Continuous monitoring of lake dynamics on the Mongolian Plateau using all available Landsat imagery and Google Earth Engine
Graphical abstract
Introduction
Lakes are primary water resources for people's livelihood as well as agricultural and industrial production on the Mongolian Plateau (MP), a typical arid and semi-arid region (Gao et al., 2017; O'Reilly et al., 2015). Meanwhile, some of these lakes are internationally important wetlands coupled with their surrounding surface runoff (e.g., Lake Hulun in Inner Mongolia of China, Lake Uvs in Mongolia). These lakes play an irreplaceable role in maintaining biodiversity by protecting threatened species and migratory waterfowls (e.g., Golden Eagle, White-headed Duck) (Liu et al., 2013; Tao et al., 2015), and provide essential supports for ecosystem services related to human wellbeing (Chen et al., 2012; Han et al., 2017; Li et al., 2017). However, due to the increasing pressure from human activities and climate change, substantial lake shrinkage and wetland degradation occurred in the past decades (Chang et al., 2015; Chen et al., 2018; Hou et al., 2017; Liu et al., 2017; Tao et al., 2015). The lake shrinkage brought considerable threats to the regional environment and ecosystems, including dust release, water salinization, and waterfowl decline (Hou et al., 2018; Liu et al., 2013; Ma et al., 2010; Wang et al., 2018). Given the potential negative effects of lake deterioration on the MP, monitoring lake changes on the entire plateau is of great significance to assess climate change impacts (Lu et al., 2011), and to protect regional ecosystems in such a typical arid and semi-arid region. However, existing efforts have been limited so far.
With the rapid development of remote sensing technology in the past several decades, satellite-based water body mapping has become a main approach to monitor water body changes (Chen et al., 2018; Hou et al., 2018; Huang et al., 2018; Rokni et al., 2015), which enables large-scale water resource monitoring particularly in remote and inaccessible mountain regions (Buchroithner and Bolch, 2015; Crétaux et al., 2011; Song et al., 2013; Song et al., 2014; Zhang et al., 2011). Among all the satellite sensors, Landsat family sensors have the longest satellite monitoring capability and medium spatial resolution (Chander et al., 2009; Hansen and Loveland, 2012; Ju and Roy, 2008; Wulder and Coops, 2014). Although there has been a series of researches on open surface water body mapping (Feyisa et al., 2014; Fisher et al., 2016; McFeeters, 1996; Xu, 2006; Yamazaki et al., 2015), only a few studies had been conducted on time series analysis of lake dynamics using historical Landsat imagery on regional scales, especially in a region (e.g., the MP) with fragile ecosystems which is sensitive to climate change. Liu et al. (2013) researched the decadal changes of lakes over 1 km2 in the semi-arid steppe region in northern China from 1975 to 2009, and found a general reduction in the water areas of lakes since the 1990s. Moreover, they found the regional-scale lake shrinkage and desiccation in the semiarid region of China was initially caused by climate drying. Tao et al. (2015) investigated the changes of lakes (>10 km2) on the MP over nine periods (every three, four, or five years for a period) from 1970s to 2010 using single period cloud-free Landsat images collected in Junes to Septembers of each period, and found a rapid loss of lake water areas on the plateau after the mid-1990s, with a doubled rate of decreases in Inner Mongolia of China than that in Mongolia. Furthermore, they found precipitation was the main driver for the lake loss in Mongolia while coal mining and irrigation were the major drivers for lake deterioration in Inner Mongolia from the 1980s to 2010. On the basis of the lake data (MP, 1976–2010) from Tao et al. (2015), Zhang et al. (2017) extended the study period by generating the lake map of the MP for the year 2013, and discussed the area changes of lakes (>10 km2) on the MP during 1976–2013. Moreover, they found that the drier climate since 1998 could have been the dominant driver of lake shrinkage on the MP.
All the previous studies related to the lake dynamics on the MP mentioned above were carried out by sparse temporal dynamic analyses. However, due to the rapid inter- and intra-annual variations of climate on the MP, epoch-based dynamic analyses could miss important interannual variation information (e.g., turning points) and seasonality of lake dynamics (L. Chen et al., 2014). In addition, lake investigation in the previous studies mainly focused on large lakes over 10 km2 (Tao et al., 2015; Zhang et al., 2017), and the climatic and anthropogenic driver analyses of lake changes were conducted based on these large lakes. Because the widely distributed smaller lakes under 10 km2 on the MP were more vulnerable to regional climate and human activities (Zhang et al., 2017), the driver analyses without considering these lakes could omit some important mechanisms of climatic effects on lake changes. Last but not least, previous studies all reported that lakes were shrinking on the MP before 2010, while it is still unclear whether the lake shrinkage continued in the recent decade.
