The YYB (90°06′-91°41′E, 28°08′-29°13′N), covering an area of about 9064 km² with an average elevation of 4500 m above sea level (Zhang et al., 2012), is the largest closed lake basin in south Tibet (Figure 1). Located in Langkazi and Gongga counties, Shannan Prefecture, Tibet, the northern basin is separated by Ganbala Mountain from the Yarlung Zangbo River, the southern basin is bounded by Mengdagangri Snow Mountain, the western basin borders Nianchu River Basin with the Karola Glacier as the watershed, and the eastern basin is adjacent to the Zheguco Basin (Guan et al., 1984). With the alpine bush-steppe semiarid climate in south Tibet, the annual average temperature and precipitation in the YYB were 2.9°C and 365.7 mm, respectively according to the monthly observations of Langkazi Meteorological Station during 1961-2014. The annual average water surface evaporation was 1219.6 mm according to the daily observations of Baidi Hydrological Station during 1975-2014. Accompanied with sparsely scattered small arbors, the main vegetation type in the YYB, namely, the meadow is composed by Gramineae, Compositae, and Ranunculaceae (Yu et al., 2010). Livestock husbandry is the dominant human activity for the YYB. Tourism is also playing an increasingly important role in local economic development.

Major lakes in the YYB include Lake Yamzhog Yum Co, Kongmu Co, Chen Co, Bajiu Co, and Puma Yum Co. As one of the holy lakes in Tibet, Lake Yamzhog Yum Co (90°22′- 91°03′E, 28°27′-29°12′N) is the largest inland lake on the northern foothills of the Himalayas with a water surface of about 588.9 km² (Sun et al., 2013a). Supplied mainly by precipitation, the lake level has dropped significantly since 1974 (Ye et al., 2007; Chu et al., 2012; Li, 2014). There are 7 major inflowing rivers around the lake, i.e., Gamalin River, Kadongjia River, Quqing River, Xiangda River, Puzong River, Kaluxiong River, and Yajian River (Figure 1). Located about 40 km southward from Lake Yamzhog Yum Co, Lake Puma Yum Co (90°13′-91°33′E, 28°30′-28°38′N) is the second largest lake in this basin with an area of about 285.7 km² (Tian et al., 2012). The Jia River, accounting for 77% of the total inflow into the Lake Puma Yum Co, is sourced from the glacial melt in the southwestern part of the basin (Zhu et al., 2006). An open channel connected to the Kadongjia River was excavated on the lake’s eastern side, through which the water drained into Lake Yamzhog Yum Co during high lake level period.

Water samples were field collected annually in the YYB from 2009 to 2014. A total of 52 sampling sites were set in this basin (Figure 1), comprising 24 surface water sites and 12 deep water sites (0.5 m, 10 m, 20 m, and 30 m, respectively, beneath the water surface along 3 profiles) in Lake Yamzhog Yum Co, 6 surface water sites in Lake Puma Yum Co, and 10 surface water sites in other 3 lakes and 7 rivers. According to the coordinates of global positioning system (GPS, Trimble Juno SB, USA) and field investigation records, sampling sites were kept as consistent as possible. In addition, historical observation data in 1979 and 1984 were extracted trying to give a whole picture of the hydrochemical evolution in the YYB during the past dacades.

All the water samples were collected using pre-washed high-density polyethylene (HDPE) bottles wrapped with parafilm immediately after sample collection. Parameters including water temperature (T), pH, and total dissolved solids (TDS) were measured in situ by utilizing a multi-parameter water probe sensor (HANNA HI9828, Italy). While hydrochemical ions were tested in the laboratory at the Institute of Geographic Sciences and Natural Resources Research (IGSNRR), Chinese Academy of Sciences (CAS), the cations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ were measured by an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin-Elmer Optima 5300DV, USA) with a precision of ±5%. An ion chromatography system (ICS, Shimadzu LC-10ADvp, Japan) with the accuracy about ±0.04% was adopted for the analysis of Cl^- and SO₄^2-. CO₃^2- and HCO₃^- were titrated by double-indicator method. Mineralization degree (MD) was summed up by all major ions. And total hardness (TH) was calculated with the following equation (Maidment, 1993):

