3.1 Area changes
3.1.1 Annual changes
The areas of Bange Co, Cuoe Lake and Selin Co showed overall increasing trends between 1988 and 2017; the areas increased by 2.56 km2, 2.83 km2 and 685.80 km2 at rates of 0.67 km2/yr, 0.11 km2/yr and 30.39 km2/yr, respectively (Fig. 2). Both Bange Co and Cuoe Lake experienced similar change trends; the lake areas presented upward trends before 2005 (decreasing during the period from 1988–1995 and increasing during the period from 1995–2005) but downward trends afterward. The area of Bange Co increased from 114.60 km2 in 1988 to 145.40 km2 in 2005, showing an increase of 30.80 km2 at a rate of 1.27 km2/yr, but its area decreased to 117.17 km2 in 2017 with a decrease of 28.24 km2 at a rate of 1.98 km2/yr after 2005. The area of Cuoe Lake first slightly increased by 3.90 km2 at a rate of 0.40 km2/yr, then decreased by 1.07 km2 at a rate of 0.19 km2/yr. Specifically, during the period from 1988–2005, the areas of Bange Co and Cuoe Lake decreased by 10.82 km2 and 3.43 km2, respectively, then increased by 41.62 km2 and 7.34 km2, respectively (Fig. 2a). Unlike the former two lakes, the area of Selin Co exhibited three increasing stages; it slowly increased by 27.11 km2 at a rate of 2.05 km2/yr from 1988–1997, quickly increased by 510.53 km2 at a rate of 64.55 km2/yr from 1997–2005 and slightly increased by 148.16 km2 at a rate of 13.25 km2/yr from 2005–2017 (Fig. 2b). The area changes of Selin Co agreed well with previous results from Meng et al. (2012a). Selin Co also experienced a slight increase, accelerated growth, and a relatively slow increase during the period from 1976–2009, with a total increase of 656.64 km2 (39.4%).
Lake changes during the period also occurred simultaneously in sizes. The shapes of the three lakes changed accordingly, especially dramatic changes in Selin Co (Fig. 3). The shape of Bange Co changed slightly; these changes occurred in the shallow areas east of Bange II. Cuoe Lake expanded southward continuously. There was an opening in the west that was closed in some years so that a large rock island in the west of the lake was inundated. Mainly, a significant change in the shape of Selin Co was observed; the lake expanded rapidly northward, southward and southeast-southward, and marked expansion occurred along the lake shoreline in the southeast direction. The maximum expansion distance was approximately 146.7 m in the northwest direction. Notably, Yagedong Co rapidly enlarged, and it connected with the main body of Selin Co in 2004. Selin Co also expanded northward in 2005. Both these factors resulted in the sharp expansion of Selin Co during the period from 2004–2005.
3.1.2 Seasonal changes
As shown in Fig. 4, the areas of Bange Co and Selin Co displayed obvious seasonal oscillations. When spring began, the areas rapidly increased and experienced peaks in May, then continuously increased and reached annual maximums in summer, then finally sharply decreased after autumn. The area of Bange Co experienced peak values in March, May and July, and the area of Selin Co peaked in January, May and September. The area experienced minimum values in April for Bange Co and in March for Selin Co. The lake areas were generally stable when the lakes froze in winter. The peaks and valleys observed in winter were due to large snow coverage, which caused overdelineations of the lake boundaries. This commonly occurred for Bamu Co and Zonag Lake on the QTP, with valleys observed in February and March, respectively (Bhasang et al. 2012; Liu et al. 2019).
3.2 Changes in the lake levels and water volumes
Despite the lack of lake level data in some years, water level elevation data of Bange Co reconstructed from the lake shorelines at different dates from previous studies (Zhao et al. 2006; Zhao et al. 2011) revealed that the lake level has risen slightly since 1959 with an increase of 1.64 m; the water surface elevations were 4520.76 m in 1959 and 4522.5 m in 2010 (Fig. 5a). Due to data discontinuities since 2003, the variation in the lake level during the period from 1959–2003 was analyzed significantly. From 1959 to 2003, the lake level slowly fell by 0.25 m from 1959–1973, slightly rose by 1.75 m from 1973–2000 and then fell by 0.72 m from 2000 to 2003. Between 2000 and 2015, the lake level of Selin Co also displayed an upward trend, with a rise of 8.138 m at a rate of 0.48 m/yr. It markedly rose by 6.92 m during the period from 2000–2005 but relatively slowly rose by 1.21 m from 2005–2015 (Fig. 5b). There was a marked correspondence between lake area and lake level in which a rapid increase occurred before 2005; the area of Selin Co significantly increased by 357.65 km2 at a rate of 70.38 km2/yr between 2000 and 2005, compared to an increase of 156.68 km2 at a rate of 16.69 km2/yr during the period from 2005 to 2015. Specifically, between 2003 and 2009, Selin Co experienced a relatively fast water-level rise of 3.80 m (0.59 m/yr), which was consistent with the increases in the water level of Selin Co estimated by numerous studies due to the use of available ICESat data since 2003, as well as the water level fluctuations of other alpine lakes. Selin Co was reported to exhibit significant water-level rises of 4.79 m (0.625 m/yr) (Song et al. 2013) and 4.37 m (0.69 m/yr) (Zhang et al. 2013) during the period from 2003–2009.
