Lake Changes in Inner Mongolia over the Past 30 Years and the Associated Factors

: Lakes are important water resources in Inner Mongolia and play essential roles in ﬂood storage, water source maintenance, aquaculture, water volume regulation, and the regional ecological balance. However, most lakes in Inner Mongolia have undergone signiﬁcant shrinkage over the past few decades. In order to quantify the lake changes in Inner Mongolia and analyze the factors associated with these changes, information about 546 lakes in seven years (1990, 1995, 2000, 2005, 2010, 2015, and 2018) was retrieved using 30 m resolution Landsat images taken of the entire region over 29 years (1989–2018). In addition, water census data from 2010 and 1:250,000 geological maps were used as references. The analysis revealed that the lakes in Inner Mongolia exhibited rapidly decreasing trends during the past three decades, with both the area and the number of lakes decreasing to a minimum by 2010. The number of lakes with areas of >1 km 2 decreased from 384 in 1990 to 301 in 2018; the total area of lakes with individual areas of >1 km 2 decreased from 4905.74 km 2 in 1990 to 4187.45 km 2 in 2018. With respect to the lake distribution among different geomorphological units, the analysis revealed that the lake shrinkage was most pronounced on the West Liaohe Plain, followed by the northern Inner Mongolian Plateau. Furthermore, in relation to different climatic zones, lake shrinkage primarily occurred in the mid-temperate semi-arid zone, wherein the lake area decreased by 776.6 km 2 . We hypothesize that the changes in the lake number and area in Inner Mongolia resulted from the combined effects of natural conditions and anthropogenic disturbances; possibly, lake shrinkage was mainly driven by the rising temperature and decreasing precipitation, along with water regulation projects, agricultural irrigation, mining development, and population growth that also had non-negligible effects on the lakes.


Introduction
Lakes are nodes for interactions between the various components of the terrestrial surface system. As they serve as important freshwater reservoirs, flood-regulating reservoirs, and species gene pools on Earth, lakes play irreplaceable roles in maintaining the ecological balance of watersheds, conserving water sources, holding soil and water, preventing floods and droughts, regulating climate, and maintaining biodiversity [1][2][3]. China has a vast territory and contains a large number of lakes, including 2759 lakes with individual areas of more than 1 km 2 and a total combined area of 91,019.6 km 2 , according to the Records of Chinese Lakes [4]. These lakes are mainly distributed in five major lake areas: the East Plain, Qinghai-Tibet Plateau, Yunnan-Guizhou Plateau, Mongolia-Xinjiang Plateau,

Description of the Study Area
Inner Mongolia is located along the northern border of China, extending obliquely from the northeast to the southwest in a narrow fashion, with a length of about 2400 km from east to west and spanning more than 1700 km from north to south at its maximum, covering a total area of 1.183 million km 2 . Due to its geographical location and topography, the entire region has a complex and diverse climate, dominated mainly by a temperate continental monsoon climate with an annual precipitation of 100-500 mm and generally more than 2700 h of sunshine per year.
There are more than 1000 rivers in Inner Mongolia, of which 107 have a watershed area of more than 1000 km 2 each. The Yellow River, Argun River, Nengjiang River, and West Liaohe River are the four major rivers. The lakes in Inner Mongolia span a large geographical area and are scattered throughout the region, but they are mainly located

Data Sources
Landsat thematic mapper/enhanced thematic mapper/orbital land imager (TM/ ETM/OLI) remote sensing images with 30 m resolution and covering the entire region were collected for 29 consecutive years . In order to estimate the maximum area of each lake in each year, the high-water period from June-August in Inner Mongolia was selected as the monitoring period for all years. To better analyze the change process and development trend of the lakes throughout a certain period of time and to consider the lake changes between different years, seven years (1990, 1995, 2000, 2005, 2010, 2015, and 2018) were selected for region-wide data analysis. The meteorological data were obtained from the China Meteorological Data Sharing Network, and the annual average precipitation and temperature data were procured from 69 meteorological stations active across the region from 1970-2017.

