**1. Introduction**

In recent decades, many lakes in the Tibetan Plateau have exhibited continued and rapid expansion [1–3], in contrast to a general trend of lake shrinkage in other regions and basins around the world. Qaidam Basin, located in the northeastern margin of the Tibetan Plateau, is an arid alpine region with scarce precipitation and intense evaporation. Thus, the regional hydrological cycle is sensitive to both climate change and human activity. As an important part of the hydrological cycle in arid areas, lakes are possibly affected by climate change processes such as increased precipitation and temperature [3,4]. For example, higher temperatures accelerate glacier melting and increase runoff into the lake, leading to lake expansion. Simultaneously, increased precipitation may partly contribute to lake expansion [5–7].

In addition to climate change, the impact of groundwater contribution to lakes should also be considered. The role of groundwater in lakes has previously been observed in Qinghai Lake [8] (China), Nalengele River [9] (China), Pyhajarvi Lake (Finland) [10], and other basins [11], although the sources of groundwater have not been thoroughly studied. A recent study [12] revealed that the endorheic Qiangtang Basin has a large amount of missing water (up to 540 × 10<sup>8</sup> m3/year), which leaks through six rifts in the south of the basin and may subsequently upwell in surrounding areas. This observation invalidates the

**Citation:** Zhang, X.; Chen, J.; Chen, J.; Ma, F.; Wang, T. Lake Expansion under the Groundwater Contribution in Qaidam Basin, China. *Remote Sens.* **2022**, *14*, 1756. https://doi.org/ 10.3390/rs14071756

 Academic Editors: Massimo Menenti, Yaoming Ma, Li Jia and Lei Zhong

Received: 11 February 2022 Accepted: 1 April 2022 Published: 6 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

traditional water budget theory of watersheds and likely also affects the water budget of surrounding areas. Qaidam Basin, which is adjacent to Qiangtang Basin, is a key area for water discharge. Studies of radon (222Rn) isotopes have revealed that lakes in the Qaidam Basin have an extensive groundwater contribution [9,11]. Therefore, in this study, we investigated lake surface area changes in response to climate change and groundwater contributions in the northeast of Qaidam Basin, which is important to understand the sources, circulation, and evolution patterns of regional groundwater.

The expansion or shrinkage of a lake directly reflects the lake water budget. In inland basins of alpine regions, mountain precipitation and/or glacial meltwater converge into rivers. When these rivers flow through the piedmont plain, some of the water infiltrates as groundwater, and some continues to flow downstream, flowing through alluvial plains to eventually form endorheic lakes. Endorheic lakes are generally located at the lowest elevation in the basin, where they form a confluence of surface water [9,13]. The regional distribution of groundwater heads determines whether the lake discharges to groundwater or groundwater contributes to the lake, which controls groundwater inflow and outflow in the water balance of the endorheic lake. The groundwater level in Qaidam Basin gradually decreases from the mountains to the plains; thus, groundwater flows from the mountains to the lakes in the plains, where it eventually contributes to the lakes [8,9,11]. Therefore, the water input component of these endorheic lakes predominantly includes surface runoff, lake precipitation, and groundwater inflow, and the water output component is principally lake evaporation; groundwater outflow can typically be ignored [14].

Thus, it is important to accurately quantify lake evaporation before exploring the causes of lake expansion. By comparing the eddy covariance system with several combined evaporation models, McJannet [15,16] concluded that the Penman–Monteith model was most suitable for estimating lake evaporation because it considers the effects of both vapor pressure gradient and wind speed. However, the meteorological monitoring network in alpine areas is typically sparse, with stations often located far from lakes; therefore, it is difficult to obtain long time-series of meteorological monitoring data near lakes. The improved Penman–Monteith model proposed a general application of wind functions and thus can be used to calculate lake evaporation from remote overland meteorological measurements [15,16], which has been applied in several subsequent studies. Lake surface area changes can also reflect changes in lake water storage [17–19]. However, because of the remote environment and lack of monitoring stations, lakes in the Tibetan Plateau lack long-term monitoring data related to water levels and lake surface area. Thus, satellite remote sensing technology is used in this study to observe long-term changes in lake surface area and water level [2,4,20,21]. Moreover, 2H and 18O isotopes in water bodies are ideal natural tracers for identifying groundwater sources and tracing hydrologic cycles [11,13]. Furthermore, tritium (3H) can determine the rate of groundwater circulation and the groundwater age [20].