TH (mg/l as CaCO₃) = 2.497×Ca (mg/l) + 4.118×Mg (mg/l)

Plotted by GW-Chart (Winston, 2000), Piper trilinear diagram (Piper, 1944) was used for elucidating the dominant ions and hydrochemical types of waters in the YYB. Distinct zones in Piper diagram display different hydrochemical facies with defined cation and anion concentrations (Cui and Li, 2014). Based on Kriging interpolation method (Stein, 1999), the contour maps of hydrochemical concentrations were drawn in ArcGIS 10.2 (ESRI, 2013) to describe the spatial variation of hydrochemical features. Meanwhile, Gibbs model (Gibbs, 1970) was introduced to reveal the natural control mechanism of hydrochemistry, namely, the influences of precipitation, rock-water interaction, and evaporation on dissolved salts in waters.

Waters in the YYB were weakly alkaline, with pH values ranging from 8.38 to 9.49 (Table 1; Figure 2a). Most waters exceeded the permitted pH values for drinking (6.5-8.5) stipulated by Chinese Ministry of Environmental Protection (2002). As the TH values of the Yajian River and Gamalin River were 127.32 and 174.68, respectively, they were classified as hard waters (121 < TH < 180 mg/l; Maidment, 1993). All other waters in the YYB were the hardest water (TH > 180 mg/l), with TH values ranging from 207.05 to 1327.68 mg/l (Table 1 and Figure 2b). Specifically, the TH values of Lake Bajiu Co and Yamzhog Yum Co were higher than the recommended value of the World Health Organization (WHO) for drinking water (500 mg/l). In addition, the lakes were slightly saline (TDS > 1000 mg/l; Maidment, 1993), with the TDS reaching 1550.67 and 1065.71 mg/l in Lake Bajiu Co and Yamzhog Yum Co, respectively (Table 1; Figure 2c). Most other waters were fresh (TDS < 1000 mg/l). In particular, the relatively low TDS values of Lake Kongmu Co (209.33 mg/l) and Puma Yum Co (227.69 mg/l) were indicative of diluting effects of glacial melt on soluble salts in water, resulting from the inflow of the Kaluxiong River and Jia River, respectively. Compared with the lakes, the rivers contained lower TDS, ranging from 110.50 to 269.17 mg/l, which probably be ascribed to lower flow velocity at the river mouth, or a higher rate of evaporation in the lakes compared with the rivers. Almost all rivers in the YYB, however, contained higher TDS than the mean value of the world’s rivers (115 mg/l; Gaillardet et al., 1999), which could be attributed to intense evaporation over the TP.

(1) For the Yamzhog Yumco Basin (YYB)

Diversified hydrochemical types with various dominant ions were obtained in the waters of the YYB (Figure 3). Mg²⁺ and Na⁺ constituted the major cations, and SO₄^2- plus HCO₃^- made up the major anions for waters in Lake Yamzhog Yum Co (Figure 3a), indicating its SO₄^2--HCO₃^--Mg²⁺-Na⁺ type. The hydrochemical types of Lake Bajiu Co, Kongmu Co, Chen Co, and Puma Yum Co were SO₄^2--Mg²⁺-Na⁺, SO₄^2--HCO₃^--Ca²⁺, SO₄^2--Na⁺-Mg²⁺-Ca²⁺, and HCO₃^--SO₄^2--Mg²⁺-Ca²⁺, respectively. While for the inflowing rivers, the hydrochemical types were HCO₃^--Ca²⁺ and SO₄^2--Ca²⁺ with Ca²⁺ concentrations accounting for more than 57% of the cations (Figure 3b), which were particularly different from that in Lake Yamzhog Yum Co and Bajiu Co. The lower Ca²⁺ concentrations in lakes should be related to the intense evaporation and subsequent precipitation of Ca²⁺ from the lake waters in the form of carbonate (Zhu et al., 2010b).

