occurs in July and the lowest in December, and the year can be divided into dry and wet seasons. On average, the wet season (May-October) delivers about 80% of annual precipitation, with only about 20% in the dry season (November-April). According to the data, the average annual precipitation was approximately 1,900 mm at HLG, and only about 900 mm at MX. In other words, precipitation tends to increase with altitude in the study region.

Annual evaporation was approximately 300 mm at HLG, accounting for only about 15.8% of the annual precipitation. The limited amount of evaporation is mainly due to low air temperatures, and foggy and cloudy days.

HLG River discharge volumes have increased in recent years due to increased ice and snow melt volumes. The hydrological observations show that the river's annual runoff depth and mean discharge rate reached about 3,000 mm and 7.8 m³/s at HLG station, respectively, owing to extensive melting and plentiful precipitation in the study area – in other words, the water resources are very rich. The annual river runoff is well distributed, with maxima in the summer months and minima in the winter. The hydrological data show that about 15% of the annual runoff occurred in spring (March to May), 52% in summer (June to August), 25% in autumn (September to November) and only 8% in winter (December to February) (Figure 2). The runoff rate is highest in summer during the main ablation period.

Craig (1961) observed the global correlation between δ¹⁸O and δ²H precipitation: δ²H = (8 × δ¹⁸O) + 10, later defined as the Global Meteoric Water Line (GMWL) . In fact, Craig's results are essentially a global average of numerous local meteoric water lines (LMWL). Each LMWL changes between regions, depending mainly on local geographic and meteorological factors (Peng et al. 2004). Meng et al. (2014) determined the LMWL for the study zone as: δ²H = (7.84 × δ¹⁸O) + 11.96‰ (R² = 0.98). The relationship between the δ¹⁸O and δ²H of surface waters in the study area is shown in Figure 3. The isotopic data points of water samples from HLG River's main stem and its tributaries tend to be on the left side of the δ²H/δ¹⁸O-relationship diagram, showing relatively low isotopic values. Precipitation contains water with the highest isotopic values; while those of groundwater are generally lower; and meltwater is the most depleted in ¹⁸O and ²H in this region (Meng & Liu 2016).

Where the isotopic values of surface waters are low, this is interpreted as indicating the presence of a heavy-isotope-depleted water component (meltwater) in river flow in the alpine proglacial zone. Analysis of the relationships between the δ¹⁸O and δ²H values of river waters can provide information about the preservation/alteration of the isotopic signature of precipitation in river waters and the role of evaporation in changing the isotopic composition. Most of the surface waters fall close to the LMWL, indicating that evaporation, whether during precipitation events from the cloud base to the ground or of river water during flow, is only minor. The isotopic composition of precipitation is well preserved in river flow.

The hydrologic regime of the HLG River changes significantly from the upper basin downward, because of many natural factors – e.g. different climatic settings, diverse topographic gradients, etc. Such changes in the hydrologic system can be reflected by the distinct isotopic signature of the river water. Figure 4 shows the changes in δ¹⁸O, δ²H and water temperature of surface waters collected in July 2011. As a typical glacial river, the isotopic compositions of water samples collected from the main stem of the HLG River usually increase substantially with travel downstream. Typically, low isotopic values were observed in the upper reaches and high values lower down. The lowest δ¹⁸O and δ²H values were observed at the highest sampling section upstream – sampling Section 7 – with values around −16.3‰ and −112‰, respectively. The highest δ¹⁸O and δ²H values were observed at the furthest section downstream – section, Section 25 – with values of −14.3‰ and −99‰, respectively. This pattern of variability cannot be explained by evaporation-induced isotopic enrichment. The increasing trend of isotopic values of river water downstream from the upper basin in the main river arises from the increasing contribution of heavy-isotope-enriched water with distance from the headwaters.

