2.1. The Study Area
Bosten Lake (41°56′ N–42°14′ N, 86°40′ E–87°26′ E) is situated in southern Xinjiang, the arid and semi-arid region in northwest China. It is the biggest lake in Xinjiang and was previously the biggest inland freshwater lake in China that has evolved into an oligosaline (subsaline) lake in the previous 60 years [
7,
30]. Bosten Lake is rich in fish, reeds, and waterfowls. The lake was referred to as the “Oriental Hawaii of Xinjiang” due to its distinct lush scenery enclosed by the rough Gobi Desert. The lake held natural outflow conditions in history until an artificial pumping station was constructed in 1983. Then, via a channel, the water of the lake has been pumped out to the Kongque River. The lake is at the beginning of the Kongque River (KQR) and the terminal of the Kaidu River (KDR) (
Figure 1).
The mean width and length of the lake is around 20–25 km from south to north and 55 km from east to west, respectively. When the water level is 1048 m a.m.s.l.(above mean sea level), the lake has an average and maximum water depth of 8.1 and 15 m, respectively, a water surface area of 1160 km
2, and storage capacity of 8.41 × 10
9 m
3. The lake is very shallow close to the shores and the deepest part is in its east-central region with a dissymmetrical bottom topography [
31]. The Kaidu River is the main perennial tributary stream of the lake, accounting for ≈83.4% of its total inflow. The annual runoff volume of the Kaidu River, on average, is 3.412 × 10
9 m
3. The Huangshuigou, the Qing-Shui River, and the Wu-La-Si-Te River are the other important tributary streams [
32]. The Caohu region is the southeastern corner of the lake. The climate here is labeled as dry with a hot summer and cold winter. The annual precipitation is only 68.2 mm, mostly falling during the summer months, and the annual potential evaporation rate reaches up to approximately 1800–2000 mm, and the mean annual air temperature is 8.4 °C [
33]. Winds principally come from the southwest, presenting the most important effect of the westerlies in the summer season. There are 14 monitoring sites for spatial salinity surveys (
Figure 1). Site 1 is situated to the lake southwest, and in front of the lake water outflow. Sites 7–12 are situated close to the agricultural wastewater discharge and tourist sites on the western lakeside. Site 13 and 14 are situated near the Kaidu River, which supplies the lake a considerable amount of fresh water. The five other sites are situated in the center of the lake.
Bosten Lake is very important for the region. It plays an important part in preventing floods from the Kaidu River and supplying water for the Kongque River basin and the downstream of Tarim River. It plays a significant part in wild reed and fish production, as well as wildlife breeding, and provides precious but limited water resources for industry, drinking consumption water for about 1.3 million people, around 190,000 ha of agriculture irrigation, and more than 50,000 km
2 downstream basin ecosystem. The lake water resource is of great importance for the societal stability and economic development of southern Xinjiang, as well as the ecological restoration of the lower reaches of the Tarim River watershed [
6,
17,
34].
However, the lake has been threatened by water salinization because of the reduction of water inflow and the increment of salt inflow. The lake water salinity has been commonly over 1.0 g L
−1 [
35,
36] and the lake has turned into a slight brackish lake since 1958 [
37]. The Bosten Lake water salinity went through three change periods because of the combined effects of climate change and anthropogenic activities in the past several decades [
38,
39]. Specifically, the lake water salinity indicated an obvious upward trend from the first observation (0.39 g/L) [
38], starting in 1956 and rising to its highest recorded salinity (1.87 g/L) in 1987. Then, the lake water salinity showed a successive downward trend and fell to its lowest record (1.17 g/L) in 2003. Thereafter, the water salinity rose dramatically from 2002 to 2010 and increased to 1.45 g/L in 2010. Increasing lake evaporation resulted in the successive increase in lake water salinity from 1958 to 1987. Increasing lake inflow and human-controlled lake outflow together resulted in the decrease in lake water salinity from 1988 to 2002. During 2003 to 2010, the lake water salinity increased sharply because the lake inflow significantly reduced due to a drop in precipitation; meanwhile, the water supply project of transferring water from Bosten Lake to Tarim River resulted in the growing of the lake outflow. The salinization of the lake is controlled by various elements, including the inflow, outflow, and water level of the lake, as well as the salt carried by agricultural drainage into the lake [
6].
Water salinization harmfully influences lake water systems, local eco-environments, and water use, and has turned into a severe environmental problem in Bosten Lake. Salinity was found to be the dominant factor that controlled the sedimentary abundance of
Betaproteobacteria and the bacterial community composition in the oligosaline Lake Bosten [
7,
40]. The spatial distribution of bacterial abundance was affected by the water salinity in Lake Bosten [
41]. Its water salinization should be managed to supply enough water resources for the surrounding arid area, and for protecting the environment of the lake and its surroundings [
6].
2.2. Model Description
In this paper, we implemented the EFDC Explorer 7, a widely used model developed by Dynamic Solutions International (DSI), for the hydrodynamic modeling of Bosten Lake. The EFDC model was originally developed by Hamrick (1992) [
42] from the Virginia Institute of Marine Science, and afterwards was financed by the United States Environmental Protection Agency (U.S. EPA). It is capable of modelling one-, two-, and three-dimensional flow; transport; and biogeochemical evolutions in surface water systems. It models topographically and density-induced circulation; wind-driven flows; and temporal and spatial layouts of temperature, salinity, and conservative/non-conservative tracers. Its reliability and validity in hydrodynamic modeling has been extensively tested in various aquatic systems at numerous sites worldwide, including reservoirs, lakes, rivers, wetlands, estuaries, and coastal ocean regions contributing to environmental management and assessment [
43,
44].
