Spatiotemporal variability of hydrogen and oxygen stable isotopes in the Han River Basin and the regional hydrological implication

Characterization of the spatiotemporal variability of stable isotopes (δ¹⁸O and δD) in the surface water of the Han River Basin (HRB) is critical for tracing basin-scale hydrological cycle processes, identifying moisture-source dynamics, and optimizing water resource management. Through systematic sampling and analysis of hydrogen and oxygen isotopes in the mainstream, tributary, groundwater, and rainwater of the HRB, we investigated the spatial and seasonal variation in the isotopic composition of water bodies in this area. The Local Meteoric Water Line (LMWL): δD = 7.72δ¹⁸O + 11.55 indicates that the study area is influenced by atmospheric precipitation and exhibits evaporative fractionation. The seasonal variation is closely related to the circulation effect and evaporative fractionation. The summer water isotope values (δ¹⁸O: −8.2‰, δD: −52.5‰) were significantly higher than those in spring (δ¹⁸O: −8.7‰, δD: −58.0‰) and autumn (δ¹⁸O: −8.6‰, δD: −56.6‰). This pattern can be attributed to two main factors: first, the moisture derived from the Western Pacific during summer exhibits inherently heavier isotopic composition (δ¹⁸O: −3.75‰, δD: −18.5‰); second, intensified evaporative fractionation further enriches heavy isotopes in surface waters. Across the Han River Basin, the spatial pattern of δ¹⁸O values follows an “increase-decrease-increase” trend from the Hanzhong Basin to the Qin-Ba Mountains, then to the middle and lower reaches. This trend is primarily controlled by the shifting dominance of three factors: groundwater discharge, tributary inputs, and direct precipitation. This study, for the first time, reveals that the seasonal variations of stable isotopes in surface water of the HRB are driven by circulation effect, providing a new isotopic tracing basis for hydrological analysis of watersheds in monsoon regions.

1 Introduction

Global climate change has become a major topic in Earth Science, with significant impacts on the water cycle (Huntington, 2006; Kundzewicz, 2008; Grover, 2014; Gosling and Arnell, 2016; Qiu et al., 2023). Rising temperatures and changing precipitation patterns are altering the spatial and temporal distribution of water resources, affecting surface and groundwater flow, recharge, and consumption (Alexander et al., 2013). Hydrogen and oxygen isotopes, as essential tracers in hydrological studies, have been widely used to examine various aspects of the water cycle, especially in the processes of evaporation, precipitation, and groundwater recharge (Gonfiantini, 1986; Gammons et al., 2006; Kendall and McDonnell, 2012; Yang and Han, 2020; Ren et al., 2024). It is also possible to effectively track the origin, flow path, and exchange processes of water, providing critical support for accurate watershed hydrological modeling and water resource management (Clark and Fritz, 1997; Risi et al., 2013; Nusbaumer et al., 2017; Hu et al., 2018).

Existing studies have well demonstrated the response of hydrogen and oxygen isotopes in water bodies to climate change, as well as their considerable potential for tracing hydrological cycle processes. Studies in regions such as the Colorado River Basin in the United States (Guay et al., 2006), the Sava River Basin between the Southern Alps, the Dinaric Mountains and the Pannonian Plain in Europe (Ogrinc et al., 2018), and the Indus River Basin (Sharma et al., 2017) have shown that isotopic signatures exhibit diverse characteristics across different climate zones. In China, particularly in major river basins like the Yangtze and Yellow Rivers (Ding et al., 2014; Li et al., 2015; Fan et al., 2017; Wu et al., 2018; Zhang et al., 2018; Peng et al., 2018; Shi et al., 2019; Li C. et al., 2020; Li Z. et al., 2020; Yan et al., 2021; Wu et al., 2022b; Chen et al., 2024), providing valuable data to quantify regional hydrological processes.

The Han River Basin (HRB), as the primary water source for the South-to-North Water Diversion Project, provides vital water supplies for surrounding regions, making it crucial for both economic and social development. Although isotope studies have been carried out in several sub-basins of the HRB, such as the Jinshui River Basin (Bu et al., 2018), Minjia River Basin (Ma et al., 2022), Guanshan River (Zhang Q. Z. et al., 2025), Hanbei Basin (Yan et al., 2022), and the middle-lower reaches (Li et al., 2016), these investigations have primarily focused on qualitative analyses of hydrological processes within individual small watersheds. Comprehensive quantitative research on regional hydrological processes is still lacking in this basin. Furthermore, due to the diverse climate, geography, and topography of the HRB, existing studies have not fully covered all key hydrological processes within the basin. For instance, the isotopic distribution and variation between different water bodies within the basin remain underexplored, indicating the need for further detailed investigations. Therefore, a more thorough study of hydrogen and oxygen isotopic characteristics in the HRB will not only contribute to a better understanding of local hydrological processes but also provide a scientific basis for water resource management and regulation in the South-to-North Water Diversion Project.