Lake Analysis
Due to the cloudy weather characteristic of the high-water period from June-August, images with low or no cloud cover for a given geographical site within this period could only be obtained by synthesizing images of the same geographical site during the same period over 3 consecutive years. For example, the synthesized images for 1995 were based on the raw images acquired in June-August of 1994, 1995, and 1996 (though mainly 1995), and the synthesized images for 2018 were based on the raw images acquired in 2017 and 2018, primarily during the period of May-August. By analyzing 7 years' image data, such as by feature image construction, water sample selection, intelligent detection of water bodies, classification post-processing (removal and correction of false spots), manual editing optimization, and summary statistics, information on 546 lakes was compiled from the lake images captured by Landsat. The water census data for 2010 and 1:250,000 geological maps were used as references. Of the 546 lakes, 468 were included in the census data and 78 were new lakes identified by interpretating the remote sensing images.

Changes in the Area of the Lakes
The total lake areas in 1990,1995,2000,2005,2010,2015, and 2018 are listed in Table 1. It is evident that the total lake area generally exhibited a decreasing trend, with a decrease of 859.43 km 2 (17.31%) during 1990-2010, i.e., from 4965.35 km 2 in 1990 to 4105.92 km 2 in 2010. The lake areas increased slightly during 2010-2018, with an increase of 186.46 km 2 (4.54%). In summary, the net decrease during 1990-2018 was 672.97 km 2 , with a mean annual decrease of 24.03 km 2 (13.55%). From 1990 to 2018, the total area of the lakes with individual areas of >1 km 2 exhibited a general decline, decreasing from 4905.24 km 2 in 1990 to 4014.97 km 2 in 2010, and then rebounding to 4187.45 km 2 in 2018, with a net decrease of 14.63%. Among these lakes, the following trends were observed: (1) The total area of one lake with an area of >500 km 2 decreased and then increased, reaching a minimum in 2010 and then rebounding and essentially recovering to its normal state. (2) The total area of six lakes with individual areas of 100-500 km 2 decreased annually and reached a minimum in 2015, with a net decrease of 563.82 km 2 (45.84%). (3) Conversely, the total area of 25 lakes with individual areas of 10-100 km 2 generally increased, with a net growth of 125.86 km (21.62%). (4) The total areas of 50 lakes with individual areas of 5-10 km 2 and of 243 lakes with individual areas of 1-3 km 2 exhibited decreasing trends, shrinking by 175.2 km 2 and 87.34 km 2 (48.85% and 21.45%), respectively. (5) Lastly, the total area of 58 lakes with individual areas of 3-5 km 2 exhibited a minimal net change, as the lakes experienced an increase from 1990-2000, a decrease from 2000-2010, and then a rebound from 2010-2018.
During the period of 1990-2018, the total area of the lakes with individual areas of <1 km 2 generally increased, from 60.11 km 2 in 1990 to 104.93 km 2 in 2018, a net increase of 74.56%.

Changes in the Number of Lakes
The analysis results (Table 2) revealed that the total number of lakes with individual areas of >1 km 2 increased and then subsequently decreased during 1990-2018, increasing from 384 in 1990 to 397 in 2000, and then decreasing to 323 in 2010 and 301 in 2018, with net decreases of 61 (15.9%) and 83 (21.6%) during 1990-2010 and 1990-2018, respectively. Additionally, the total number of lakes in 2018 was 29.5% lower than that reported in 1987 [13]. Compared with the data for 1987, the following changes were observed in 2018: (1) The number of lakes with individual areas of >500 km 2 remained unchanged (i.e., one). (2) Conversely, the number of lakes with individual areas of 100-500 km 2 decreased from six to three. (3) The number of lakes with individual areas of 10-100 km 2 remained basically unchanged. (4) Additionally, the number of lakes with individual areas of 5-10 km 2 decreased annually, with a total decrease of 24 since 1990. (5) The number of lakes with individual areas of 3-5 km 2 fluctuated. (6) Lastly, the number of lakes with individual areas of 1-3 km 2 decreased the most (by 57), accounting for 68.67% of the total decrease in the combined number of lakes. From 1990 to 2018, the number of lakes with individual areas of <1 km 2 increased, from 129 in 1990 to 226 in 2018, a net gain of 75.19%. In particular, the number of lakes with water surface areas of <0.5 km 2 decreased from 69 in 1990 to 59 in 2000, and then rose to 119 in 2018, a net increase of 72.46%. The number of lakes with water surface areas of <0.2 km 2 (nearly or completely dried-up) also exhibited a decreasing-increasing trend, i.e., initially decreasing from 44 in 1990 to 31 in 2000, and then rebounding to 63 in 2018, with an overall net increase of 43.18% in 1990-2018.

