Distribution of δD and δ18O in the Changjiang river water
The water on the land surface is predominantly replenished by precipitation during the global water cycle. As a result, δD and δ18O in the river water are determined by the isotopic composition of precipitation and thus may serve as a good proxy for precipitation isotopic composition [1]. However, in some cases, the δD and δ18O in river water are different from the local precipitation due to evaporation, transpiration from different altitudes [4]. In particular, for the large rivers, the δD and δ18O of river water may vary significantly from the precipitation as a result of the “catchment effect” [29]. In Fig. 3, the regression line of δD and δ18O for the Changjiang river water is δD = 8.5 × δ18O + 15.5 (R2 = 0.98), displays a more pronounced slope and higher intercept than GMWL [30]. However, the correlation of river water δD and δ18O in accordance with local meteoric water line (LMWL) observed in specific cities in the Changjiang catchment, e.g., δD = 7.34 × δ18O + 2.56‰ (R2 = 0.98; Kunming site), δD = 8.47 × δ18O + 15.46‰ (R2 = 0.99; Changsha site) and δD = 8.43 × δ18O + 17.46‰ (R2 = 0.98; Nanjing site), which are calculated based on archived precipitation data retrieved from GNIP [31].
Compared to the riverine samples, the lake samples show the highest δD and δ18O values and significant variations, which suggest that the lake water in the Changjiang catchment suffers from more significant evaporative fractionation than river water. The river water samples from Chongqing in the upstream are more depleted in both δD and δ18O than those from the lakes in the middle reaches and from Nantong and Datong downstream. δ18O depletion in the Changjiang river water with increasing elevation is probably due to the “continent effect/elevation effect”, which has been thoroughly investigated by [10, 19].
These substantial differences in isotopic ratios between Changjiang sub-basins are primarily fashioned by different isotopic compositions in the local precipitation, and partly by the combined effects of evaporation, groundwater replenishment, and perhaps snow melting in the mountain area in each specific sub-basin [10, 19, 25].
The seasonal variations of δ18O in Nantong river water from lower Changjiang reaches show a similar time series with other large rivers in the world, e.g., the Amazon [32], Danube [33], Mekong [34]. The seasonal variations of water isotopes for large rivers are believed to be dominated by the water isotopic composition of precipitation [9, 35]. Except the precipitation, snowmelt water can also have significant impact on large river water isotopic composition. For example, runoff with snowmelt water having a low δ18O causes clear seasonality with low δ18O in Lena river water isotope time series, while contribution of summer precipitation to Lena river water is not significant [36]. The overall seasonal variations of water isotopes for large rivers are complex and can be influenced by many factors like precipitation, snowmelt, groundwater and so on.
Seasonal variation of δ18O for the precipitation in a basin-scale
A deep insight into the spatial distribution of precipitation isotopes will improve our understanding of large-scale catchment water cycling dynamics. In the 1980s and 1990s, GNIP has been systematically monitored over the precipitation δD and δ18O in the Changjiang catchment. Among the 37 GNIP stations in China, 8 stations were set up within the Changjiang catchment to collect the primary isotopic data of precipitation across the basin. In most of the mid- and high-latitude land areas, the precipitation δ18O is enriched in summer and depleted in winter because both the δD and δ18O are positively correlated with temperature [37]. However, this seasonal isotopic trend was reversed in the coastal region of East China, e.g., the Changjiang catchment (Fig. 4a), where δD and δ18O values are higher in winter than in the summer [28, 38]. During the past decades, many investigators have attempted to explain the isotopic characteristics of precipitation observed in East China, and most agree that the isotopic characteristics of precipitation are strongly regulated by the distinct monsoon system and topography in East Asia [28, 38,39,40,41,42].
The Changjiang River basin is located primarily in the subtropical and temperate climate zone, and most parts of the basin prevailed by the subtropical monsoon climate. Two types of monsoon current flow through the basin in a year, the Siberian northwest winter monsoon and the Asian southeast summer monsoon (or Indian southwest summer monsoon in the upper Changjiang reaches) [28]. Changjiang River discharge sourced primarily from precipitation. The mean annual precipitation was 1057 mm, made up of an approximate total discharge of 1912 × 109 m3/year. In general, the summer monsoon initiated in April and impacts the Changjiang basin then retreats in October, which determines the timing of the rainy season from May to October, and most precipitation concentrates from July to August [43]. However, the spatial and temporal variations of precipitation in the Changjiang catchment are rather intricate and highly associated with the prevailing monsoon system and diverse topography. As a whole, about 70–90% of the total annual precipitation occurred from May to September, with significant temporal variations, but no apparent long-term trend observed over the last five decades (Fig. 2a). On the other hand, the spatial distribution of rainfall is particularly irregular [44]. Overall, the rainfall in general decreases from the southeast (lower reaches) to the northwest (upper reaches) of the basin because of the migration of monsoon-induced rain front from the southeast to the northwest of the coastal region in China [43].