Based on this, the present study was conducted to collect precipitation, surface water, and groundwater samples in the basin in spring, summer, and autumn, and to analyze the composition of water stable isotopes. The aim is to systematically reveal the spatial and temporal evolution of isotopic composition of the water in the HRB, to quantify the recharge relationship between different water bodies, and to clarify the key factors influencing the isotope distribution, thus providing solid data support and scientific information for the in-depth understanding of the basin’s water cycle process in HRB and guaranteeing the sustainable use of water resources at the water source of the South-to-North Water Transfer Central Route Project.

2 Materials and methods

2.1 Study area

The Han River is the largest tributary of the Yangtze River (Figure 1), originating from the southern foot of the Qinling Mountains in Shaanxi Province. Its mainstream flows through Shaanxi and Hubei provinces and empties into the Yangtze River at Hankou. The river is 1,577 km long, with a total drainage area of 159,000 km² (Li and Zhang, 2008; 2009; Xu et al., 2011). The upper reaches (above Danjiangkou, about 925 km long) are dominated by medium mountains and low mountains, with deep canyons as the main landform. The middle reaches (from Danjiangkou to Zhongxiang, 270 km) pass through hilly areas, with several intermountain basins distributed between them. The lower reaches (below Zhongxiang, 382 km long) gradually narrow due to artificial embankments. This longitudinal zonation of landforms has shaped the spatial heterogeneity of the hydrological environment in the basin and influenced the temporal and spatial distribution patterns of the hydrological cycle and water resources.

Figure 1

Figure 1. Sampling sites in the Han River Basin (HRB).

The Han River Basin (HRB) is located in the transitional zone of hydrological and climatic characteristics between northern and southern China, showing typical seasonal variations of the subtropical monsoon region (Chen et al., 2019). Meteorological observation data (1951–2020) indicate a significant temperature gradient across the basin, with monthly average temperatures ranging from −2 °C in the northwest source area to 27 °C in the southeast lowlands. The annual average precipitation in the basin is 873 mm, with a highly uneven distribution throughout the year. About 75% of precipitation occurs during the flood season (May to October) (Li et al., 2016; Yuan et al., 2019), as shown in Figure 2, often leading to summer and autumn floods. The discharge of the Han River is mainly replenished by precipitation, with abundant water volume. The intra-annual distribution of the discharge is highly consistent with precipitation, with 5–10 months accounting for 78% of the annual total (Wang et al., 2020).

Figure 2

Figure 2. Monthly variations in precipitation and temperature during the sampling year in the HRB. Data was acquired from https://rp5.ru/.

The lithologic distribution of the HRB is highly complex, with strata from all geological periods ranging from the Precambrian to the Quaternary almost all exposed. Paleozoic and older strata are mainly developed in the upper reaches of the basin, forming the main body of the Qinling Mountains and Daba Mountains (Zhang et al., 1996). The Qinling Mountains are dominated by granite and metamorphic rocks (Deng, 1981), whereas the Daba Mountains are mainly composed of limestone with intercalated metamorphic rocks (Zheng et al., 2013). The Cenozoic and Mesozoic strata are primarily distributed in intermountain basins, grabens, and low-lying areas, corresponding to the formation of medium- to low-relief mountain and hilly landforms (Deng, 1981). Structurally, the Han River Basin spans multiple first-order tectonic units, including the Qinling Fold Belt, the Yangtze Platform, and the Songpan-Ganzi Fold System. The tectonic movement is characterized by folding, block movement, and uplift (Deng, 1981; Wu et al., 2025). The Ankang Fault developed in the study area (Figure 1) is a regional fault located in the southern part of the South Qinling Tectonic Belt, with an overall strike of northwest and north-northwest, and serves as the boundary fault of the Daba Mountain Tectonic Belt (Xie et al., 2022).

2.2 Sampling and analysis

In this study, samples were collected continuously from downstream to upstream in 2023 during spring, summer, and autumn. Sampling points were deployed in the mainstreams, tributaries, groundwater, and rainwater, distributed across different geomorphological units as much as possible. The distribution of sampling points is shown in Figure 1. A total of 67 water samples were collected, including 29 from the mainstreams of the Han River, 18 from tributaries, 17 from groundwater, and 3 from rainwater. River water samples were generally collected below 10 cm of the water surface. Groundwater samples were collected after 10 min of pumping. Two rainwater samples were collected in summer and one in autumn, all of which were single precipitation samples. Rainwater was collected using high-density polyethylene bottles equipped with funnels, and a ping-pong ball was placed at the funnel opening to prevent evaporation. The bottles were immediately sealed with parafilm after precipitation ceased to avoid fractionation. All of the samples were filtered through a 0.22 μm cellulose membrane, and the filtered samples were transferred into dry, clean 50 mL centrifuge tubes.