Based on lake area and lake surface elevation data, the lake volume was calculated as follows:, where is the lake volume change from area (A1) and lake surface elevation (H1) to area (A2) and lake surface elevation (H2) (Zhang et al. 2013). From 1988 to 2010, the lake volume of Bange Co slightly increased by 0.088 km3, during which increases of 0.025 km3 from 1988 to 2003 and of 0.065 km3 from 2003 to 2010 were detected (Table 2). During the period from 2000–2015, as the lake water level increased by 8.13 m and the lake area increased by 514.33 km2, the lake water volume of Selin Co dramatically increased by 17.47 km3. Likewise, the trend in the lake water volume before and after 2005 was similar and highly consistent with those of the lake area and lake level; the water volume substantially increased by 14.63 km3 during the period from 2000–2005 but slightly increased by 2.84 km3 from 2005–2015. This was well explained by the significant shape change described above; Yagedong Co merged into the body of Selin Co in 2004, and the lake rapidly expanded northward from 2014–2015.
Table 2
Changes in the lake volumes of Bange Co and Selin Co.
Lake
|
Period
|
Elevation change (m)
|
Area change (km2)
|
Volume change (km3)
|
Bange Co
|
1988–2010
|
0.69
|
8.55
|
0.088
|
1988–2003
|
0.2
|
13.35
|
0.025
|
2003–2010
|
0.49
|
-4.79
|
0.065
|
Selin Co
|
2000–2015
|
8.13
|
514.33
|
17.47
|
2000–2005
|
6.92
|
357.65
|
14.63
|
2005–2015
|
1.21
|
156.68
|
2.84
|
3.3 Influential factors
3.3.1 Climate factors
The average annual precipitation (AP), mean annual air temperature (MAAT), and annual evaporation (AE) during the period from 1988–2017 recorded at the Bange Co and Shenzha meteorological stations were 345.5 mm and 345.1 mm, 0.003°C and 0.63°C, and 1,926.2 mm and 1,914.2 mm, respectively. Both the AP and MAAT at the two stations displayed overall increasing trends; the AP increased at rates of 1.38 mm/yr and 2.95 mm/yr, and the MAAT rose at rates of 0.05°C/yr and 0.04°C/yr at the Bange Co and Shenzha stations, respectively (Fig. 6a); however, during the same period, AE declined at rates of 10.68 mm/yr and 16.12 mm/yr, respectively (Fig. 6b), which was in accordance with the quantitative estimation that a nonsignificant decrease in actual evaporation of 4.17 mm/yr was observed for the lake area of Selin Co (Zhou et al. 2016). This climatic characteristic is consistent with the observations that the climate in most regions on the QTP experienced rapid warming-wetting in the late 20th century and early 21st century (Yang et al.2011; Li et al. 2018). It is also obvious that the variation trends in AP, MAAT and AE were generally similar for the two meteorological stations, with consistent peak and valley values, especially maximum values in 2009 and minimum values in 1997, which commonly represent local climatic conditions within the Selin Co basin.
The increasing trends in AP and MAAT exhibited three stages. The AP sharply decreased from 1988–1995, gradually increased from 1995 to 2005 and then slowly decreased (Fig. 7a). The MAAT gradually fell from 1988 to 1997, rapidly rose from 1997 to 2007 and then slowly rose (Fig. 7b). A significant decreasing trend in AE was observed from 1988 to 2017, which also displayed three stages (Fig. 7c). AE gradually declined from 1988 to 1997 but increased from 1997–2007; these variations were attributed to the decreasing and increasing MAAT, respectively. After 2007, AE sharply declined, and the decrease in wind speed may have been responsible for these changes. The area changes of Bange Co and Cuoe Lake corresponded well to the variations in the AP. The lake areas decreased slightly from 1988–1995, which agreed with the decrease in the AP. The lake areas continuously increased from 1995 to 2005 with increasing AP. After 2005, the lake areas decreased with increasing AP. This result suggested that the changes in the areas of Bange Co and Cuoe Lake were closely associated with the AP, and precipitation was the most critical influencing factor. However, the area change of Selin Co coincided with the increasing MAAT. The area of Selin Co slowly increased from 1988 to 1997 with a slight increase in the MAAT, quickly increased from 1997 to 2005 due to a rapid increase in the MAAT and slowly increased from 2005 to 2017 with a slight increase in the MAAT. This indicates that the rapid expansion of Selin Co had a close relationship with the continuous increase in the MAAT, as the MAAT has important impacts on glacier ablation processes and permafrost thawing. The area changes of the three lakes were inconsistently correlated with AE; the areas decreased slightly from 1988–1995 as AE declined, continuously increased from 1995 to 2005 with increasing AE and then decreased in spite of the declining AE.