Previous studies on precipitation water isotopes in the Changjiang catchment were based primarily on the data from the individual meteorological station, while the basin-scale isotopic characteristics have been scarcely investigated. As the spatial distribution of the rainfall is exceptionally irregular in the Changjiang basin, the rainfall amount and its H and O isotopic composition can vary by region, especially between the upper and lower reaches [28, 38, 40]. The Changjiang river water, in particular from the lower reaches, integrates the water from the entire basin. Therefore, a more comprehensive comparison of δD and δ18O between river water and basin-scale precipitation is critical to understand the influence of precipitation on river water. In this regard, our study provides the first practical synthesis of basin-scale precipitation δ18O. As shown in Fig. 4a, the basin-scale δ18O in the precipitation is nearly the same in each site, but slightly varies due to variable climate and topography in the catchment. Our calculation indicates that the precipitation δ18O is high (~ − 4‰) from January to May, but decreases rapidly in June and remains in a lower value (~ − 10‰) until October. The δ18O then rises back to a higher value (~ − 4‰) and maintains till the next year. Although our calculation may somewhat underestimate the complex precipitation processes in each sub-basin (Fig. 5 and Eq. (1)), dividing the basin into different sub-catchments based on its key tributaries does allow for a first-order estimation of δ18O in a basin-scale, made a significant difference compared to the previous attempts.
In comparison, the 2-year time series Changjiang river water δ18O in Nantong reveals similar seasonal variations with the precipitation, yielded high δ18O in winter and low δ18O in summer (Fig. 4a), suggests a potential correlation between river water and precipitation. The river water δ18O variation in Datong also displays a similar trend as that observed in Nantong, while the δ18O variation in Chongqing is different from those in Nantong and Datong. The river water δ18O in Chongqing is exceptionally high in May and June 2004. The reason behind the phenomenon remains unclear, but may be attributed to some specific climate or local events. Regardless of the several abnormal values observed in May and June, the river δ18O in Chongqing exhibits similar seasonal variations with those in Nantong and Datong, though the variation between winter and summer is not as notable as the latter two sites.
In contrast, the seasonal variations of δ18O in Poyang Lake are remarkably different from the other sites, showing a V-pattern with the lowest δ18O observed in June and July in the first monitoring year (Fig. 4a). This feature to some extent may be determined by evaporation depending on the local temperature and humidity. Apart from the trends, there are differences observed in absolute δ18O values between the precipitation and Changjiang river water. For instance, both the absolute values and the δ18O range in the Changjiang river water are smaller than that of the precipitation. This is probably due to the derivation of river water, primarily from precipitation upstream of the sampling location (i.e., at higher elevations), though the upstream δ18O values are often lower than local precipitation δ18O, particularly in catchments with high elevation gradients [29]. Different isotopic ranges between river water and precipitation could be resulted from the catchment which receiving water tracer (e.g., δ18O) inputs that were transported across diverse flow paths through the unsaturated and saturated zones as tracers migrate through the sub-surface toward the stream network [45], thus the amplitudes of δ18O can be significantly dampened in river water relative to those of precipitation.
Despite the discussion above, the seasonal variations of δ18O in the Changjiang river water are generally consistent with the basin-scale pattern of precipitation δ18O. Combined with the findings in this research, it is believed that the long-term δ18O time series in the Changjiang river water is mainly dominated by local precipitation. Nevertheless, it is worth mentioning that the isotopic signature of a river, especially for the enormous river, is multifaceted, and can be altered by many hydrological processes, such as evaporation in the lakes and river surface [46], transpiration of vegetation [47, 48], and recharge from groundwater [49]. These hydrological processes, however, are beyond the scope of this study that focuses on the temporal variation of stable isotopes rather than the constraint of absolute isotopic value in the given catchment.
