Lake Ulaan is located in the southeastern most tip of the Valley of Lakes between the Khangai and Govi Altai Mountain Ranges (Figure 1, Figure 2). The morphometric values including altitude, width, length, depth and shoreline are presented in Table 1. Because the Govi sediment filled in the lake basin radiates red ray from the water surface, the lake was named Ulaan meaning red [21]. The Ongi River draining the Khangai Mountain Range flows into Lake Ulaan, but the river cannot permanently feed the lake today, therefore depth and area of the lake are unstable. There is a record noting that the lake was almost completely dried out in 1952-1953, the lake bottom exposed, and salt marshes were bleached on the bottom [21]. In the lake, thick alluvial and lacustrine sediments deposited during the past [21]. The lake level is strongly dependent on precipitation, i.e., the lake water covers a large area from more than ten to more than hundred sq. km during heavy rainfall and is almost dried out during low precipitation. This phenomenon implies the shallowness and flat bottom of the lake [21]. Shores are mostly in plains and there are sandy beach ridges located on the northern and northwestern margins of the lake. During torrential rains, flash floods transport the sands and gravels into the lake along the Ongi River and some other branch mountains. The lake sediments consist of mud and sand.

Lake Taatsiin Tsagaan is located in the Valley of Lakes between the Khangai and Govi Altai Mountain Ranges (Figure 1, Figure 2). The morphometric values including altitude, width, length, depth and shoreline are presented in Table 1. The lake is fed by rainfall, overflow and ground water. During the heavy rainfall, the Taats River draining the Khangai Mountain Range flows into the lake.

Lake Orog is located extending from west to east in the Valley of Lakes between the Khangai and Govi Altai Mountain Ranges (Figure 1, Figure 2). The morphometric values including altitude, width, length, depth and shoreline are presented in Table 1. In the lake basin there are numerous bays and estuaries, and its shores are relatively flat plains [21]. In the north of the lake, a crescent sand dune field extends for more than 10 km long, 1.0 - 1.5 km wide, and it is 20 - 50 m higher than the lake level. The beach ridges (10 - 15 m high) around the lake imply that the lake level repeatedly fluctuated in the past due to shallowness, high evaporation of lake water and annually oscillating overflow [21].

According to Pevtsov (1878-1879), Lake Orog was contained in a large paleolake basin between the Khangai and Govi Altai Mountain Ranges, and widely distributed lake deposits imply that a large fraction of the Mongolian Govi was a bottom of the large lake [21]. The altitudes of the paleostrandlines in the east and west of modern Lake Orog are probably lower than 1250 m a.s.l., and the level of the present-day lake lies at 1210 - 1220 m a.s.l. [10]. The lake bottom is divided into two parts, and the bottom in the west of the lake is deeper and larger than that in the east [21]. Lake deposits consist of mud, clay and sand. Main sources of the lake water are precipitation, overflow and ground water. The Tuin River draining the Khangai Mountain Range flows into the lake and there are abundant marshes along the Tuin River and other small rivers flowing into the lake. Due to the lack of outflow, lake water is saline.

Lake Buun Tsagaan is located in the Valley of Lakes between Khangai and Govi Altai Mountain Ranges (Figure 1, Figure 2). The morphometric values including altitude, width, length, depth and shoreline are presented in Table 1. The southern and northern margins of the lake border with the Khoid Tal, a wide plain and the Khar Argalant Mountain Range, a branch of Govi Altai Mountain Range, respectively [21]. Beach ridges are found along the northern and eastern shorelines, and a distance between these beach ridges ranges from 40 - 50 m to 200 - 250 m. Lacustrine deposits consist of clay, mud and sand. The Baidrag River draining the Khangai Mountain Range flows into the lake and there is no any outflow [21]. The present-day lake level is estimated to be about 1310 m a.s.l. [10]. The paleolake Buun Tsagaan was likely connected to the paleolake Tsagaan (or Adgiin Tsagaan), now a small ephemeral lake 60 km to the east, at the 1350 m a.s.l. during the late Quaternary Period [10]. A recent decadal rise in lake level formed numerous small lakes with the depths of 1 - 2 m and a total area of about 10 km² [21].

Lake Biger is located in an intermontane basin surrounded by high mountains between the Khangai and Govi Altai Mountain Ranges (Figure 1, Figure 2). The morphometric values including altitude, width, length, depth and shoreline are presented in Table 1. The lake is a saline lake and is fed by rainfall and ground water. Numerous streams flow into the lake. Shorelines are situated on the lowlands and are abundantly covered by marshes [21].