δ¹⁸O and δD values of all samples were determined by the Liquid Water Isotope Analyzer (IWA-35EP) in the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. The precision of δ¹⁸O and δD measurements was better than ±0.1‰ and ±1.0‰, respectively. Results were reported as relative to the standard V-SMOW (Vienna Standard Mean Ocean Water).

3 Results

3.1 The hydrogen and oxygen isotope composition of waters in the HRB

The δ¹⁸O and δD values for river water, groundwater and rainwater in the HRB are shown in Table 1. The δ¹⁸O value of river water of the whole basin varied from −9.8‰ to −5.3‰ with an average value of −8.7‰. The δD value for the whole basin ranged from −65.0‰ to −38.0‰, with an average of −58.0‰. These results are similar to those of previous studies in the basin: δ¹⁸O: −10.3 to −7.2‰, δD: −70.6 to −48.0‰ (Ding et al., 2014; Li et al., 2015; Deng et al., 2016; Li et al., 2016; Zhang et al., 2016). The ranges of δ¹⁸O and δD for groundwater are relatively narrow (−8.4‰ to −6.2‰ and −55.4‰ to −40.9‰, respectively), with an average δ¹⁸O value of −7.2‰ and δD of −47.3‰. The hydrogen and oxygen isotopic compositions of rainwater showed that the δ¹⁸O values were −5.5‰ and −2.0‰, and the δD values were −30.2‰ and −6.8‰ in summer; while in autumn, the δ¹⁸O value was −6.6‰ and the δD value was −34.4‰.

Table 1

Table 1. Stable isotope compositions and d-excess in the HRB.

The deuterium excess (d-excess) in precipitation is defined as the y-intercept when the slope = 8, expressed by d-excess = δD - 8 × δ¹⁸O (Dansgaard, 1964). This equation could reflect differences in humidity between the water vapor source and evaporation (Rozanski et al., 1993; Clark and Fritz, 1997). The d-excess values in river water ranged from 4.2‰ to 16.1‰ with a mean value of 12.2% (Figure 3).

Figure 3

Figure 3. Relationship between the d-excess and δ¹⁸O of the water samples in the HRB.

3.2 Seasonal variations of δ¹⁸O and δD of waters in the HRB

As shown in Figure 4 and Table 1, the hydrogen and oxygen isotope compositions of samples collected across different seasons were highly variable. Most river water samples exhibited relatively higher δ¹⁸O and δD values in summer (−8.2‰ ± 0.7‰ and −52.5‰ ± 4.2‰, respectively) compared to spring (−8.7‰ ± 0.9‰ and −58.0‰ ± 6.0‰, respectively) and autumn (−8.6‰ ± 0.7‰ and −56.6‰ ± 4.2‰, respectively). Meanwhile, the stable isotope composition of tributaries showed a greater range of variation than that of the mainstream, most evident in spring, reaching 4.5‰ and 27.0‰, respectively. The stable isotopic composition of groundwater showed no significant seasonal variation (Table 1), with average δ¹⁸O and δD values of −7.2‰ ± 0.5‰ and −47.3‰ ± 3.7‰, respectively. The coefficient of variation (CV) for hydrogen and oxygen isotopes in the mainstream and groundwater of the HRB exhibits minimal seasonal fluctuations, maintaining within a narrow range of −9% to −5% throughout three seasons. The CV of tributaries in spring peaks at −16% for δ¹⁸O and −14% for δD, significantly higher than those in summer and autumn (−12% for δ¹⁸O and −11% for δD). This characteristic is highly consistent with the information presented in Figure 4. The outlier points in Figure 4 are distributed in the middle and lower reaches, where human activities are intensive. These human disturbances locally disrupt the isotopic signal of tributaries, resulting in abnormally high δ¹⁸O values (−7.1‰ to −5.8‰) and δD values (−47.4‰ to −38.3‰) and thus increasing the CV in this region.

Figure 4

Figure 4. Seasonal variations of δ¹⁸O (A) and δD (B) in mainstreams and tributaries. The circled outlier samples are all located in the middle and lower reaches, an area characterized by intense human activity.