Changes in Lakes in Different Geomorphological Units
Inner Mongolia is divided into eight geomorphological units, namely, the Daxinganling Mountains, the West Liaohe Plain, the northern Inner Mongolian Plateau, the Yinshan Mountains, the Hetao Plain, the Ordos Plateau, the Alxa Plateau, and the deserts [24,25], with each unit having a characteristic pattern of temporal variations in the number and area of its lakes ( Figure 1).  2010  519  323  1  3  25  37  44  213  196  105  60  2015  520  335  1  3  24  37  54  216  185  98  59  2018  527  301  1  3  27  27  57  186  226  119  63 From 1990 to 2018, the number of lakes with individual areas of <1 km 2 increased, from 129 in 1990 to 226 in 2018, a net gain of 75.19%. In particular, the number of lakes with water surface areas of <0.5 km 2 decreased from 69 in 1990 to 59 in 2000, and then rose to 119 in 2018, a net increase of 72.46%. The number of lakes with water surface areas of <0.2 km 2 (nearly or completely dried-up) also exhibited a decreasing-increasing trend, i.e., initially decreasing from 44 in 1990 to 31 in 2000, and then rebounding to 63 in 2018, with an overall net increase of 43.18% in 1990-2018.

Changes in Lakes in Different Geomorphological Units
Inner Mongolia is divided into eight geomorphological units, namely, the Daxinganling Mountains, the West Liaohe Plain, the northern Inner Mongolian Plateau, the Yinshan Mountains, the Hetao Plain, the Ordos Plateau, the Alxa Plateau, and the deserts [24,25], with each unit having a characteristic pattern of temporal variations in the number and area of its lakes ( Figure 1). As for the temporal variations in the number of lakes within the different geomorphic units (Table 3), during 1990-2018, the number of lakes with individual areas of >1 km 2 decreased in the northern Inner Mongolian Plateau region, the desert zone, the West Liaohe Plain, the Yinshan Mountains, and the Daxinganling Mountains. Among these regions, the number of lakes in the West Liaohe Plain experienced the most significant decrease of 79 to 36 (54.43%), followed by that on the northern Inner Mongolian Plateau, which declined from 149 to 117 (21.47%). The temporal trend associated with the number of lakes within the desert zone consisted of two stages. Stage 1 involved an annually increasing trend during 1990-2000, with an overall increase of eight in the entire desert re- As for the temporal variations in the number of lakes within the different geomorphic units (Table 3)  As is shown by the temporal distribution of the lake areas throughout the different geomorphological units (Table 4), the total area of the lakes with individual areas of >1 km 2 decreased during 1990-2018, except for that on the Hetao Plain and the Ordos Plateau, where the total area remained generally stable. In particular, the lake area on the West Liaohe Plain decreased most significantly, by nearly 50% (i.e., 49.18%). During 1990-2010, the total area of the lakes with individual areas of >1 km 2 decreased by different degrees depending on the geomorphological units, with the greatest reduction occurring on the northern Inner Mongolian Plateau, where the lake area decreased from 3121.81 km 2 to 2527.85 km 2 (19.03%). During 2010-2018, the lake area continued to decline in the Daxinganling Mountains, the West Liaohe Plain, the Yinshan Mountains, and the desert zone. Specifically, the Yinshan Mountains experienced the largest decrease of 54.98 km 2 (24.41%), from 225.25 km 2 to 170.27 km 2 . Contrastingly, on the northern Inner Mongolian Plateau, the total lake area increased by 189.09 km 2 (7.48%), from 2527.85 km 2 to 2716.94 km 2 .