Lake Shargiin Tsagaan is located in a wide depression of Sharga Govi in an intermontane basin surrounded by high mountains, i.e., it is situated in the northwesternmost tip of the Valley of Lakes between the Khangai and Govi Altai Mountain Ranges (Figure 1, Figure 2). The morphometric values including altitude, width, length, depth and shoreline are presented in Table 1. The lake is fed by rainfall and ground water, and the Bayan, Sharga and Khoid Rivers flow into the lake [21]. The lake level is unstable because the lake level highly oscillates during the heavy rainfall. Shores are swampy and marshy due to its position on lowland.

In this study, the results from the six lakes (Table 1) from the Valley of Lakes in the Govi region (Figure 1, Figure 2) show decreases in lake areas for all lakes during the past 44 or 45 years (Table 3).

For Lake Ulaan (Figure 3(a), Figure 4(a)), the lake area reduced by 18.2 km² from 1970 to 2014 (Table 3). The decrease in lake area during the past 44 years may have been related to the local climate change of rising temperature since 1995 and decreasing precipitation since 1987 (MO in Figure 5) observed in the Mandal-Ovoo meteorological station (Figure 2) locating 23.1 km from the lake. Lake Ulaan was identified in 1991 but did not appear in the Landsat images since 2000 [11], however reappearance of Lake Ulaan was detected in Landsat 8 in 2014 (Figure 3(a)). It is shown that a gradual decrease in lake area for the Lake Ulaan started since 1987 and strengthened since 1995 (MO in Figure 5).

For Lake Taatsiin Tsagaan (Figure 3(b), Figure 4(b)), the lake area decreased by 7.9 km² between 1970 and 2015 (Table 3). The decrease in the lake area may have been connected to the intensification of

rising temperature since 1997 although precipitation increased in 1987-1997 and since 2006 (BU in Figure 5) observed at the Baruunbayan-Ulaan meteorological station (Figure 2) positioning 1.7 km from the lake. It is shown that the lake area is highly dependent on temperature, and not on fluctuation in the precipitation, i.e., the lake is very sensitive to the present warming.

For Lake Orog (Figure 3(c), Figure 4(c)), the lake area has decreased by 95.5 km² from 1970 to 2015 (Table 3). This large decrease during the past 45 years may indicate that the lake is very sensitive to climate change. The local climatic data at the Bogd meteorological station (Figure 2) locating 11.6 km from

the lake was found for a short-term period between 1994 and 2015. According to the data at the Bogd meteorological station, the temperature rise with fluctuations since 1997 (BO in Figure 5) may have impacted on the decrease in lake area, i.e., drought before 2003 affected the lake area although precipitation fluctuated to increase since 2003 (BO in Figure 5). The Baruunbayan-Ulaan meteorological station (Figure 2) is the next nearest one at 41.3 km. According to the data of this last station, Lake Orog is similar to Lake Taatsiin Tsagaan, i.e., its area has been decreasing since 1997 when the temperature has been rapidly increased and the rise continues to present, although precipitation increased slightly since 2003. This is similarly consistent with the result by [24] showing that the lake area decreased in 1986-1989 and 1998-2004, and the lake disappeared in 2004-2011.

For Lake Buun Tsagaan (Figure 3(d), Figure 4(d)), lake area reduced by 83.2 km² between 1970 and 2014 (Table 3). The large decrease in lake area may imply a direct impact of rising temperature during the past 45 years, although there is no climatic data in the Baatsagaan meteorological station situating 0.8 km from the lake. Instead, the Bogd meteorological station is the nearest one situating 119 km from the lake. At the station, temperature and precipitation rises were observed since 1997 and 2003, respectively (BO in Figure 5). In addition, the Biger meteorological station (Figure 2) is the second one locating 138.6 km from the lake. According to the data at the Biger meteorological station, continuous rise in temperature and intermittent fall in precipitation were observed since 1980 and 1987 (BI in Figure 5), respectively. The large decrease in lake area may have been started since 1987 and strengthened since 1997. This is similarly consistent with the result by [24] who showed that the lake area decreased in 1987-1991 and since 1995. The large loss in the lake occurred in the eastern part and it has been also confirmed by [24] indicating that after 1996, the water diminished until the eastern part disappeared entirely.