3.3.2 Glacier contribution
Bange Co and Cuoe Lake are nonglacier-fed lakes, and their water budgets mainly depend on precipitation and evaporation. As mentioned above, precipitation was the main driver behind their observed changes. Selin Co is a glacier-fed lake with high glacier coverage in its basin. There were 297 glaciers with a total area of 423.09 km2 in 1980, accounting for approximately 1% of the basin area. Lakes supplied by many glaciers within their catchments may be more affected by increasing glacial meltwater than by precipitation (Zhang et al. 2011). As the MAAT recorded at Tanggula meteorological station increased at a rate of 0.03°C/yr between 2005 and 2015 (Fig. 8a), glaciers in the Selin Co catchment considerably retreated. The statistical results showed that from 1980 to 2010, the total glacier area decreased by 165.1 km2 (39%); during this period, it decreased by 25.9 km2 (6%) from 1980 to 2001 and by 139.1 km2 (35%) from 2001 to 2010 (Fig. 8b). For Selin Co, a highly glacierized lake in a catchment on the QTP, such considerable glacier ablation generated a large amount of surface runoff from glacier meltwater, which contributed to the extension of Selin Co. Strong positive relationships between lake area change and glacier area change were observed for most large lakes on the QTP (Liu et al. 2020). In addition, a more rapid decline in glacier area was observed during the period from 2001–2010, corresponding well to a much larger rise since 2000. Meng et al. (2012a) derived lake water level variations from topographic maps and GPS measurements and found that the water level of Selin Co increased by 8.2 m between 2000 and 2010 at a rate of 0.82 m/yr compared to a rise of 4.3 m between 1976 and 2000 at a rate of 0.18 m/yr. This further suggested that increasing meltwater supply from glaciers is the dominant factor behind the rapid expansion of Selin Co.
The lake supply coefficient was defined as the ratio of the basin area to the lake area, as follows: (AC=AB/AL=(EL-PL)/(rPB) + 1), where AC is the lake supply coefficient; AB and AL are the basin area and lake area, respectively; EL and PL are the lake surface evaporation and precipitation, respectively; PB is the precipitation in the basin; and r is the runoff coefficient. For an endorheic lake, PL is equal to PB, which means that the lake supply coefficient (AC) mainly depends on the lake surface evaporation (EL) and runoff coefficient (r) (Li et al. 2011). For glacier-fed lakes, positive correlations between water-level rising rates and supply coefficients were also reported for lakes in the Pamir and Tienshan Mountains and on the QTP (Li et al. 2011; Song et al. 2014). Due to the large lake watershed area, Selin Co has a large supply coefficient of 18.9 based on a basin area of 45,530 km2 and a lake area of 2,405.65 km2 as measured in 2017; this large coefficient also accelerated lake growth to a certain extent. In comparison with Nam Co, which has a supply coefficient of 5.36 (Song et al. 2014), although the two lakes have similar glacial meltwater supplies, the much larger supply coefficient of Selin Co may partly account for its more rapid growth rates than those of Nam Co.
3.3.3 Permafrost degradation
The largest permafrost degradation on the QTP has occurred in the marginal zones of the permafrost region (Pang et al. 2011; Qin et al. 2017), and permafrost over Selin Co was distributed only in its southern marginal zones. The island of permafrost was distributed in the southeastern basin with an area of 76.38 km2, accounting for only 3% of the whole Bange Co catchment. The ground ice content was only 0.39 km3. For Selin Co, continuous permafrost was distributed in the northeastern basin with permafrost coverage of 1.3×104 km2, and an island of permafrost was distributed in the southern basin with a small area. The ground ice content over the basin was 148.4 km3 with a range of 2.0 ~ 85.2×106 m3 (Fig. 9a). As evidence of significant permafrost degradation, both the ALT at the QT04 observation site and the ground temperature of permafrost at a 15-m depth at the QTB15 borehole site drilled upstream of the Selin Co basin experienced increasing tendencies. The ALT increased by 58.7 cm at a rate of 7.44 cm/yr from 2006–2019, and the soil temperatures at a 15-m depth rose at a rate of 0.0346°C/yr from 2005–2017 (Fig. 9b). The increase in the ALT here was much larger than that of the ALT (1.29 cm/yr) on the whole QTP (Xu et al. 2017), and an increase of 1.96 cm/yr was determined for the ALT along the Qinghai-Tibet Highway from 1981–2017 (Liu et al. 2020b). This result suggested that accelerated permafrost degradation around Selin Co occurred.