3.3 Spatial variations of δ¹⁸O and δD of waters in the HRB

In general, from the source region to the river mouth, the δ¹⁸O and δD values exhibited evident spatial variation (Figure 5; Table 1). Topography and geomorphology directly regulate the spatial differentiation of river water isotopic compositions by altering the altitude effect of precipitation recharge and the retention and mixing processes of water bodies within the basin. Based on topographical features, the Han River Basin is divided into three zones: the Hanzhong Basin, the Qin-Ba Mountain, and the Middle and Lower reaches. We employed Principal Component Analysis (PCA) and One-way Analysis of Variance (ANOVA) to validate the credibility of this spatial classification (Figure 6). The results confirm significant isotopic heterogeneity among the units (ρ < 0.001) and illustrate a clear evolutionary trajectory from the headwater basin through the mountainous transition zone to the downstream plains, providing a solid statistical foundation for the division. The δ¹⁸O and δD values of the mainstream exhibited similar trends in spring and autumn across the upper reach (Hanzhong basin and Qin-Ba Mountain) (Figures 5A,B). In the Hanzhong Basin, the values initially rose, then continuously declined in the Qin-Ba Mountain area until they reached the lowest point (at Ankang, spring: −9.7‰ for δ¹⁸O, −64.5‰ for δD and autumn: −9.5‰, −61.6‰), and then slowly rose again. The δ¹⁸O and δD values of the mainstream flowing through the Hanzhong Basin fluctuated in summer, remaining at a relatively high level. Upon reaching the Qin-Ba Mountain area, the values increased rapidly and then stabilized. Upon entering the middle and lower reaches of the Han River, the isotope values in all three seasons increased sharply at HJ9 (Xiangyang) (spring: −8.9‰, summer: −7.2‰, and autumn: −7.1‰). Subsequently, they remained stable in spring but showed a decreasing trend in summer and autumn. Tributaries in the Hanzhong basin exhibited a trend opposite to the mainstream, decreasing first and then increasing (Figures 5C,D). In the Qin-Ba Mountain area and the middle and lower reaches, they showed a continuous increasing trend. Similar to seasonal variations, the spatial CV for mainstream and groundwater is relatively low (−9% to −5%), indicating a uniform spatial distribution. The CV for tributaries is significantly higher (−16% to −11%), reflecting pronounced spatial differentiation and high dispersion among different tributaries.

Figure 5

Figure 5. Variations of hydrogen and oxygen isotopes in mainstreams (A, B), and tributaries (C, D) along the flow path in the HRB. The filled circles and squares represent the δ¹⁸O and δD of the mainstream, respectively and the empty circles and squares represent the δ¹⁸O and δD of the tributary, respectively.

Figure 6

Figure 6. (A, B) Boxplot of original δ¹⁸O values and δD values for the three regions, with One-way ANOVA results indicating significant differences (p < 0.001). The boxes represent the 25th–75th percentiles, and the whiskers indicate the range. (C) Principal Component Analysis (PCA) biplot of the sampling sites based on standardized variables (elevation, river gradient, δ¹⁸O, and distance to the source). The three clusters correspond to the geomorphological divisions, illustrating the longitudinal evolution from the headwaters to the plains.

4 Discussion

Rivers are complex dynamic systems in which δ¹⁸O and δD values record key water cycle signals. These isotopic signatures are controlled by water mixing, recharge-discharge relationships, and evaporative fractionation. Analyzing their correlation with environmental factors enables quantification of different water sources and reveals the impact mechanisms of hydrological and meteorological processes (Hitchon and Krouse, 1972; Gat, 1996; Kendall and Coplen, 2001; Bershaw et al., 2012).

4.1 The relationship between the δ¹⁸O and δD values of the river water and meteoric water in the HRB

As the primary source of surface water, precipitation imparts a close relationship between the hydrogen and oxygen isotopic composition of river water and that of the precipitation within its catchment (Poage, 2001; Dutton et al., 2005; Reckerth et al., 2017). Based on the analysis of precipitation samples from around the world, Craig (1961) established the Global Meteoric Water Line (GMWL): δD = 8δ¹⁸O + 10. During the transport of water vapor sources in atmospheric precipitation across regions, environmental changes cause imbalances in gas-liquid isotope fractionation, resulting in the phenomenon of linear deviation of the Local Meteoric Water Line (LMWL) from the GMWL (Dansgaard, 1964; Clark and Fritz, 1997). Given the absence of systematic rainwater sampling and the lack of atmospheric precipitation isotope monitoring data covering the entire HRB in the present study, we conducted a systematic review of prior researches: specifically, studies targeting the upper reaches (Zhang, 1989; Bu et al., 2018) and those focusing on the middle-lower reaches (Wang, 2021) have each completed year-long (or longer) systematic analyses of atmospheric precipitation isotopes. By synthesizing these published datasets, the LMWL for the HRB was determined as δD = 7.72 δ¹⁸O + 11.55 (Figure 7). Compared with the GMWL, the LMWL of the HRB shows a slightly lower slope and a slightly higher intercept. This phenomenon indicates that isotopic nonequilibrium fractionation occurs during the precipitation process in the basin, which is induced by sub-cloud secondary evaporation, mixing of multiple water vapor sources, and the evaporative environment in certain regions (e.g., the middle and lower reaches of the basin).

