Online system, http://www.scar.ac.cn Sciences in Cold and Arid Regions 2011, 3(6): 0463–0467 DOI: 10.3724/SP.J.1226.2011.00463

Climatic changes have led to significant expansion of endorheic lakes in Xizang (Tibet) since 1995

YinSheng Zhang *, TanDong Yao, YingZhao Ma

Key Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of Tibetan Plateau Re- search, Chinese Academy of Sciences, Beijing 100085, China

*Correspondence to: Dr. YinSheng Zhang, Key Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of Tibetan Plateau Research, Chinese Academy of Sciences. No. 18, Shuangqing Road, Beijing 100085, China. Email: [email protected]

Received: 12 June 2011 Accepted: 10 August 2011

ABSTRACT

Robust climate warming has led to significant expansion of lakes in the central Tibetan Plateau. Using remote sensing data, our quantitative analysis indicates that Siling Co, a saline lake in a characteristic endorheic basin in the central region of the Plateau, has expanded more than 600 km2 in area since 1976. Particularly since 1995, the lake has signif- icantly expanded in response to increasing precipitation, decreasing water surface evaporation caused by weaker winds and less solar radiation, and increased glacier meltwater draining to the lake. Glacier–lake interactions are important in governing lake expansion and are also part of a feedback loop that influences the local climate. Worsening climatic conditions (decreased precipitation and increased temperatures) that could have caused the lake to shrink during 1976–1994 were offset by increasing glacier meltwater feeding the lake, which made the lake nearly stable. We demonstrate that this pattern changed during 1995–2009, when glacier meltwater actually decreased but participation runoff increased and evaporation decreased, leading to expansion of the lake. If climatic conditions became suitable for further lake development, which would be indicated by expansion in lake area, glacier meltwater could be saved in a stable reservoir. Keywords: endorheic lake; climatic change; Xizang (Tibet)

1. Introduction crease over the Plateau during the period of 1955–1996 were about 0.16 °C/decade for the annual mean, which There are more than one thousand lakes in the Qing- exceeded those for the Northern Hemisphere and the hai-Xizang (Tibet) Plateau with a total area of 44,993.3 same latitudinal zone in the same period. Such robust km2, accounting for 50% of the total area of lakes in warming will lead to significant changes in the water China (Wang and Dou, 1998). This region is an ideal site cycling system in this cryosphere dominant region, in- in which to evaluate the effects of climatic changes on cluding novel glacier–lake interactions (Yao and Zhu, the water resource systems on the Plateau and in the 2006). neighboring regions. Changes in lake levels may lead to Systematic studies have shown that lakes are im- a series of environmental problems and an alteration of portant hydrological and climatological components of the hydrologic regimes on various time scales. the Tibetan Plateau terrain system. The changes in lake Recent work analyzing recorded temperature (Liu area and the variation in lake chemical properties can and Chen, 2000) showed that the major portion of the greatly alter the earth surface conditions and can influ- Plateau has experienced statistically significant warming ence the general atmospheric circulation (Yu and Harri- since the mid-1950s. The linear rates of temperature in- son, 1995; Qin and Huang, 1998). However, because of 464 YinSheng Zhang et al., 2011 / Sciences in Cold and Arid Regions, 3(6): 0463–0467