Our calculation on δ18O in precipitation over the Changjiang catchment may yield significant uncertainty due to limited isotopic observation stations available (mostly from GNIP) and antiquity of dataset (mainly in the late 1980s and 1990s). Nevertheless, this work provides the first attempt to estimate the seasonal variability of precipitation δ18O of the basin-scale and offers a quantitative isotopic comparison between river water and precipitation. As shown in Fig. 2a, the monthly precipitation average from Nanjing (GNIP station) in the lower Changjiang River is generally constant albeit the large seasonal fluctuations from 1987 to 1993, while a longer basin-scale precipitation record (Fig. 2b) in the Changjiang River also reveals a similar trend since the 1950s [15]. At the same time, the nationwide precipitation amount in China increases only by 2% from 1960 to 2000 [50] that conforms with the findings in the Changjiang catchment. Similar to the precipitation pattern, the stable H and O isotopic compositions in precipitation (e.g., δ18O of precipitation in Nanjing, Fig. 2a) yielded significant seasonal variations but an overall constant annual average during the past several decades. Similar observations have also been recently reported [38] based on the dataset provided by the Chinese Network of Isotopes in Precipitation (CHNIP). In this case, our estimation of basin-scale δ18O in precipitation over the Changjiang catchment may serve as a long-term dataset, provides essential background for the future isotopic study in the Changjiang River.
Water mixing determines the seasonal variation of δ18O in the lower Changjiang river water
The seasonal variations of δD and δ18O in river water from large rivers made a useful proxy to investigate the catchment hydrology, impacts of climate change and human activities on river discharges, and can be used to structure and validate on newly proposed hydrological models [22, 51]. However, the reasonable interpretation of seasonal stable isotopes variation depends mostly on the understanding of the hydrological setting and meteorological conditions of the river. For instance, the low-resolution isotopic sampling or hydrological monitoring may hinder the discovery of subtle relations between river water isotopes and local hydrological settings.
For the Changjiang river system, major tributaries are primarily located in the upper reaches of the Three Gorges except for the Hanjiang River, while many lakes, including the two largest lakes, Dongting Lake and Poyang Lake, are located in the middle and lower reaches. The Changjiang water discharge into the East China Sea is hence largely determined by the upstream contribution (regulated by Yichang gauging station) and the discharges from the Dongting and Poyang lakes (Fig. 4b). The two most significant freshwater lakes, Poyang Lake and Dongting Lake, are located in the middle, and lower Changjiang reaches, with basin coverage up to 4125 km2 and 4040 km2, respectively. Both lakes receive river water from several tributaries as well as the Changjiang mainstream (Fig. 1), which exerts an essential role in buffering the flood from the Changjiang upstream during the flood season. Here, we employ a simple conceptual model to quantitatively estimate the relative water contributions from the upper reaches and from the lakes to the lower Changjiang mainstream.
The water discharges data from Dongting Lake, Poyang Lake, and Hanjiang River were obtained from the gauging stations in Chenglingji, Hukou, and Huangzhuang, respectively. The river water discharge in Nantong is derived from the Datong gauging station as there is no regular gauging station in Nantong, and no significant tributaries exist between these two sites. Locations of each gauge station are shown in Fig. 1. Daily river water discharge data (November 2012 to December 2014) for all these stations are sourced from Changjiang Wuhan Waterway Bureau (http://yu-zhu.vicp.net/) and plotted in Fig. 4b. Apparently, the Hanjiang River only accounts for a small proportion (less than 5%) of total discharge in Datong, which will not be considered in the following calculation.
The total water discharge from the Dongting Lake, Poyang Lake, and Yichang upstream is very similar to that of Datong (Fig. 4b), suggesting that these three end-members predominantly supply the water discharge to the lower Changjiang mainstream in Datong. As Dongting and Poyang lakes have very similar temporal variations in water discharges (Fig. 4b) and isotopic compositions (Fig. 3), their total water discharges to the Changjiang mainstream are categorized under the same unit, as lake contribution. The river water across Nantong, therefore, can be simplified to merely two main end-members, i.e., the source (1) from the Yichang upstream and (2) from the two lakes in the middle reaches (Fig. 6). In this case, the daily water contribution from the lakes to water discharge in Nantong can be simply calculated by Eq. (2):
$${\text{Lake}}\;{\text{contribution}} = \frac{{{\text{Dis}}_{\text{L}} }}{{{\text{Dis}}_{\text{L}} + {\text{Dis}}_{\text{YC}} }},$$
(2)
where DisL and DisYC represent the water discharges from the lakes and the Yichang station, respectively. The calculated lake contribution to Changjiang discharge is presented in Fig. 4c. From the periods of November 2012 to June 2013 and March to July 2014, the Changjiang water discharge in Nantong is primarily sourced from the lakes. In other months during the monitoring period, the upper catchment upstream of Yichang has supplied much water to the lower Changjiang mainstream.