Changes in Lakes in Different Climatic Zones
According to the climate zoning scheme for China, Inner Mongolia is divided into five climate zones: mid-temperate arid, mid-temperate semi-arid, mid-temperate semihumid, mid-temperate humid, and cold temperate humid zones [26]. The studied lakes were mainly distributed in the mid-temperate arid, mid-temperate semi-arid, and midtemperate semi-humid zones ( Figure 2); the temporal variations in the number and area of the lakes exhibited different trends depending on the climate zones (Table 5).
In the mid-temperate arid zone, the number and area of the lakes displayed increasing trends. During 1990-1995, the number of lakes increased rapidly from 106 to 119 (10.9%), and the area of the lakes increased from 627.86 km 2 to 642.80 km 2 . Conversely, during 1995-2010, the number of lakes fluctuated, diminishing slightly from 119 to 116, and the total area of the lakes decreased from 642.80 km 2 to 583.82 km 2 . During 2010-2018, the number of lakes increased from 116 to 125 (7.2%), and the area of the lakes grew by 25 temperate semi-humid zones ( Figure 2); the temporal variations in the number and area of the lakes exhibited different trends depending on the climate zones (Table 5).  In the mid-temperate arid zone, the number and area of the lakes displayed increasing trends. During 1990-1995, the number of lakes increased rapidly from 106 to 119 (10.9%), and the area of the lakes increased from 627.86 km 2 to 642.80 km 2 . Conversely, during 1995-2010, the number of lakes fluctuated, diminishing slightly from 119 to 116, and the total area of the lakes decreased from 642.80 km 2 to 583.82 km 2 . During 2010-2018, the number of lakes increased from 116 to 125 (7.2%), and the area of the lakes grew by 25.13% or 146.71 km 2 , from 583.82 km 2 to 730.53 km 2 .
In 1990, the number and area of the lakes in the mid-temperate semi-arid zone accounted for 75.8% and 91.4%, respectively, of those in the entire region.   In 1990, the number and area of the lakes in the mid-temperate semi-arid zone accounted for 75.8% and 91.4%, respectively, of those in the entire region. indicating increased precipitation in these areas. In contrast, the precipitation inclination rate was negative in central Inner Mongolia and in the Hailar district in the northeast, indicating decreased precipitation in these areas. The specific rates of increase or decrease in the precipitation varied among the areas.

Precipitation
Over the past 50 years, the precipitation inclination rates at 69 meteorological stations in Inner Mongolia ( Figure 3) were positive in the western region and in parts of northeast, indicating increased precipitation in these areas. In contrast, the precipitation inclination rate was negative in central Inner Mongolia and in the Hailar district in the northeast, indicating decreased precipitation in these areas. The specific rates of increase or decrease in the precipitation varied among the areas.

Temperature
The temperature inclination rates in Inner Mongolia over the last 50 years ( Figure 5) were positive across all the meteorological stations, indicating that, throughout the entire region, the temperature continuously increased over the nearly 50-year period from 1970-2017. Notably, the temperature increased at an average rate of 0.043 • C/a, and it increased by more than 2 • C in some areas. The slope of the increasing temperature trend varied among the regions. It was relatively high (0.3-0.5 • C/10 a) in central and western Inner Mongolia, where the temperature increased rapidly, whereas in the northeastern region, the slope of the increasing trend was relatively small, and the temperature rose slowly.