The weathering of rock material also contributes to the hydrochemical composition of water, and this contribution can be estimated from carbonate hardness, represented by the concentrations of Mg²⁺ and Ca²⁺ in the water, as carbonate hardness is in equilibrium with dissolved carbonates (Li et al., 2013). For the waters of Lake Puma Yum Co, Gamalin River, and Yajian River, the predominant impact factors were weathered carbonates with their carbonate hardness exceeding 50% distributed in Zone I in Piper diagram (Figure 3). While weathered evaporates were dominating factors with non-carbonate hardness exceeding 50% for the waters of Lake Bajiu Co, Kadongjia River, Quqing River, and Puzong River located in Zone II. Weak water-rock interactions were detected in Lake Yamzhog Yum Co, Kongmu Co, and Chen Co, in view of their samples situated in Zone V with no anion-cation pair exceeding 50% (Figure 3).

(2) For the Lake Yamzhog Yum Co

The complex zigzag shoreline of Lake Yamzhog Yum Co, coupled with mixed inflow from various river systems, had resulted in inhomogeneous mixing of the waters, giving rise to significant spatial differences in the hydrochemistry of its surface waters. The MD values, as well as the molar concentrations of (Na⁺+K⁺) and Mg²⁺, were low in the river mouth entering into Lake Yamzhog Yum Co (Figures 4a-4c). Obviously, inflowing river water played an essential role as a diluting agent. Moreover, the narrow shape and steep northwestern lakeshore made slope runoff an important factor contributing to the low MD values in this region (Figure 4a). In contrast, higher Ca²⁺ molar concentrations occurred proximal to river mouths, due to the high Ca²⁺ values of the inflowing river waters (Figure 4d). While pervasively low Ca²⁺ concentrations were maintained throughout Lake Yamzhog Yum Co because of the large amount of Ca²⁺ precipitated in carbonates during periods of intense evaporation and concentration. These hydrochemical characteristics are similar to those of most alpine lakes on or near the TP (Ju et al., 2010). The concentrations of Cl^-, SO₄^2-, and (HCO₃^-+ CO₃^2-) ranged among 7.78%-8.18%, 52.20%-53.71%, and 38.19%-39.98%, respectively.

The irregularity of hydrochemical variations with depth (Figure 5) was related to disturbances caused by strong winds over the TP, and abundant rainfall during the sampling period. Additionally, the warming climate accelerated glacial melt, resulting in less pronounced variations with water depth during 2009-2014 than was observed in 1979. In summary, waters in Lake Yamzhog Yum Co were mixed uniformly in the vertical direction, which was in agreement with the previous studies about the water spatial variation based on stable isotopes (Zang et al., 2014).

Ion source and climatic change were the two primary impact factors of the hydrochemical characteristics.

(1) Source of ions

The source of hydrochemical ions was explored through a Gibbs diagram (Figure 6). Water samples from Lake Yamzhog Yum Co, Chen Co, and Bajiu Co contained relatively high TDS and yielded high Na⁺/(Na⁺+Ca²⁺) values, plotting in the evaporation-crystallization zone in Figure 6. The influence of weathered rock material introduced to the lakes by inflowing rivers was likely the cause of the low Cl^-/(Cl^-+HCO₃^-) values, as shown by samples from Lake Kongmu Co and seven rivers, which plotted in the rock dominance zone (low TDS; Figure 6). Similar position was observed for the samples from Lake Puma Yum Co in the Gibbs diagram. Higher Na⁺/(Na⁺+Ca²⁺) ratios, however, reflected the combined effects of evaporation and crystallization.