As permafrost degradation occurred, a large amount of ground ice content in the catchment melted; this melt was not only more likely to provide more water resources but also increased aquifer activation to enhance hydrological processes in the basin and further supply rivers and lakes, given the rise to a marked expansion of Selin Co. Meltwater directly influenced the groundwater recharge and water levels of Selin Co or increased the amount of groundwater discharge as surface drainage. Some of the meltwater even directly drained to become surface runoff and supplied Selin Co. Moreover, the presence of ice-rich permafrost in the Selin Co basin served as a barrier layer due to its low hydraulic conductivity and permeability (Yang et al. 2003; Woo et al. 2008; Wang et al. 2009). Ice-rich permafrost impeded liquid water infiltration and the interaction between surface water and groundwater, which finally resulted in a large amount of direct surface runoff due to both rain and snow-glacier melting due to the lack of a water storage buffer effect.
As shown in Table 3, precipitation from May-September mainly accounted for 93% of the annual precipitation in the study site. Both precipitation and the air temperature increased from April, reached maximum values in July, and decreased after July. Evaporation showed an obvious increase from January to April and then decreased, but it continued to increase after September and appeared to peak in October. Large areas of Bange Co and Cuoe Lake observed from May-August were mainly attributed to the most significant increases in precipitation. For Selin Co, glacier ablation runoff mainly occurred from July to September due to rising temperatures (Zhang et al. 2009). According to thawing and freezing processes of the active layer of permafrost near the Tanggula region, the active layer begins to thaw downwards from the ground surface at the end of April, and the thawing process reaches its maximum depth in late autumn (Zhao et al. 2000; Hu et al. 2014). Glacial meltwater and ground ice meltwater significantly participate in mountainous discharge in glacial regions, and this discharge is greater than precipitation in this basin (Zhang et al. 1997). Therefore, glacier melting and permafrost thawing with high air temperatures during the period from June-August can account for the large area of Selin Co observed during the period from August-November well, with a corresponding response lag. At the seasonal time scale, the relationship between lake group areas and influencing factors further confirmed that changes in the areas of Bange Co and Cuoe Lake were mainly related to increasing precipitation, and glacial and permafrost meltwater were significant factors influencing Selin Co growth.
Table 3
The statistics of monthly lake areas and meteorological elements from 2015–2017.
Climate factors
|
Jan
|
Feb
|
Mar
|
Apr
|
May
|
Jun
|
Jul
|
Aug
|
Sep
|
Oct
|
Nov
|
Dec
|
Precipitation (mm)
|
3.93
|
0.2
|
5.07
|
7.66
|
32.1
|
73.1
|
95.96
|
92.56
|
37.83
|
5.2
|
0.2
|
0.16
|
Percent (%)
|
1.11
|
0.05
|
1.43
|
2.16
|
9.06
|
20.64
|
27.10
|
26.14
|
10.68
|
1.46
|
0.05
|
0.04
|
Air temperature (°C)
|
-11.19
|
-6.27
|
-3.60
|
0.12
|
3.71
|
8.03
|
9.73
|
9.30
|
7.56
|
1.74
|
-3.40
|
-5.86
|
Evaporation (mm)
|
81.86
|
117.93
|
144
|
173.4
|
166.8
|
130.76
|
141.1
|
124.76
|
120.96
|
175.5
|
129.6
|
127.66
|
Percent (%)
|
5.00
|
7.21
|
8.81
|
10.60
|
10.20
|
8.00
|
8.63
|
7.63
|
7.40
|
10.73
|
7.92
|
7.81
|
Bange Co area (km2)
|
111.1
|
111.5
|
115.1
|
108.9
|
116.4
|
116.9
|
117.9
|
117.8
|
115.5
|
113.0
|
112.0
|
108.6
|
Selin Co area (km2)
|
2400.53
|
2400.40
|
2400.39
|
2400.53
|
2400.70
|
2400.73
|
2400.82
|
2400.97
|
2401.00
|
2400.96
|
2400.89
|
2400.75
|