the arid climate over most of the Tibetan Plateau, rain- The development of remote sensing technology pro- fall- and precipitation-generated runoff evaporates rapid- vides an effective means for research of lake and glacier ly. Lake stability is closely related to glacier meltwater dynamics. Especially in recent years, the development of flowing into them (Zhu et al., 2006). Research has high-resolution remote sensing satellites provides robust shown that an increase in lake area and rise in water level technical support (Khromova et al., 2006). Using remote resulted from glacier meltwater supply in, for example, sensing data, we made a quantitative analysis of lake Nam Co (Lu et al., 2005). The sequential changes in the variation, and explored lake area response to climate Plateau lakes have shown that, at an early stage, down- change using meteorological observation data. stream lake shrinkage was caused by the increased evap- oration, but lake areas soon enlarged due to an increase 2. Expansion of Siling Co Lake in glacier meltwater supply caused by climate warming (Zhu et al., 2006). However, with continued climate In Qiangtang Basin, a characteristic endorheic basin warming, glacier meltwater discharge will eventually in the central region of the Tibetan Plateau (30°N–35°N, tend to decrease, and the quantity of lake water will also 80°E–95°E), there are 497 lakes with areas larger than 1 decrease (Kang et al., 2002). km2. Table 1 lists the locations of the three largest en- By influencing the hydrological cycle of surface dorheic lakes in the area and their changes during water and water phase variety, temperature and precipi- 1976–2009, which were estimated from TM images from tation variations strongly affect lake–glacier interac- the U.S. Geological Survey (http:// edcsns17.cr.usgs.gov/ tions, and also impose some feedback on the climate EarthExplorer/). During 1976–2000, Namu Co, Siling Co, system (IPCC, 2007). In the Tibetan Plateau, most of and Zharinanmu Co expanded their areas at rates of 5.4, the lake areas are sparsely populated, with very poor 11.0, and 2.4 km2/yr, respectively, while Namu Co re- natural conditions due to natural and physical con- mained the largest lake in the region. In the decade from straints. Lacking data of systematic observations, it is 2000 to 2009, increases in lake area reached 2.7%, difficult to acquire details of lake–glacier alterations. 24.1%, and 1.2%, for Namu Co, Siling Co, and More importantly, lakes and glacier catchments have Zharinanmu Co, respectively. Siling Co has expanded not yet been considered together (Liu, 1995), which more than 700 km2 in area since the mid-1970s, becom- prevents an adequate understanding of the links among ing the largest endorheic lake in the region and currently lake and glacier changes. the second largest saline lake in China.

Table 1 Change in area of the three largest endorheic lakes on the central Tibetan Plateau

Location Change in lake area (km2) Lake Long. (E) Lat. (N) a.s.l. (m) 1976 1990 2000 2009 Namu Co 89°21′–91°23′ 29°56′–31°07′ 4,724 1,824.8 1,939.1 1,959.8 2,013.4 Siling Co 88°33′–89°21′ 31°34′–31°51′ 4,535 1,617.0 1,709.3 1,887.0 2,341.2 Zharinanmu Co 85°20′–85°54′ 30°44′–31°05′ 4,611 917.6 976.8 977.7 989.2

Siling Co is located in Bangoin County, Tibet station in the basin, Shenzha, which is located at 88°38′E, (88°33′–89°21′E, 31°34′–51°00′N), with an elevation of 30°57′N at 4,672 m a.s.l. Records from 1961–2009 show 4,535 m a.s.l. and an area of 2,341.2 km2 (from this study). that average annual mean temperature, precipitation, Figure 1 illustrates detailed changes of Siling Co during wind speed, and relative humidity were 0.7 °C, 315 mm, 1976–2009, and its connection to Geladandong Glacier. 3.9 m/s, and 42%, respectively. Like other regions of The Siling Co catchment is a characteristic endorheic ba- Tibet, the region is characterized by strong radiation and sin with an area of 45,530 km2 in the central Plateau, with powerful evaporation, with average annual sunshine the Tanglha and Nyamqentanglha mountains toward the duration of 2,950 hours and annual pan-evaporation of south. Its topography is characterized by high peaks, a 2,080 mm. broad lake, and square-shaped altiplanos. There are 642 glaciers with a total area of 593.09 km2 in the basin, 3. Climatic and glacier changes which serve as major water sources for the lake. The region is little affected by human activity, so the For the period 1970–2009, Figure 2 plots: (a) lake and glaciers evolve mainly through natural condi- changes in the area of Siling Co, which were deduced tions and climate change. The climate is typical for the from TM images and previous reports (Chen et al., Plateau, dry and cold, with its annual precipitation main- 2001; Bian et al., 2010); (b) annual precipitation; (c) ly occurring from May to September (Liu and Chen, annual pan-evaporation; (d) wind speed; (e) lower 2000). Due to inclement natural conditions, there are few cloudiness; and (f) air temperature. Estimates of (b) to (f) hydrological observations and only one meteorological were deduced from data collected at Shenzha Station in YinSheng Zhang et al., 2011 / Sciences in Cold and Arid Regions, 3(6): 0463–0467 465

the Siling Co Basin. between 1995 and 2009. Pan-evaporation, an indicator of Figure 2a reveals significant expansion of the Siling lake water evaporation, has decreased since 1995, and Co lake after 1995, with an increase in area of 619 km2, totaled 1,898 mm. The decreased water evaporation can accounting for 85% of the expansion during 1976–2009. be explained by weaker winds (Figure 2d) and atmos- Correspondingly, annual precipitation changed from pheric optical dimming indicated by positive changes in negative trends to positive, and accumulated 846 mm lower cloudiness (Figure 2e).