As noticeable effects on the hydrochemical compositions of lake waters in the YYB, the source of ions in river waters was further analyzed. Weathered rocks are the major source of dissolved salts in river waters due to limited precipitation and waste pollution in this region (Smolders et al., 2004). 50% of solutes in the world’s rivers originate from carbonates, 17.2% from evaporates, and 11.6% from silicates (Meybeck, 1987). The rock types that have been weathered to yield the solutes can be determined from the major ions in the waters, e.g., HCO₃^- in river waters is derived mainly from the weathered carbonates, Cl^- and SO₄^2- from evaporates, Na⁺ and K⁺ from both evaporates and silicates, and Ca²⁺ and Mg²⁺ may from all three of these rock types under natural conditions (Chen et al., 2002). The types of weathered rock affecting the hydrochemistry of rivers in the YYB hence were uncovered by analyzing the ratios of major ions in waters (Figure 7). Considering the relatively high concentrations of (Cl^-+SO₄^2-) compared with HCO₃^-, solutes in the waters of Xiangda River, Quqing River, Kaluxiong River, Puzong River, and Kadongjia River were derived from weathered evaporates where they flowed above (Figure 7a). Conversely, the high HCO₃^- concentrations in the waters of Gamalin River and Yajian River indicated that ions in these waters were derived predominantly from the weathering of carbonates. Weathered evaporates constituted a source of solutes in Yajian River, which contained almost equal proportions of (Cl^-+SO₄^2-) and HCO₃^-. In addition, all the rivers contained almost equal proportions of (HCO₃^-+SO₄^2-) and (Ca²⁺+Mg²⁺) (Figure 7b), indicating that the dissolution of calcite, dolomite and gypsum could be crucial reaction in river systems. Relatively high (Ca²⁺+Mg²⁺) values revealed weathered carbonates were not the primary source of ions in the river systems (Figure 7c), and additional sources of Ca²⁺ and Mg²⁺ were balanced by Cl^- and SO₄^2- (Zhang et al., 2011). The greater proportion of (Ca²⁺+Mg²⁺) and SO₄^2- relative to alkalis metal ions (Figure 7d) and Na⁺ (Figure 7e) , respectively, were indicative of the influence of Ca²⁺-Mg²⁺-SO₄^2--rich rocks (e.g., gypsum) on the hydrochemistry of river waters. The relationship between SO₄^2- and Na⁺ suggested mirabilite was a potential source of Na⁺ in Gamalin River owing to the coordinate relationship between SO₄^2- and Na⁺ (Figure 7f). And halite was responsible for Na⁺ in Puzong River, Kaluxiong River, Quqing River, and Kadongjia River, because they contained equal proportions of Na⁺ and Cl^- (Figure 7f). Na⁺ in Xiangda River was probably originated from silicates, as no correlation between SO₄^2-, Cl^-, and other metal ions is observed.

(2) Climatic change

Climatic change contributes to hydrochemical variations in the YYB, particularly in Lake Puma Yum Co. The annual average temperature and annual precipitation increased at a rate of 0.4°C/10a and 22.1 mm/10a, respectively, between 1984 and 2014. Compared with field hydrochemical investigations in 1984 (Chen, 1990), the salinity of Lake Puma Yum Co for the period of 2009-2014 had been decreased obviously (Zone I in Figure 3a), which should be a result of accelerated glacial melt driven by a warming climate. While both ionic compositions and hydrochemical types had changed little in the other waters of the YYB (Figure 3a), which can be explained with water balance of the closed lake system. Specifically, inflow runoff and lake-surface precipitation were the major inputs, lake evaporation was the major output, and water leakage and inaccuracy were the residuals. Sun et al., (2014) reported that variations in the water volume of Lake Yamzhog Yum Co showed no clear relationship with system outputs, but strongly correlated with inputs, including increased precipitation and melting water. In addition, underground water leakage and the operation of the Yanzhog Yumcuo Pumped Storage Power Station probably exacerbated water loss. Therefore, although evaporation was greatly enhanced under the climatic warming, little change was detected in the hydrochemical properties of the closed lake system of the YYB. To better identify the influences of climatic change on the hydrochemical regime and its mechanism of alpine lakes, more regular water sampling should be conducted at a higher spatial resolution. Moreover, future studies should focus on quantifying each component of the lake water balance.