Figure 7

Figure 7. δD vs. δ¹⁸O values for natural water in the HRB. The black line is the Local Meteoric Water Line (LMWL, δD = 7.72 δ¹⁸O + 11.55) (Zhang, 1989; Bu et al., 2018; Wang, 2021). The black dashed line is the Global Meteoric Water Line (GMWL, δD = 8δ¹⁸O + 10) (Craig, 1961). The green dashed line represents the River Water Level (RWL) in spring, the red dashed line represents the RWL in summer, and the blue dashed line represents the RWL in autumn.

Based on the results obtained in the surface water, the relationship between δD and δ¹⁸O of Han River water in different seasons is as follows:

$\delta \mathrm{D}=6.25\times {\mathrm{\delta }}^{18}\mathrm{O}-3.68\left(\text{Spring}\right)(1)$

$\delta \mathrm{D}=5.74\times {\mathrm{\delta }}^{18}\mathrm{O}-5.64\left(\text{Summer}\right)(2)$

$\delta \mathrm{D}=5.77\times {\mathrm{\delta }}^{18}\mathrm{O}-6.89\left(\text{Autumn}\right)(3)$

The δD and δ¹⁸O values of water in the Han River are all distributed near the LMWL (Figure 7), indicating that the primary water source is atmospheric precipitation recharge. The slopes of the δD-δ¹⁸O relationship Equations 1–3 are all less than 8, suggesting that precipitation-recharged water has been subject to superimposed evaporative fractionation during its transport and retention.

4.2 Controlling factors of seasonal variation

4.2.1 Seasonal scale circulation effect

In this study, we observed that the δ¹⁸O and δD values of river water in the HRB were significantly higher in the summer compared to the spring and autumn (Figure 4). However, previous studies on the Yangtze River Basin (Chen et al., 2024; Li C. et al., 2020) and the Mekong River Basin (Noipow, 2015; Le Duy et al., 2018), which are also located in the monsoon region, have shown that summer precipitation is regulated by the “amount effect,” with its δ¹⁸O and δD values generally being negative. Since rivers are mainly recharged by precipitation, the stable isotope values of river water also exhibit the characteristic of being the lowest in summer, which is exactly contrary to the results of this study. Tan et al. (2016) reported in the eastern China monsoon region that δ¹⁸O variation in atmospheric precipitation is mainly controlled by circulation effect and shows no significant correlation with precipitation amount. When precipitation water vapor originates from the Western Pacific Ocean, the δ¹⁸O value of precipitation is relatively high, whereas if the water vapor comes from the Indian Ocean, the δ¹⁸O value of precipitation is low. The water vapor transport in the Han River Basin happens to have both of these two sources (Sun et al., 2011; Li et al., 2023). It can be concluded that the seasonal differentiation of hydrogen and oxygen isotope compositions of river water in the study area can be attributed to the differences in the sources of water vapor.

Notably, the southeastern moisture channel exhibits a significant enhancement exclusively during the summer months (Deng et al., 2024). The Western Pacific water vapor is typically enriched in heavier hydrogen isotopes, with δ¹⁸O values between −5.7‰ and −4.8‰ (Tao et al., 2021; Zhang J. et al., 2025). This is consistent with the higher δ¹⁸O isotope values (−5.5‰ to −2.0‰) observed in the rainwater samples collected during the summer. Therefore, the higher isotope values of river water in the summer in the HRB are likely related to the Western Pacific water vapor, which is transported to the basin through the southeast monsoon. In contrast to the Western Pacific water vapor in the summer, the water vapor in the spring and autumn is mainly sourced from the Indian Ocean (Deng et al., 2024). According to the studies by Tao et al. (2021) and Zhang J. et al. (2025), the δ¹⁸O values of Indian Ocean water vapor generally range from −6.4‰ to −5.1‰, which are lower than those of the water vapor in the Western Pacific. This isotopic distinction leads to a lower δ¹⁸O value (−6.6‰) in precipitation during autumn compared to summer, which is subsequently reflected in the river water, exhibiting correspondingly lower δ¹⁸O and δD values during these seasons.