Figure 1 Location of Siling Co and Geladandong Glacier. Hatched lines indicate changes in the lake’s area between 2000 and 2009.

Figure 2 The time period of 1975–2009: (a) changes in area of Siling Co, which were deduced from TM images and previous reports (Chen et al., 2001; Bian et al., 2010), (b) annual precipitation, (c) annual pan-evaporation, (d) wind speed, (e) lower cloudiness, and (f) air temperature 466 YinSheng Zhang et al., 2011 / Sciences in Cold and Arid Regions, 3(6): 0463–0467

An additional cause of lake expansion might be an ladandong Glacier (Figure 3). Results indicated that the increase in glacier meltwater drainage to the lake, as il- meltwater at ELA on Geladandong Glacier has increased lustrated in Figure 1. Employing global glacier meltwater since 1976 in accordance with rising temperatures (Fig- equations (Kotlyakov and Krenke, 1982), we estimated ure 2), and accumulated to a total of 9,790 mm w.e. (wa- annual melting at equilibrium line altitude (ELA) on Ge- ter equivalent) between 1976 and 2009.

Figure 3 Annual and accumulated meltwater at equilibrium line altitude (ELA) on Geladandong Glacier

4. Discussion 2 and 3. We summarized the periods between 1976–1994 and 1995–2009, and verified that a significant increase in The water balance of a lake is governed by various lake area has occurred since 1995. We accounted for water fluxes (e.g., on-lake precipitation, lake evaporation, contributions of increased precipitation to changes in surface runoff, groundwater) and these fluxes are con- lake area by converting the amount of precipitation (mm) trolled by many climatic and hydrologic processes. Pre- to lake drainage water. We determined the runoff coeffi- cipitation is certainly an important source of water for cient by the method of Chen et al. (2009), who measured Tibetan lakes, but other water fluxes and other climate water levels of Namu Co during 2004–2008. We esti- variables can be equally important. For example, changes mated the annual runoff coefficient to be 0.33 on a simi- in air temperature and in ice cover can have a significant lar ground surface. We estimated lake surface water effect on lake evaporation (Hostetler 1991; Hostetler and evaporation from pan-evaporation observed at Shenzha Giorgi 1995; Qin and Huang 1998; Small et al., 2001). Station, and this was converted to lake surface area by The relative importance of these factors is as yet un- using a ratio of 0.614, which was deduced for Qinghai known. Lake by Qu (1994). The glacier meltwater at ELA was In Table 2 we elucidate lake changes in response to treated as an average value of the entire glacier. Here, we the climatic and glaciological changes shown in Figures assumed the meltwater was fed into the lake with no loss.

Table 2 Possible contributions of changes in precipitation, lake surface evaporation, and meltwater from the glacier

Factor 1976–1994 1995–2009 Notes Change in area of Siling Co (km2) +106 +619 −4,904 +7,381 Runoff coefficient = 0.33 (Chen et al., Drainage to lake (mm, ratio) −83% +53% 2009) −256 +1,981 Calculated by precipitation minus Lake surface water fluxes (mm, ratio) −17% +14% evaporation 5,208 4,582 Glacier melt to lake changes (mm, ratio) 100% 33% Total area of glaciers = 593 km2 Total (mm) −2 +13,944

As shown in Table 2, 1976–1994 was a period of total of 5,208 mm after drainage water was fed into the lake area stability, when decreases in precipitation and lake. During the period of significant lake expansion increases in water evaporation were balanced by large between 1995–2009, although contribution of glacier amounts of glacier meltwater, which accumulated to a melt was decreased more than 600 mm in drainage wa- YinSheng Zhang et al., 2011 / Sciences in Cold and Arid Regions, 3(6): 0463–0467 467

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