Temperature
The temperature inclination rates in Inner Mongolia over the last 50 years ( Figure 5) were positive across all the meteorological stations, indicating that, throughout the entire region, the temperature continuously increased over the nearly 50-year period from 1970-2017. Notably, the temperature increased at an average rate of 0.043 °C/a, and it increased by more than 2 °C in some areas. The slope of the increasing temperature trend varied among the regions. It was relatively high (0.3-0.5 °C/10 a) in central and western Inner Mongolia, where the temperature increased rapidly, whereas in the northeastern region the slope of the increasing trend was relatively small, and the temperature rose slowly.
The decade-average temperature was calculated for each meteorological station in Inner Mongolia, and the differences between the average values for two successive decades were calculated to compare the trend in temperature changes within each region over the different decades ( Figure 6). Compared with the 1970s, the 1980s saw temperature increases throughout most of Inner Mongolia, with an average increase of less than 0.5 °C; however, some of the meteorological stations recorded temperature decreases Compared with the 1980s, in the 1990s, the temperature increased throughout the entire Inner Mongolia, with an average increase of more than 0.75 °C. This phenomenon was more pronounced in Hailar, Manzhouli, Xinbaerhuzuoqi, Xinbaerhuyouqi, and other areas in the northeast, where the increases were large (>1 °C). Unlike in the 1990s, in the 2000s, the temperatures continued to increase throughout most areas in Inner Mongolia however, the increase was relatively small (mostly below 0.5 °C). After 2010, western Inner Mongolia experienced a slight increase in temperature, whereas the temperature de-  The decade-average temperature was calculated for each meteorological station in Inner Mongolia, and the differences between the average values for two successive decades were calculated to compare the trend in temperature changes within each region over the different decades ( Figure 6). Compared with the 1970s, the 1980s saw temperature increases throughout most of Inner Mongolia, with an average increase of less than 0.5 • C; however, some of the meteorological stations recorded temperature decreases. Compared with the 1980s, in the 1990s, the temperature increased throughout the entire Inner Mongolia, with an average increase of more than 0.75 • C. This phenomenon was more pronounced in Hailar, Manzhouli, Xinbaerhuzuoqi, Xinbaerhuyouqi, and other areas in the northeast, where the increases were large (>1 • C). Unlike in the 1990s, in the 2000s, the temperatures continued to increase throughout most areas in Inner Mongolia; however, the increase was relatively small (mostly below 0.5 • C). After 2010, western Inner Mongolia experienced a slight increase in temperature, whereas the temperature decreased in both central and northeastern Inner Mongolia. In general, the temperature in Inner Mongolia has continuously risen in almost all areas of the region since the 1980s, with particularly drastic increases in the 1990s, yet some areas experienced a certain temperature decrease after 2010. As described above, a warm-dry climate trend characterized by significant and sustained increases in temperature and decreases in precipitation was typical post 1990s and served as the main driver of the lake changes [27]. The climate change pattern coincided with the overall pattern of the lake changes.  As described above, a warm-dry climate trend characterized by significant and sustained increases in temperature and decreases in precipitation was typical post 1990s and served as the main driver of the lake changes [27]. The climate change pattern coincided with the overall pattern of the lake changes.

Impact of Human Activities on Lakes
Human activities are one of the important drivers of lake changes in arid and semi-arid regions; the main activities inducing the lake decline in Inner Mongolia include the high scale of water conservancy projects (such as reservoirs), increased water consumption, excessive industrial water consumption (such as coal mining), and increased water consumption for irrigation [12]. This study discusses the impact of these activities on lake changes from three perspectives.

Water Resource Regulation
Water regulation has both positive and negative impacts on lakes [35,36]. For example, there have been successful examples of anthropogenic interventions maintaining lake ecology; however, ecological problems associated with severe lake shrinkage after reservoir construction have also been reported. In Inner Mongolia, water regulation has become a powerful intervention measure for the artificial regulation of storage for most lakes. Based on field investigation, relevant national policies and news reports, and analysis of the driving factors of typical lake changes, we concluded that the seven large lakes with individual areas of >100 km 2 are mainly regulated artificially, except for Dalinuoer Lake, which is still chiefly controlled by natural factors (Table 6). The construction of reservoirs in endorheic basins imparts an important impact on the lakes. As of 2007, there were 61 large reservoirs in Inner Mongolia, including 10 reservoirs in endorheic basins. In 1990, the Wulagai Gobi Lake had a water area of 257.91 km 2 ; however, by 2005 it had completely dried up due to the construction of the Wulagai reservoir. The lake area was maintained at about 140 km 2 after the construction [37,38]. Water conservancy projects in the upper reaches of the Heihe River in the Gansu Province caused the East Juyanhai Lake and West Juyanhai Lake on the Alxa Plateau to disappear in 1992 and 1961, respectively [39]. In 2000, the national inter-provincial water diversion policy was implemented for the Heihe River and led to a significant increase in the areas of these two lakes [40]. The East Juyanhai Lake has maintained a near constant water area since August 2004. Notably, by 2016, its water area was stable at about 40 km 2 [41]. The area of West Juyanhai Lake reached 128.65 km 2 in 2005 and has been maintained at 60.36 km 2 since 2018. In the 1950s, the Huangqihai Lake was a natural freshwater lake with an area of 130 km 2 . After 37 reservoirs with a total storage capacity of 51.18 million m 3 were built on the inflowing rivers in the 1960s and 1970s, the surface water supply to the Huangqihai Lake was basically cut off. Combined with the decrease in precipitation, this led to the lake's disappearance in 2008. This dried-up state lasted until 2012, when the lake water area recovered to 26.75 km 2 due to heavy precipitation [42]. However, the salinity increased to more than 7.79 g/L, turning the one freshwater lake into a saltwater lake [43].
The Hasuhai Lake, Wuliangsuhai Lake, and Hulun Lake are all maintained through water diversion projects. Both the Hasuhai Lake and Wuliangsuhai Lake are oxbow lakes of the Yellow River. The Hasuhai Lake is replenished using the Minsheng Canal to divert water from the Yellow River. The Wuliangsuhai Lake is maintained using key water control projects of the Hetao Irrigation District, which divert the ice-melt flood water of the Yellow River and intermittent irrigation water. Alternately, the Hulun Lake is refilled through a water resource allocation and water environment management project (which diverts river water to the lake). Notably, these efforts have restored the wetlands and improved the ecological environment surrounding the lake.