4.2.2 Evaporation effect

Evaporation plays a critical role in shaping the isotopic composition of river water, particularly during warmer months (Martinelli et al., 1996; Taniguchi et al., 2000; Hua et al., 2019; Ren et al., 2024). During evaporation, lighter isotopes (¹⁶O and ¹H) are preferentially vaporized, leaving the heavier isotopes (¹⁸O and ²H) concentrated in the remaining water (Dansgaard, 1964; Simpson and Herczeg, 1991). In summer, the high temperatures increase the evaporation rate, which results in a higher concentration of heavier isotopes in the river water (Figure 8). In contrast, temperatures in spring and autumn are lower, and evaporation is less intense, leading to lower isotopic values in river water. But in spring and autumn, the δ¹⁸O values exhibited a more pronounced temperature effect. This is because precipitation is relatively scarce in spring and autumn (Cao et al., 2024), allowing temperature to play a more dominant role in controlling isotopic fractionation. In contrast, during summer, the correlation weakens due to the complex input of marine moisture. Similar findings are reported by Tang et al. (2015) and Lin et al. (2024), both of which indicate that the temperature effect dominates the variation in precipitation isotope values during the non-monsoon season. Table 2 summarizes the controlling factors of seasonal variations in the isotopic values of atmospheric precipitation in monsoon regions, and a growing body of research has confirmed that the “amount effect” is no longer the dominant factor governing such seasonal variations.

Figure 8

Figure 8. The relationship between water temperature (WT) and δ¹⁸O in river water. The gray line indicates a positive correlation between δ¹⁸O and WT.

Table 2

Table 2. Summary of seasonal variations in precipitation isotopes in regions affected by circulation effect.

4.3 Controlling factors of spatial variation

In this study, the river water isotopes exhibited high spatial heterogeneity from the upper to lower reaches (Figure 5). As hydrogen and oxygen isotopes show a strong correlation (Figure 7) and exhibit identical spatial variation patterns (Figure 5), this indicates that both are governed by the same environmental factors and fractionation mechanisms. Therefore, we selected δ¹⁸O to describe this common trend.

4.3.1 Controlling factors in the Hanzhong Basin

In the Hanzhong Basin, the δ¹⁸O values in the mainstem exhibited a clear seasonal pattern: a consistent increase was observed during spring, elevated values with fluctuations were maintained in summer, and an initial rise followed by a subsequent decline occurred in autumn (Figure 9). These variations reflected shifts in dominant hydrological processes across seasons. The δ¹⁸O variation in spring and autumn was controlled mainly by changing mixing ratios between groundwater and tributary inputs. In summer, it was governed primarily by the combined effects of precipitation recharge and evaporative fractionation.

Figure 9

Figure 9. Spatial variations of δ¹⁸O values of the mainstream, tributary and groundwater in the HRB: (A) Spring, (B) Summer, and (C) Autumn.

Groundwater displayed higher and more stable isotopic values (Spring: −7.5‰; Summer: −7.4‰; Autumn: −7.3‰) due to its integrated nature from historical precipitation. During spring and autumn, reduced precipitation and lower tributary discharge - exemplified by the Xushui River’s spring flow being only 10% of its summer volume (Jia, 2021) - resulted in increased groundwater contribution as baseflow. This led to rising δ¹⁸O values in the mainstem. Similar groundwater influences on riverine isotopes have been documented in several major river systems, including the Tarim River (Zuo et al., 2025), Yarlung Tsangpo River (Yan et al., 2021), and Nenjiang River (Cui et al., 2022). The autumn decline in δ¹⁸O values resulted from enhanced tributary contributions. The Xushui and Youshui Rivers delivered substantially higher discharges in autumn (18 and 6 m³/s, respectively, Cjh.com.cn, 2025) compared to spring (8 and 2 m³/s), increasing the input of isotopically depleted water to the mainstem and consequently reducing δ¹⁸O values.

During summer, precipitation served as the main recharge source under the East Asian Summer Monsoon influence. Summer precipitation contained relatively enriched δ¹⁸O signatures (see Section 4.2.1), while high temperatures intensified evaporative fractionation from the mainstem. Although tributaries brought depleted waters, these dilution effects were counterbalanced by the dominant enriching processes, maintaining high and fluctuating δ¹⁸O values in the mainstem.

4.3.2 Controlling factors in the Qin-Ba Mountains

After the mainstream of the Han River flowed out of the Hanzhong Basin and entered the terrain dominated by the Qin-Ba Mountains, the δ¹⁸O values of the mainstream first decreased and then increased. As shown in Figure 9, during spring and autumn, this variation in the mainstream aligned with that of the tributaries, indicating that tributary inflows primarily drove the changes in isotopic composition in this segment. However, this correlation was almost absent in summer, suggesting the influence of other dominant factors.