We investigated three lakes in this study (Tuosu Lake, Keluk lake, and Gahai Lake), all of which are located in the northeast of the extremely arid Qaidam Basin. Remote sensing techniques, model calculations, and statistical analyses were used to analyze lake surface area changes in response to climate change and groundwater contribution in the study area, and potential groundwater sources were identified using stable isotopes. The aims of this study were to (1) provide scientific support for the utilization and managemen<sup>t</sup> of water resources in Qaidam Basin and (2) propose measures for mitigating future environmental and geological problems related to continued lake expansion, thereby protecting inhabitants and production in the study region.

### **2. Study Region**

The study area (96◦34–97◦54E, 36◦58–37◦40N) is located in the northeast of Qaidam Basin, China, which is surrounded by the Buhete Mountains to the east, the Delingha uplift to the west, the Denan hills to the south, and the Zongwulong Mountains to the north (Figure 1). The landscape is predominantly mountainous, alluvial–proluvial plain,

and alluvial lacustrine plain. The study area has a typical plateau continental climate, with annual average temperature, precipitation, and pan-evaporation values of 4.7 ◦C, 211 mm/year, and up to 1845 mm/year, respectively.

The main river (Bayin River) in the study area originates from the Zongwulong Mountains and has a total length of 188 km and a catchment area of 7281 km2. The Bayin River flows east to west between Zongwulong and Buhete Mountains and then north to south after flowing through the Heishishan reservoir [11]. The Bayin River seeps underground in the middle of the alluvial–proluvial plain and then upwells as springs at the front edge of the alluvial plain. Because of the influence of the Denan hills, the Bayin River flows westward downstream across the alluvial lacustrine plain and eventually flows into Keluke Lake. Keluke Lake is connected to Tuosu Lake by the Lianshui River, forming a terminal endorheic lake. Gahai Lake is another terminal endorheic lake located in the southeast of the study area, which has a weak hydraulic connection with the Bayin River through southeast groundwater runoff.

**Figure 1.** Spatial map representing the distribution of samples, including river, lake, spring, confined groundwater, and phreatic groundwater in the northeast of the Qaidam Basin, China. Squares represent samples from the literature [11].

Groundwater in the Delingha area mainly occurs in porous quaternary loose sediments in the plain areas. At the top and middle of the alluvial–proluvial plain, the single-layer alluvial aquifer is more than 300 m thick and mainly composed of sand and gravel; the depth of the groundwater level is 80–120 m and 10–30 m, respectively. At the end of the alluvial–proluvial plain, the aquifer system changes from a single-layer structure to a multi-layer structure, sediments are mostly fine-grained, and the groundwater level is shallow (<10 m) [13]. In the lacustrine plain downstream of Bayin River, the aquifer system consists of a multi-layer aquifer with interbedded clay and fine sand. In the study area, groundwater flows from south to north in the upper part of the alluvial–proluvial plain and then flows westward again because of the effect of the Denan hills [21], which are predominantly tertiary clastic rocks interspersed with mudstone and gypsum with limited infiltration capacity. In addition, the scarce precipitation and low precipitation intensity (a single precipitation event is less than 10 mm) hinders groundwater formation. In the western part of the alluvial–proluvial plain, groundwater flows from north to south and

southeast because of the Delingha uplift, before finally flowing westward [11]. In the eastern part of the alluvial–proluvial plain, a small amount of groundwater flows southeast along the ancient river channel and converges at Gahai Lake.

Since 2000, Tuosu and Gahai Lakes have exhibited rapid expansion. The continuous expansion of Gahai Lake has caused groundwater levels to rise in the vicinity, which threatens the safety of inhabitants and their livelihoods. Therefore, the causes of lake expansion have become a key area of research for local governments. Delingha has a permanent population of 88,200 and a population density of only 2.88 people/km<sup>2</sup> (the seventh National Census). Because of their remote environment and minimal human impact, the lakes in the study area are suitable for studying the effects of climate change and groundwater on lake expansion.

The Bayin River has two hydrologic stations (Figure 1), the Delingha (96◦16E, 37◦22N) and Zelinggou stations (97◦48E, 37◦25N). Zelinggou station is located 7 km upstream of Delingha station, near the exit of the mountain. The runoff measured at this station is mainly glacial meltwater from the mountainous area; hence, Zelinggou station runoff represents the amount of glacier melt in the lakes. The catchment between Delingha and Zelinggou station is defined as the upstream area. The runoff measured at Delingha station includes glacial meltwater and runoff generated by precipitation in the upstream area. The monitoring period for Zelinggou station was 1959–1984, as the station was withdrawn after 1984. Data from these hydrological stations were used for subsequent statistical analysis of runoff trends.