The injection of the lake water with higher δ18O due to evaporation [25], to the Changjiang mainstream, causes elevated δ18O in the Changjiang river water downstream of the lakes (Figs. 3 and 4a) [10, 19, 26]. From the periods of July 2013 to late February 2014, and from July to December 2014, the river water δ18O values in Nantong are relatively low, which corresponds to the reducing water contribution from the lakes. It is interesting to note that the δ18O in the lower Changjiang river water does not vary synchronously with the lakes’ water contribution, but with a time lag (Fig. 4b). A non-linear correlation analysis between the δ18O time series and daily lake contribution indicates that a forward shift of the δ18O curve by about 17 days yields the best correlation (R2 = 0.69) between these two curves (Fig. 4c). This finding suggests that the seasonal variations of river water δ18O in Nantong are closely related to the water mixing from the Changjiang upstream and the lakes (Dongting and Poyang Lakes). In addition, the 17-day time lag between river water isotopic signals and lake water contribution indicates that it takes about 2 weeks for the river water to travel from the middle reaches to the river mouth. Consequently, the δ18O signal in Nantong was 2 weeks lagged after the river water mixing in the middle reaches. It is notable that the actual water traveling time in Changjiang River may vary in different seasons and the 17-day water traveling time is only an average river water traveling time from the middle reaches to the river mouth.
In conclusion, the time-series investigation of river water isotopic signatures and water discharges at the four key hydrometric stations in the Changjiang River demonstrates that the river water δ18O variation in Nantong is in general defined by local precipitation, but more directly related to the river water mixing in middle-lower reaches. The water contributions from the upstream and tributary lakes thus determine the daily δ18O variation in the Changjiang river downstream.
Damming impacts on the water cycle and river water isotopes in the lower Changjiang mainstream
Nowadays, the water discharge in the Changjiang mainstream at Yichang station is controlled mainly by the anthropogenic regulation (impounding/releasing) of the Three Gorges Reservoir (TGR) since its first impoundment in 2003. The impact of TGR on the Changjiang water discharge is not confined to only annual scale [16, 52, 53], but also seasonal scale [21, 54]. The regulation of TGR impoundment determines the water discharge downstream in Yichang and consequently controlled the water mixing between the Changjiang mainstream and the lakes [26]. From August to December 2013, the TGR was impounded (indicated by the high water level in TGR) to lower the flood risk to the downstream region, which subsequently resulted in a higher contribution of the lake water to the lower mainstream. On the contrary, the TGR released the water from December 2013 to May 2014 for shipping and irrigation, which resulted in a higher upstream contribution relative to the lake contribution.
To better expose the damming effect on the water mixing and stable isotopes in the lower Changjiang river water, we examined the relationship between the water samples δ18O and the frequency (%) of its occurrence in the mid-lower Changjiang (Fig. 6). The data indicate that δ18O variability (δ18O range in X-axis) in the upper mainstream (Chongqing) above the TGR is more substantial than that in the mid-lower Changjiang mainstream (Nantong and Datong) and Poyang Lake. The Nantong and Datong sites are located downstream of TGR. The river water sourced from the upper Changjiang basin retained in the vast reservoir is well-mixed during the TGR impoundment may result in a homogenized isotopic signal. Furthermore, there are two prominent δ18O peaks observed in the river water in Nantong and Chongqing in Fig. 6 that correspond to the two notable δ18O values measured during the summer and winter (Fig. 4a). However, these temporal features are less pronounced in the water samples from Datong and Poyang Lake; values generated were probably insignificant statistically due to fewer water samples available.
It is noteworthy that apart from the damming effect, other environmental factors such as evapotranspiration and the mixing of groundwater can also impact the δ18O in river water [47, 48]. These influences are, however, hard to be quantified thus clarified in this paper due to limited stable H and O isotopic data retrieved from the soil, vegetation, and underground water in the catchment. Further modeling work may help in making a quantitative assessment of the evapotranspiration contribution to the river water δ18O possible in the future.