The spatial variation in δ¹⁸O values of the tributaries cannot be explained by a single isotopic effect but rather results from the combined influence of moisture sources and topographic structure. Regional studies and observations indicate that during the rainy season, the Qin-Ba region receives monsoon moisture from different pathways, including the southwestern channel from the Indian Ocean/Bay of Bengal and the southeastern channel from the Western Pacific/South China Sea (Song et al., 2021). Differences in moisture sources and transport paths affect precipitation isotopes through Rayleigh distillation and rainout processes (Sun et al., 2022; Wu et al., 2022a). In this context, the Ankang Fault developed within this region can be regarded as a topographic/tectonic transition zone (Xie et al., 2022). To the west of the fault, the steep North Daba Mountain Thrust Belt experiences intense tectonic uplift, which strongly blocks moisture with depleted isotopic composition transported from the southwestern direction (Song et al., 2021). Additionally, the forced uplift of water vapor induces initial condensation here, resulting in relatively negative δ¹⁸O values in both precipitation and tributary runoff. To the east of the fault, precipitation and tributaries are primarily influenced by moisture derived from the southeastern direction (Song et al., 2021), which is characterized by relatively enriched isotopic values. Furthermore, the river channel gradually widens in this segment, leading to prolonged water exposure, increased retention and evaporation, all of which contribute to the relative enrichment of δ¹⁸O in the tributaries.

When these tributaries converged into the mainstream, they significantly influenced the δ¹⁸O values of the mainstream through conservative mixing process in spring and autumn. Combined with measured oxygen isotope values, the impact of tributary inflow on the isotopic composition of the mainstream was calculated using the mass conservation formula (Equation 4).

${\mathrm{\delta }}^{18}{\mathrm{O}}_{mix}=x·{\mathrm{\delta }}^{18}{\mathrm{O}}_t+\left(1-x\right)·{\mathrm{\delta }}^{18}{\mathrm{O}}_m(4)$

where x represents the contribution rate of tributaries. δ¹⁸O_mix represents the δ¹⁸O values of the mainstream after tributary confluences, while δ¹⁸O_t and δ¹⁸O_m represent the isotope values of the tributary and mainstream, respectively. As shown in Table 3, the average contribution rate of tributaries is 47%, with a maximum of up to 93%.

Table 3

Table 3. Calculation results of mass conservation formula.

However, during summer, as the river flowed out of the Hanzhong Basin and entered the Shiquan area, it often received strong convective rainfall from the Qin-Ba Mountains, as well as surface runoff generated by short-duration intense mountain storms (Li et al., 2018; Xiao et al., 2024; Wang et al., 2025). Summer storm events were frequently accompanied by the “amount effect” – the input of large amounts of fresh precipitation often led to lighter isotopic values in runoff, causing a decrease in δ¹⁸O values of the mainstream (Li C. et al., 2020; Li Z. et al., 2020; Ma et al., 2022; Zhang et al., 2023; Chen et al., 2024). Additionally, when the river flowed out of the basin into gorges or narrow channels, the current velocity could increase instantaneously, reducing evaporative enrichment, causing a short-distance decline in the mainstream’s δ¹⁸O values. However, on a larger spatial scale, the influx of isotopically heavier moisture from the Western Pacific, combined with high evaporation effect and significant water retention (e.g., valley widening and reservoir storage), collectively leads to the relative enrichment of heavier isotopes in this river segment (Ding et al., 2014; Li C. et al., 2020; Tao et al., 2021; Zhang J. et al., 2025). On the other hand, although tributaries are numerous, the increased volume of the mainstream during the high-flow period reduced the dilution effect of tributary inputs on the overall isotopic composition (i.e., decreased flow proportion), thereby diminishing the dominant role of tributaries on the mainstream’s δ¹⁸O (Chen et al., 2008; Gao et al., 2018).

4.3.3 Controlling factors in the middle and lower reaches

As the mainstream enters the middle and lower reaches of the Han River, the δ¹⁸O values exhibit an increasing trend, with a smaller increase in the spring and a more pronounced rise in the summer and autumn. The highest values are observed in Xiangyang (Figure 9). The isotopic values of tributary and groundwater (−6.6‰ and −7.0‰, respectively) also showed an increasing trend and were significantly higher than those of the mainstream (−8.5‰), suggesting that tributary and groundwater in this section were not the primary factors causing the isotopic value changes in the mainstream. In Wang’s (2021) study, the multi-year average δ¹⁸O value for precipitation (−9.0‰) within this region aligns with those of mainstream (−8.5‰), indicating a dominant influence from atmospheric precipitation.