Groundwater Extraction and Agricultural Irrigation
The groundwater distribution is extremely uneven in Inner Mongolia [44]. The West Liaohe Plain and the Hetao Plain are the richest in groundwater resources and also have the highest degree of groundwater exploitation. In contrast, the northern Inner Mongolian Plateau and the Alxa Plateau are the areas with the poorest groundwater resources.
The lakes on the Hetao Plain are mainly located in the Yellow River Diversion Irrigation Zone, where the extent of groundwater exploitation is limited, and thus, groundwater extraction has a small impact on the plain's lakes. Conversely, the Hubao area is subject to a high degree of groundwater exploitation. In this area, the number of lakes is small, and the hydraulic link between the lakes and groundwater is weak. Therefore, groundwater extraction has a minimal impact on the lakes.
The West Liaohe Plain contains a large number of lakes, and its groundwater is the main source of drinking water for humans and animals. Furthermore, the plain's groundwater is the primary source for agricultural irrigation, with agricultural water consumption accounting for about 81% of the total groundwater extraction [45]. Notably, there is a close hydraulic link between the groundwater and surface water on the West Liaohe Plain. Additionally, as the surface water runoff gradually declined over the years, the exploitation of groundwater increased. This groundwater extraction has impacted the plain's lakes, but the degree varies among lakes.
In addition to the continuous temperature increases and precipitation decreases (i.e., natural factors) [46], the severe shrinkage of Daihai Lake is largely attributed to the development of irrigated agriculture within the area (an anthropogenic factor). Irrigation water consumption was once as high as 40 million m 3 /a or more, which is nearly the multi-year average natural runoff recharge received by the lake. This resulted in severe water imbalance within the Daihai Lake and rapid shrinkage of the lake area.

