Abstract:
The
inland lakes in the Tibetan Plateau (TP), with closed catchments and
minimal human disturbance, are important indicators to climate change.
However, examination of the spatiotemporal patterns of the Tibetan lake
changes, especially for water level variation, was usually limited by
inadequate measurement accessibility. This obstacle has been remedied by
the developing satellite altimetry observations. The more recent studies
revealed the growth tendency of lakes in the central TP had been
decelerated or reversed during the period 2010-2016. It has not been
systematically investigated whether the deceleration or hiatus would
last in the following years thus far. This study aims to combine the
traditional and recently-advanced altimetry observations to update our
understanding of Tibetan lake changes in recent years. The results
reveal that water level changes of the 22 examined lakes showed abrupt
rises during the period 2016-2018, but the onsets and magnitudes of the
rises varied among the lakes. During the study period, the water level
of the nine lakes in the northern TP displayed a drastic rising trend
with an average rate of 0.82 m/a. In the central TP, the lake level
changes were generally divided into two categories. The water levels for
the lakes in the western CTP rose rapidly, while in the eastern CTP, the
lake water levels rose slowly with an average rising rate less than 0.40
m/a. The water levels for lakes in the northeastern TP and northwestern
TP kept a stably rising tendency. According to the results of climate
analysis, the spatial differences of the lake level rise rates were
primarily caused by the spatial and temporal changes of precipitation
over the TP, which may be related to the large-scale atmospheric
circulation affected by the El Niño and La Niña events.
Key words :
Tibetan
Plateau, lake, water level, satellite altimetry, climate change,
precipitation
1 Introduction
Lakes are an important part of global hydrological and biogeochemical
cycles, as well as an essential resource for human societies (Lehner and
Döll 2004; Palmer et al. 2015; Sheng et al. 2016). These inland
waterbodies are quite sensitive to climate change and human activities.
The last several decades have witnessed the widespread and tremendous
changes in lakes in the context of global changes at regional and global
scales (Shi et al. 2014; Smith et al. 2005; Song et al. 2013; Vörösmarty
2002; Wang et al. 2014). Lake changes have profound effects on regional
water balance, ecosystems, biogeochemical cycles, exchange of energy and
tracing gases with the atmosphere, and human water consumption.
Therefore, investigation of lake variability is critical for a wide
range of socioeconomic, political, and scientific interests. Several
recent efforts such as Pekel et al. (2016), Donchyts et al. (2016), Yao
et al. (2019), and Busker et al. (2019) elucidate lake dynamics at
global scale using multi-temporal
satellite
optical and altimetry observations. The prominent characteristics
revealed in these studies suggest that many large lakes, such as Caspian
Sea, Aral Sea, Urmia Lake in endorheic basins of Eurasian hinterland,
shrank remarkably in the near decades(Wang et al. 2018). On the
contrast, widespread lake growths are observed on Earth’s highest
plateau, the
Tibetan
Plateau (TP).
The TP, known as “Asian Water Tower”, accommodates vast distributions
of glaciers and lakes. There are more than 1200 lakes larger than 1
km2, covering an inundation area of
~43,000 km2 (recorded around 2000)
(Zhang et al. 2014). The lack of historical in situ data in the remote
and sparsely populated TP has greatly limited the investigation of the
hydrological and climate responses of lakes to the changing climate.
However, this obstacle has been reduced by the rapidly emerging
satellite observations from various sensors. Many prior studies employed
optical satellite images to examine lake changes across space and time
in the TP (Huang et al. 2011; Lei et al. 2014; Lu et al. 2017; Mao et
al. 2018; Qiao et al. 2019; Song et al. 2014a; Song et al. 2014b; Wan et
al. 2016; Zhang et al. 2019; Zhu et al. 2010). However, the acquisition
of high-quality optical images is usually impeded by unfavorable weather
conditions, and the derived lake area variations cannot directly
indicate the water budget changes. Therefore, more and more satellite
altimetry measurements have been synergized for the investigation of the
lake dynamics across the TP.
Phan
et al. (2012), Song et al. (2014b), Wang et al. (2013), and Zhang et al.
(2011) have examined the changes in the lake water levels in the TP and
found that most Tibetan lakes experienced rapid water level rises during
2003–2009 by using the Ice, Cloud and land Elevation Satellite (ICESat)
laser altimetry data. However, the long revisit cycle and sparse
sampling of the ICESat mission is insufficient for tracing long-term or
abrupt changes (e.g. sudden shift) in water level, resulting in an
incomplete misunderstanding of the trends and heterogeneous
characteristics of lake changes in the TP. Favorably, the height
measurements of satellite radar altimeters that have been launched
continuously since the early 1990s can extend lake level observations to
a longer temporal coverage. The advantages are applications that are not
affected by weather and a reasonable high frequency of revisit from
monthly to ten-day time scales, so that we can better solve the spatial
and temporal patterns of water level changes and driving mechanisms in
the TP. The combination of multi-radar altimetry missions, including
TOPEX/POSEIDON, ERS-1/-2, Envisat, Cryosat-2 and Jason-1/-2/-3, at the
height accuracy ranging from decimeters to centimeters, have been used
to investigate the decadal water level changes of lakes in the TP (Gao
et al. 2013; Kleinherenbrink et al. 2015; Song et al. 2015a). However,
these radar altimeters are only suitable for large water bodies and
because of their large-size footprint and along-track/cross-track
spacing, many lakes are only accessible by one or two satellites.
The recent advances in satellite altimeters, e.g. CryoSat-2 (2010−),
SARAL/AltiKa (2013−), Jason-3 (2016−) and Sentinel-3 (2016−), provide
alternative options of measuring relatively smaller water bodies. The
more recent studies benefiting from the longer and finer observations
revealed the growth tendency of lakes in the central TP had been
decelerated or reversed during the period 2010-2016 (Hwang et al. 2019;
Jiang et al. 2017; Lei et al. 2019; Song et al. 2015b).
However, whether the deceleration or
hiatus would last in the following years remains unclear. It has not
been systematically investigated thus far. Thus, this study aims to
combine the traditional and recently-advanced radar altimetry
measurements to update our understanding of the changing characteristics
of Tibetan lakes in the near past several years. Furthermore, we will
explore the potential climate driving mechanism of the recent varying
tendency of lake changes in the TP.
2 Materials and methods
2.1 Study region
The TP is located in central Eurasia with a geographic area about 3
million km² (Song et al. 2014b). It has a mean altitude of approximately
4,000 m. The climate in the TP is marked by low temperature and strong
solar radiation (Piao et al. 2011). The precipitation in the TP is
characterized by strong seasonality. Most of the precipitation (60%
-90% of annual total precipitation) occurs between June and September,
while the rainfall from November to February is less than 10%. (Xu et
al. 2008). The TP and its surroundings are sometimes known as “Earth’s
Third Pole” and “Water Tower of Asia” because of the the rich storage
of water resources. The geographical environment of this area is
complex, and the water system is developed. It is the birthplace of many
large rivers (Yangtze River, Yellow River, Mekong River, etc.) in China
and throughout Asia. In addition, there are many lakes in the TP, which
is one of the most densely distributed regions in China, and these lakes
are currently undergoing significant changes due to the impact of
climate changes (Ma et al. 2010). In this study, a total of 22 lakes
larger than 100 km² in the TP were analyzed by combining the traditional
and recently-advanced radar altimetry measurements. The characteristics
of the studied 22 TP lakes covered by satellite altimetry are presented
in Fig. 1 and Table 2.