In view of the above-mentioned issues, improved understanding of lake variations and their drivers could be considered from the following perspectives: 1) continuous long-term lake monitoring at inter- or intra-annual scale. Annual water body dynamics have been detected in previous studies such as the yearly and monthly global water body maps at 30-m resolution from 1984 to 2015 produced by the Joint Research Center (JRC) of the European Commission (Pekel et al., 2016) and the annual water body maps for the United States from 1984 to 2016 (Zou et al., 2018). However, continuous monitoring of annual lake dynamics needs further analyses including extraction of lakes from water body maps; 2) climatic and anthropogenic driver analyses based on the continuous lake monitoring could provide a more promising understanding than that based on epoch-based analyses. For example, Zou et al. (2017) conducted the driver analyses of annual water body dynamics in the Oklahoma state of the United States, and found that precipitation had statistically positive effects on water body area, while temperature, and surface water withdrawals for public water supply and agricultural irrigation had negative effects; 3) in addition to the big lakes over 10 km2, smaller lakes (<10 km2) should also be considered in lake dynamic analyses (Liu et al., 2013; Zou et al., 2017; Zou et al., 2018).
In this study, we aimed to draw a whole picture of the annual lake (>1 km2) dynamics on the MP from 1991 to 2017. In doing so, the objectives of this paper are mainly three parts: (1) to extract annual lake maps on the MP from the water body maps which was generated by using a water and vegetation indices-based water body mapping algorithm, all the available Landsat images, and the cloud computing platform Google Earth Engine (GEE); (2) to investigate the areas and numbers of the lakes over 1 km2 on the MP, and then compare the results with those according to the JRC water body map datasets (Pekel et al., 2016); (3) to investigate the driving factors of lake changes on the MP including both natural and anthropogenic factors. This study provides an unprecedented lake dataset for the MP since 1991 (see Text S1, Fig. S1, Tables S1 and S2 for detailed information), and also expects to provide updated understanding of the interannual dynamics of lakes on the MP and its drivers, which would contribute to regional water resource management and protection.
Section snippets
Study area
The Mongolian Plateau (MP) geographically includes the Inner Mongolia Autonomous Region of China and the Mongolian People's Republic, with an area of approximately 2.7 million km2 at an average elevation higher than 1500 m, and a population of about 28 million (Bao et al., 2014; John et al., 2018; Tao et al., 2015). Because of its large spatial domain and high elevation, the plateau and its surroundings play a vital role in the Earth's climate system through their unique atmosphere interactions
Changes in lakes >1 km2 in Inner Mongolia and Mongolia from 1991 to 2017
Based on the statistical analyses on the interannual variations of both areas and numbers of lakes >1 km2, we found that the entire MP experienced a significant decrease in lakes from 1991 to circa 2009 and then followed by a recovery process since circa 2009 (Fig. 4a and b). Both Inner Mongolia and Mongolia experienced similar trends, but Inner Mongolia showed more drastic changes than Mongolia (Fig. 4c–f). That was proved by both our generated maps as well as the results from JRC (2000–2015),
Conclusions
As important water sources of agricultural and industrial production as well as human livelihood, lake dynamics would lead to a series of consequences on the environment and terrestrial ecosystems on the MP. Based on the Landsat-based annual water body maps generated in this study, we investigated the interannual dynamics of lakes in terms of both lake area and numbers on the plateau. We found the lake areas and numbers of both Inner Mongolia and Mongolia experienced decreasing trends from 1991
Acknowledgements
This study is funded by the Strategic Priority Research Program (XDA19040301) and Key Research Program of Frontier Sciences (QYZDB-SSW-DQC005) of Chinese Academy of Sciences (CAS), and the “Thousand Youth Talents Plan”. We thank Zhiqi Yang, Nanshan You, and Rui Zhao for their valuable comments during the study. We acknowledge the Joint Research Center (JRC) of European Commission for making the water body maps available (https://global-surface-water.appspot.com/). The CRU TS V4.01 climate data
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