The river water in the upper reaches features relatively low δ¹⁸O and δ²H values, while the middle and lower parts are quite different. The isotopic values from the main stream are usually lower than those of water in the HLG River's non-glacial tributaries. For example, the δ¹⁸O and δ²H at Section 16 – from Chuanxinheba Gulley, a non-glacial tributary – with values around −12.9‰ and −81‰, respectively, are relatively higher than those from the river's main stream nearby at Section 15, with values of −15.4‰ and −103‰, respectively. This pattern of variability is induced by non-glacial tributary water gain with heavy-isotope-enriched stream waters. Thus, the tributary-input enrichments increase the δ¹⁸O and δ²H values of the river water in the middle and lower parts of the main stream.

The pattern of water temperature variations is similar to that of the river water's isotopic composition, the water temperature increasing significantly from the upper basin downward (Figure 4(c)). The lowest temperature on the main stream of the HLG River was observed at Section 7, the highest sampling point, with a water temperature of 0.5 °C. The highest temperature was observed at Section 25, the furthest downstream, where the water temperature was as high as 10.3 °C. The water temperatures observed in the river's non-glacial tributaries are generally higher than those of the main stream. In the reach between Reshui (Section 12) and Qingshiban (Section 14), for example, the water temperatures in the main stream at Section 13 was 11.48 °C, which is lower than that in the tributaries, Reshui Gulley – Section 12 (13.61 °C) – and Qingshiban Gulley – Section 14 (14.80 °C). The temperature increases in the main stream with increasing distance from the upper basin arise from the higher temperature of the tributary water inputs.

Most of the HLG River's non-glacial tributaries are characterized by relatively high values of δ¹⁸O and δ²H, and high water temperatures (6.6 to 13.6 °C). In the headwater region, for example, HBL Gulley is a typical non-glacial tributary of the river. Water samples collected at Sections 1, 2, 3, 4, 5 and 6 from the gulley, with δ¹⁸O ranging from −15.7‰ to −13.2‰ and δ²H from −103‰ to −86‰, are more enriched in ²H and ¹⁸O than those collected at Sections 7, 8 and 9 from the river's main stream, with δ¹⁸O ranging from −16.2‰ to −16.3‰ and δ²H from −109‰ to −112‰. Correspondingly, the water temperatures observed from the gulley (6.6 to 8.2 °C) are higher than those in the main stream (0.5 to 3 °C). Thus, the isotopic composition and temperature of waters collected from the gulley and the main stream differ due to their different geographical origins in the headwater region.

Generally, δ¹⁸O and δ²H both increase from the upper basin downward in the proglacial zone on the east slope of Mount Gongga. There are some irregularities, however. For example, the river water's isotopic composition decreases slightly between Section 25 and Section 27 (Figure 4(a) and 4(b)). This can be explained by surface water input from YZG River, which is glacial and whose waters are heavy-isotope-depleted. Relatively low δ¹⁸O and δ²H values were observed at Section 26 from the YZG River, with values around −15.4‰ and −104‰, respectively, while relatively high values were observed at Section 25 (HLG River) with values of −14.7‰ and −99‰, respectively. The isotopic signature of river water in the proglacial zone is thus quite variable, depending mainly on the time and location.

Surface waters at higher elevations tend to be more depleted in ²H and ¹⁸O than those from lower elevations in the study area. The relationship between altitude and river water δ²H observed in the river's main stream in July 2011 is shown in Figure 5(a), with that between altitude and δ¹⁸O in Figure 5(b). The correlation between altitude and river water isotopic composition from the river's main stream is statistically significant. Regression analysis shows altitude effects of −0.1‰ per 100-m rise for δ¹⁸O, and −0.8‰ per 100-m rise for δ²H, for river waters collected from 12 sites between 1,421 (Section 27) and 2,954 m asl (Section 7). The negative correlation between altitude and river water isotopic composition may be induced by varying degrees of mixing of water components contributing to river flow. The flow in the HLG River appears to be dominated by meltwater in the headwater region, while receiving a substantial portion from local monsoonal precipitation in the lower valley.