Water Consumption for Mining and Power Plants
Coal mining has the following major impacts on lakes [47,48]. (1) Coal mining drains groundwater, and the subsequent decline in the groundwater table may lead to lake water leakage in areas where the lakes have close hydraulic links with the groundwater. (2) Coal mining causes surface subsidence and/or fractures throughout the lake area, thereby leading to lake water leakage. (3) Mine drainage pollutes the lake water.
Analysis of the lakes' spatial distribution in relation to the coal mining zones (Figure 7) revealed that the average distance between coal mines and lakes was only 43.72 km, with 54 lakes located less than 10 km from the nearest mining sites. Furthermore, 132 lakes were within 5 km of nationally planned mining zones, and 166 lakes were within 10 km ( Table 7). The Hulun Buir Plateau, the northern Inner Mongolian Plateau, the Ordos Plateau, and the West Liaohe Plain all contain a large number of coal mines, as well as the largest number of lakes within 5 km and 10 km of these mining zones.
Analysis of the lakes' spatial distribution in relation to the coal mining zones ( Figure  7) revealed that the average distance between coal mines and lakes was only 43.72 km, with 54 lakes located less than 10 km from the nearest mining sites. Furthermore, 132 lakes were within 5 km of nationally planned mining zones, and 166 lakes were within 10 km ( Table 7). The Hulun Buir Plateau, the northern Inner Mongolian Plateau, the Ordos Plateau, and the West Liaohe Plain all contain a large number of coal mines, as well as the largest number of lakes within 5 km and 10 km of these mining zones.  According to the annual statistical bulletins on national economic and social development in Inner Mongolia during 2000-2018 [49], the amount of coal mined in Inner Mongolia increased continuously during 2002-2012, and then decreased after 2014 (Figure 8). The areas of the lakes within 10 km of the mining zones on the Hulun Buir Plateau, the  According to the annual statistical bulletins on national economic and social development in Inner Mongolia during 2000-2018 [49], the amount of coal mined in Inner Mongolia increased continuously during 2002-2012, and then decreased after 2014 (Figure 8). The areas of the lakes within 10 km of the mining zones on the Hulun Buir Plateau, the Northern Plateau of Inner Mongolia, the Ordos Plateau, and the West Liaohe Plain (Figure 9) decreased to various extents after 2000, though this decreasing trend slowed or even slightly reversed after 2010. This indicates that, to some degree, the lake area within 10 km of mining zones coincided with the fluctuations in the amount of coal mining.
The impact of a coal mining zone on a nearby lake is closely related to the hydrogeological conditions of both the lake and the mining zone. Thus, there is a need to conduct investigations and research on these coal mining zones. Regarding the use of water by power plants, as an example, the Daihai power plant uses water from Daihai Lake as the cooling circulating water, which consumes 10 million m 3 per year and has become one of the main drivers of the shrinkage of Daihai Lake [46]. investigations and research on these coal mining zones. Regarding the use of water by power plants, as an example, the Daihai power plant uses water from Daihai Lake as the cooling circulating water, which consumes 10 million m 3 per year and has become one of the main drivers of the shrinkage of Daihai Lake [46].    investigations and research on these coal mining zones. Regarding the use of water by power plants, as an example, the Daihai power plant uses water from Daihai Lake as the cooling circulating water, which consumes 10 million m 3 per year and has become one of the main drivers of the shrinkage of Daihai Lake [46].

1.
From 1990-2018, the number and area of the lakes in Inner Mongolia significantly decreased, reaching minimums by 2010. Specifically, from 1990 to 2018, the total lake area exhibited a net decrease of 672.97 km 2 (13.55%), and the number of lakes with individual areas of >1 km 2 decreased by 83 (21.61%). This decreasing trend in lake area has slowed in recent years; however, the long-term decreasing shifts in both the number and area of the lakes are still apparent.

2.
With respect to the distribution of the lakes throughout the different geomorphological units, lake shrinkage mainly occurred on the northern Inner Mongolian Plateau, in the desert zone, on the West Liaohe Plain, in the Yinshan Mountains, and in the Daxinganling Mountains. Among these regions, the lake shrinkage was the most severe on the West Liaohe Plain, where the number and area of the lakes decreased by 54.43% and 49.18%, respectively, followed by the northern Inner Mongolian Plateau, where the number and area of the lakes decreased by 21.47% and 19.03%, respectively.

3.
With respect to the distribution of the lakes within the different climatic zones, the lake shrinkage was primarily seen in the mid-temperate semi-arid zone, decreasing by 776.6 km 2 (18.09%). In contrast, the number and area of the lakes in the mid-temperate arid zone increased by 19 lakes and 102.67 km 2 , respectively. The lakes in the midtemperate semi-humid zone remained relatively stable, with only small changes in both the number and area of the lakes.

4.
The combined effects of the natural conditions and anthropogenic disturbances were the main causes of the lake shrinkage. The warm-dry climate trend characterized by increasing temperatures and decreasing precipitation was the main driver of the lake changes; however, the additional impacts of water regulation projects, agricultural irrigation, mining development, and population growth cannot be ignored. The factors associated with the lake shrinkage varied geographically, and thus there is a need to conduct more detailed lake-specific analysis and research. Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.