For Lake Biger (Figure 3(e), Figure 4(e)), the lake area reduced by 0.5 km² between 1970 and 2014 (Table 3). The small decrease in lake area shows that Lake Biger is similar to other lakes in the Valley of Lakes being affected by the rising temperature since 1987 and decreasing precipitation since 1980 (BI in Figure 5) observed in the Biger meteorological station (Figure 2) locating at 3.3 km from the lake. The data shows that decrease in lake area may have started since 1980 and strengthened since 1996 (BI in Figure 5). It is implied that although the gradual increase in precipitation appeared since 1980, the lake area sensitively responses to the rising temperature.

For Lake Shargiin Tsagaan (Figure 3(f), Figure 4(f)), the lake area decreased by 5.0 km² between 1970 and 2015 (Table 3). The decrease in lake area indicates a relationship with rising temperature since 1987 and abruptly dropping precipitation since 2004 (SH in Figure 5) observed at the Sharga meteorological station (Figure 2) situating 18.2 km from the lake. In other words, the decrease in lake area may have begun since 1976 when precipitation fluctuated with frequencies of ~5 years and intensified since 1987 (SH in Figure 5).

Generally speaking, the lake area changes for the large lakes in the Govi region indicate a common trend in decrease between 1970 and 2014 or 2015 in relation to the continuous rise in temperature (Figure 5), not with the gradual increase in precipitation with fluctuations (Figure 5). The three largest lakes among the six lakes have lost their large areas during the past 44 or 45 years, i.e., Lakes Ulaan, Orog and Buun Tsagaan have experienced of decreases of 18.2, 95.5 and 83.2 km², respectively (Table 3), whereas Lakes Taatsiin Tsagaan, Biger and Shargiin Tsagaan have gradually decreased by 7.9, 0.5 and 5.0 km², respectively (Table 3). The decrease in lake area shows that the three larger lakes more directly and significantly response to the present warming (Figure 5) than the smaller three lakes. Nevertheless, the similar decreases in lake areas for the six lakes suggest that the lakes are very sensitive to increasing temperature (Figure 5) resulting in evaporation and lake shrinkage. Climatologically, data between 1975 and 2015 observed at the meteorological stations near the lakes show that the decrease in lake areas is consistent with the trends in temperature rise since 1995 and precipitation fluctuations since 1975 (Figure 5). Spatially, the large lakes in the eastern part of the Valley of Lakes are more sensitive to the rising temperature and dropping precipitation than the other lakes in the western part of the Valley of Lakes, i.e., Lakes Ulaan, Orog and Buun Tsagaan (Figure 2) have lost their large areas during the past 45 years (Table 3, Figure 4). Geomorphologically, the large lakes in the eastern part of the Valley of Lakes are surrounded by mountains elevated at 1200 - 2700 m a.s.l., while the lakes in western part are surrounded by higher mountains elevated at 2200 - 3800 m a.s.l. (Figure 2).

Trend in decrease of the lake areas shows that the lakes in Valley of Lakes face the challenge of the present warming, depending on local geomorphic features and climatic conditions in the lacustrine basins. The result from this study is in agreement with results by [23 , 24] and [25]. Davaa [23] noted that the lake area in Mongolia has reduced by 373 km² in 1940-2000, specifically the increase in lake area for mostly large lakes located in the permafrost zone, fed by glacier melt waters, and the shrinkage and shift to the class of very small lakes for small lakes. Szumińska [24] showed a tendency for decreasing in surface area, intermittent with short episodes of resupply, observed in Lakes Buun Tsagaan and Orog during the last 40 years in the relation to climatic factors. Moreover, the number of lake change; most of the large lakes over 6 km² in 1991 had disappeared by 2000, all of the lakes had disappeared from the Govi region by 2002, but 9 small lakes were detected again in 2006, i.e., the lakes in the Govi region showed frequent disappearance and reappearance with that average of 17.3 lakes, compared with 2.3 lakes for the other regions in Mongolia [11].