In the middle and lower reaches, the terrain is relatively flat, allowing water vapor from the western Pacific to penetrate far inland. Ingraham and Taylor (1991) conducted an analysis of isotopes in precipitation in coastal regions and found that during the transport of water vapor, heavy isotopes are continuously removed by precipitation along the pathway, leading to progressive depletion of heavy isotopes in the remaining vapor. Accompanied by the continental effect (Dansgaard, 1964; Rozanski et al., 1993), the δ¹⁸O of river water becomes progressively lighter from the estuary toward upstream areas. Furthermore, as rivers enter the plains, the channel slope decreases, flow velocity slows, and water residence time increases, which enhances evaporation (Deng et al., 2016; Hua et al., 2019; Ren et al., 2024). This process preferentially removes lighter isotopes from the water, concentrating heavier isotopes and further driving an increase in δ¹⁸O values. Seasonal variations in precipitation also play a critical role in controlling isotopic changes in the mainstream. Precipitation in summer and autumn is generally larger and typically exhibits lighter isotopic characteristics (lower δ¹⁸O values). However, enhanced evaporation preferentially removes the lighter isotopes, causing δ¹⁸O values to rise, which accounts for the difference in isotopic increase between spring and summer–autumn seasons.

At the specific confluences where the Tangbai River joins the Han River, the higher δ¹⁸O values (−6.6‰ in spring and −6.0‰ in autumn) of the tributary cause a localized anomaly, leading to a sharp increase in the δ¹⁸O of the mainstream, with a contribution rate reaching 36% (Table 3). However, this change is confined to a small area and does not significantly affect the overall spatial trend of the isotope composition. According to end-member mixing analysis theory, recharge source only exerts a notable influence on the isotope composition of the mainstream when its flow constitutes a substantial proportion of the mainstream’s flow (Hewlett and Hibbert, 1967; Liu and Kao, 2007; Ma et al., 2022). Given that tributaries in the middle and lower reaches contribute little to the mainstream (Table 3, average 18%), they primarily disrupt isotopic signals at the local scale rather than acting as dominant factors. The variation in δ¹⁸O of the mainstream, with its larger flow, is predominantly controlled by precipitation and evaporation.

5 Conclusion

In this study, we investigated the stable isotope characteristics of water bodies in the HRB. The results indicated that the hydrogen and oxygen isotopic compositions of water bodies in this watershed exhibited distinct spatial and seasonal variation patterns. Overall, stable isotope variations in the HRB were influenced by precipitation and underwent evaporative fractionation. Seasonally, influenced by atmospheric precipitation circulation effects and evaporation, the δ¹⁸O and δD values are higher in summer than in spring and autumn. Spatially, hydrogen and oxygen isotopic compositions in the mainstream are higher in the Hanzhong Basin, primarily controlled by shifts in the mixing ratio of groundwater and tributaries. However, since our study did not obtain groundwater flow data, we cannot quantify the contribution ratio between the two sources. Future research may consider incorporating hydrological models (such as SWAT) to conduct quantitative studies on this controlling factor. The Qin-Ba Mountains, the δ¹⁸O and δD values in mainstream first increase and then decrease. This trend is controlled by tributary inflow during spring and autumn, while in summer the stable isotope values rise due to heavy rainfall and intense evaporation. Entering the middle and lower reaches, the δ¹⁸O and δD values generally show an increasing trend, primarily influenced by precipitation and following the continental effect. By clarifying the recharge distribution patterns across different periods and the recharge types in distinct sub-regions, the spatiotemporal variations in the isotopic compositions of river water within the Han River Basin can furnish a core scientific underpinning for cross-seasonal water regulation, inter-regional water allocation, and long-term sustainable water resource planning.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

NW: Data curation, Formal Analysis, Investigation, Methodology, Writing – original draft, Writing – review and editing. Y-NY: Conceptualization, Formal Analysis, Methodology, Resources, Supervision, Writing – original draft, Writing – review and editing. J-WZ: Conceptualization, Formal Analysis, Methodology, Resources, Supervision, Writing – review and editing. M-LH: Data curation, Formal Analysis, Investigation, Writing – original draft. DZ: Conceptualization, Formal Analysis, Investigation, Supervision, Writing – review and editing. Y-CF: Data curation, Formal Analysis, Investigation, Writing – original draft. G-SZ: Conceptualization, Formal Analysis, Investigation, Resources, Supervision, Writing – review and editing. Z-QZ: Conceptualization, Formal Analysis, Funding acquisition, Investigation, Project administration, Resources, Supervision, Validation, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was supported by the National Natural Science Foundation of China (No. 42373058, 42573054), Special Fund for Basic Scientific Research of Central Colleges, Chang’an University (No. 300102274203), and the Henan Province Science and Technology Research Project (No. 252102320220).

Acknowledgements

The authors thank Xue Tian, Chen Hao, and Gao Zhenpeng for helping in the field work.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer DM declared a shared affiliation with the authors NW, J-WZ, M-LH, G-SZ, Z-QZ to the handling editor at time of review.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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