Figure 6(a) and 6(b), respectively, show the correlation between δ²H and δ¹⁸O, and temperature in samples collected from the main stream in July 2011. Both are statistically significant – that for δ²H and temperature is δ²H = 1.25T-112.77 (R² = 0.85), while that for δ¹⁸O is δ¹⁸O = 0.15T-16.47 (R² = 0.87). The positive correlations indicate that river water with more negative isotopic values tends to be colder. It is noted in this context that the water temperature increased from 0.5 °C at Section 7 to 12 °C at Section 27, in July 2011. This is, at least in part, due to the decreasing fraction of meltwater, with its lower water temperature, contributing to river flow from the upper basin downward. River water upstream typically has low temperature and more negative isotopic values, while that in the river's lower valley is characterized by relatively high temperatures and isotopic values.

The runoff from alpine watersheds, strongly depending on altitude, is influenced by glacier melt, snow accumulation and snowmelt (Gurtz et al. 1999). The hydrologic systems of mountain river basins with significant proportions of glaciated area are largely controlled by the storage and melting processes, and exhibit maximal runoff during the summer melt season because of the high glacier melt rates and the more moderate melt rates of the snow covered areas (Verbunt et al. 2003). Water from HLG River's main stream in the headwater region is characterized by relatively low values of δ¹⁸O and δ²H – the isotopic fingerprint of glacial- and snow- meltwaters.

• is the δ¹⁸O of river water

• is the δ¹⁸O of groundwater

• is the δ¹⁸O of meltwater, and

• f_M is the proportion of meltwater inputs over total river flow.

To determine the fraction of meltwater inputs in the upper proglacial reach, the δ¹⁸O values observed during the sampling period were assigned – i.e., = −16.3‰ at Section 7, = −16.2‰ at Section 8, = −15.6‰ at Section 11, = −13.5‰ and = −16.7‰. The fraction of meltwater inputs over the total river flow was thus calculated as ranging from 89% at Section 7 to 86% at Section 8 to 67% at Section 11. This confirms that a substantial portion of stream flow in the proglacial zone is derived from meltwater sources, particularly in the upper headwater region.

The study's results show that the waters of the HLG River, which is glacial, and selected non-glacial tributaries, contain a wide range of spatial variation in δ¹⁸O and δ²H. Surface waters tend to be more depleted in ¹⁸O and ²H than groundwater, but more enriched than meltwater. River waters tends to fall on or very close to the LMWL, indicating that evaporation, during both rainfall and streamflow, is quite limited.

The complex hydrologic system in this proglacial zone is reflected by the distinct isotopic composition of surface water in stream flow. Both the isotopic values and temperature of the water from the river's main stream are usually lower than those of its non-glacial tributaries. As a typical glacial river, HLG River has increasing isotopic composition and temperature trends from the upper basin down in its main stream, induced mainly by non-glacial tributary water gain. At Section 27, however, the water becomes more depleted in the heavy isotopes ²H and ¹⁸O than at the nearby Section 25, because of the surface water input from YZG River, whose waters are heavy-isotope-depleted.

Typically, surface waters at higher elevations were more heavy-isotope depleted than those lower down. Regression analysis revealed altitude effects of −0.1‰ per 100-m rise for δ¹⁸O-, and −0.8‰ per 100-m rise for δ²H. The river water's isotopic composition is positively correlated with its temperature – i.e., river water with more negative isotopic values tends to be colder. The isotope hydrology of the alpine proglacial zone depends strongly on altitude and is influenced mainly by ice-snow melt and tributary water inputs.

Finally, in the headwater region, the proportion of meltwater input to total river flow ranges from 67% at Section 11 to 89% at Section 7. This confirms that a substantial portion of stream flow in the proglacial zone is derived from meltwater sources, particularly in the upper headwater region.

This work was supported by the Youth Foundation of Sichuan University under Grant No. 2015SCU11048 and the Key Projects of Natural Science Foundation of China under Grant No. 40730634. The authors are grateful to the Gongga Mountain Alpine Ecosystem Station, Chengdu Institute of Mountain Hazard and Environment, the Chinese Academy of Sciences, Sichuan, China, for supplying necessary meteorological and hydrological data.