It is shown that lakes in the Govi region are very sensitive to today’s climatic warming (Figure 5 to Figure 7). The average annual temperature in Mongolia has been increasing recently [e.g., 28 , 29]. The observed warming trend from 60 sites across Mongolia shows that the annual mean air temperature has risen by 2.17˚C overall, and by 1.9˚C - 2.28˚C in the high mountains and 1.6˚C - 1.7˚C in the Govi desert from 1940 to 2001, and the intense drought spells in recent years are most likely due to increased temperature and decreased precipitation [28]. Figure 6 shows decrease in lake areas coinciding with temperature increases for the lakes in the Govi region. Dulamsuren et al. [29] noted the increase in trends of both annual mean maximum and minimum temperatures, and showed that the annual minimum (TXn) values of daily maximum temperatures (Figure 7(d)) and the annual minimum (TNn) values of daily minimum temperatures (Figure 7(c)) have increased significantly since the year 1981, while the annual maximum (TXx) values of daily maximum temperatures (Figure 7(b)) and annual maximum (TNx) values of daily minimum temperatures (Figure 7(a)) have increased rapidly since the year 1995. These temperature increases and precipitation changes are likely to have driven observed changes in lake area, river discharge, and glacier extent [30]. For the future, climate modeling based on the greenhouse gas (GHG) A1B scenario estimates an increase in average temperature in the next 100 years of 2.6˚C in winter and 2.4˚C in summer [31]. This temperature rise, coupled with changes to short heavy rain, will likely increase the stress on the hydrologic regime in Mongolia.

The decreasing trend of the lake areas in the Valley of Lakes, Govi region, southern Mongolia due to increasing temperature since 1995 and fluctuating precipitation since 1975 is consistent with other results [e.g., 23 , 24 , 32 ). According to MARCC [32], the surface water flow in Mongolia increased from the mid-1970s until the 1990s and then precipitously dropped beginning in 1993 and 1994, while Davaa [23] noted that average annual river flow in Mongolia increased from the late 1970s to the early 1990s, and then decreased from 1994 to 2010. In addition, the 2003 surface water inventory indicated that in Mongolia there are 4.193 lakes and ponds, of which 760 have dried up during the last few years, whereas, the 2007 water inventory reveals that 1.181 lakes and ponds out of a total of 3.747 have dried up [32]. Kang & Hong [25] showed the considerable seasonal and inter-annual lake area variations in Mongolia with a decrease by −9.3% at an annual rate of −53.7 km²/yr of total lake area from 2000 to 2011. According to Kang et al. [11], the temporal variations of lake area showed a generally decreasing pattern in Dalanzadgad, Govi region with that the most dramatic decrease in lake area occurred between 1991 and 2000 when the lake area decreased over 80% from 47.1 to 5.9 km², i.e., for the Dalanzadgad, the lake area change was highly correlated with temperature, but poorly with precipitation changes at both monthly (R² = 0.26) and annual (R² = 0.028) time scales, respectively. Szumińska [24] showed that the rapid decrease in the surface areas of the Lakes Buun Tsagaan and Orog since 1995 is related to the increase in annual mean air temperature detected at the Bayankhongor meteorological station.

The regionality of lake area change agrees with evidence from radar-altimetry lake level data that indicate distinct but spatially different inter-annual variations of lake level for inland lakes in the Central Asia regions for the last two decades [11]. The water level fluctuations in lakes in Mongolia show that water level was generally rising from 1960 till the middle of 1990, and has been continuously falling since 1996 [23]. In northern Mongolia, the progressive reduction of the Egiin River headwaters results in the lowering of Lake Khuvsgul levels [19] where the hydrometric data of the annually averaged values show that up to 60 cm [33] and 80 cm [34] rises of the level occurred in 1973-1977 and 1982-1987 [34]. In western Mongolia, three lakes (i.e., Lakes Uvs, Khyargas and Khar) showed distinct drawdown in late 2000s [11]. The water level of the Lake Ugii located in the steppe zone dropped by 144 cm since 2004, while the Lake Buun Tsagaan in the Govi desert registered a drop of 603 cm from 1998 to 2009 [23]. Regarding ground water level fluctuations, they seem to decrease in correlation with current changes occurring in surface water resources. For instance, ground water level dropped in the Murun (forest steppe zone) by 0.55 m, in Arvaikheer (steppe zone) by 0.3 m and in Ekhiin Gol (oasis in the Govi desert) by 0.50 m in 1997-2009 [23]. The decreasing trend in lake areas in southern Mongolia linked with local geomorphic features and climatic conditions is consistent with other results. For instance, Kang et al. [11] indicated that the regional patterns of precipitation-driven lake area changes varied considerably (R² = 0.028 - 0.950), depending on the regional climate regime and hydromorphological characteristics. Kang & Hong [25] showed a certain regional hydrology-induced significant decreasing trend in the annual mean lake area for big lakes in Govi region between 2000 and 2011.

It should be noted that lake area change depends on many factors including precipitation, evaporation, run off from the catchment, outlet discharges, groundwater balance, glaciers and permafrost [9 , 10]. A relationship between these factors and lake areas fluctuations in the Valley of Lakes would be investigated in further work.