The hydrodynamic portion of EFDC uses the three-dimensional continuity, vertically-hydrostatic, free-surface Reynolds-averaged Navier Stokes equations formulated using the turbulent-averaged motion equations for a changeable-density fluid with the Boussinesq approximation and Mellor–Yamada [
45] turbulence closure [
42,
46]. The level 2.5 turbulence closure scheme of Mellor–Yamada is applied to calculate the vertical turbulent viscosity and diffusivity [
45,
47]. The code works out scalar transport equations in the aquatic column (e.g., salinity, temperature). Density-dependent vertical flows are simulated by ensuring mass conservation at every grid with a given flow boundary condition at the surface due to evaporation/condensation and precipitation, as well as at the bottom due to groundwater exchange. Horizontal flows are modelled by momentum equations without flow boundary conditions at lateral walls. The EFDC hydrodynamic model includes equations of continuity, momentum, state, and transport for salinity and temperature (see the
Appendix A for equations). Details of the hydrodynamic model and mathematical schemes of the EFDC model are documented by Hamrick (1992; 2007a; 2007b) [
42,
43,
44] and Hamrick and Wu (1997) [
46].
The model of Bosten Lake was built with a curvilinear, orthogonal horizontal coordinate system and a sigma vertical coordinate system. The horizontal plane contained 3737 active cells with a grid size of 500 × 500 m. Vertical sigma coordinate could model the bottom terrain better. The vertical sigma layers number was fixed and set to a constant and equal fraction within the whole model domain, whereas the regional thickness of every layer varied with the bathymetry of the model. The bathymetry data of the model from field measured water depth were applied to the model grids by kriging interpolation (
Figure 1). Ten equal thickness vertical layers were set to form the bathymetry. The simulation was performed from 1 April to 30 September 2005.
EFDC is forced by atmospheric conditions (e.g., wind shear, evaporation, and precipitation) and tributary inflow/outflows, among other factors. Hourly meteorological data of 2005 at Yanqi weather station (
Figure 1) in the vicinity of Bosten Lake, including air temperature, wind speed and direction, atmospheric pressure, cloud cover, relative humidity, pan evaporation, and precipitation, were obtained from the China Meteorological Administration. The wind rose map of the station is presented in
Figure 2. Due to the absence of solar radiation observations at Yanqi Station, or neighboring areas in 2005, solar radiation was calculated by the method of Rosati and Miyakoda (1988) [
48] and presumed to be the same in the whole model area. The solar attenuation coefficient was set as 0.45. The model flow boundary conditions included two inflows from the main stream, the Kaidu River at the southwest and agriculture drainage at the Huangshuigou region at the northwest, and one outflow to the Kongque River at the southwest of the lake (
Figure 1). Daily volumetric flow rate of river (
Figure 3) and water temperature data specified for the inflow of the Kaidu River at Yanqi hydrological station and outflow of the Kongque River at Tashidian (TSD) hydrological station were acquired from the Hydrological Yearbook of the People’s Republic of China [
49]. The time series of the Kaidu River discharge was taken as the inflow discharge into the Bosten Lake. In 2005, the Kaidu River discharges varied from 13.2 to 341 m
3/s, having a mean of 67.1 m
3/s. The time series of the Kongque River discharge was set as the outflow discharge out of the Bosten Lake. To simulate the real inflow and outflow of the lake, some factors were applied to the river inflow and outflow. Salinity of the inflows was set by the data obtained from interpolation of several monthly observations acquired at a nationally-managed environmental monitoring station.
The initial conditions involving water temperature and water salinity, and water level were set as constant values. The lake water surface was assumed to be horizontally flat. The initial water level was specified as 1047.5 m of the daily value of the Bohu hydrological station on 1 April 2005. The initial water flow rate was specified as zero.
Parameters associated with the Mellor–Yamada turbulence model [
45] were specified as the suggested values that was used in the Princeton Ocean Model [
50]. Similarly, the dimensionless horizontal viscosity in the Smagorinsky equation [
51] was specified as a given value of 0.1 [
52]. Bottom roughness (Z0) was specified as a representative value of 0.0036 m [
42,
50] for water level calibration. Critical wet and dry depth were specified as 0.1.
A two-time level explicit finite-difference scheme was used in the time integration of the model. Through an internal/external mode-splitting procedure, the internal shear (or baroclinic mode computed across each sigma layer) separated from the external free-surface gravity wave (or barotropic mode computed on the depth average). The model was executed using a 25 s time step to meet the needs of the stability criterion of Courant–Fredrichs–Lewy condition.
Model calibration and verification was dependent on water level data observed at Bohu hydrological station, as well as water temperature and salinity measures from 14 sites across the lake (
Figure 1). The 2005 daily averaged water levels were obtained from hydrologic yearbooks of the People’s Republic of China. The water surface elevation varied substantially within a year, with the highest value of 1047.46 m during the winter, and the lowest value of 1046.87 m during the summer. The observations of lake spatial salinity and temperature were obtained from the Xinjiang Environmental Protection Academy of Science for 14 sites on 11 May, 10 June, 7 July, 5 August, and 3 September of 2005. Of these sites, sites 13 and 14 are situated near the Kaidu River, the freshwater region. The other sites are situated in oligosaline regions, where they are under the impact of human activities in terms of tourism or agriculture. Site 1 is situated to the southwest of the lake. Sites 7–12 are impacted by eutrophic and saline agriculture wastewater drainage from the western lakeshore and tourist sites on the western lakeside. Sites 5 and 4 are close to the tourist region of the northern lakeshore (
Figure 1).
Because we are concerned with the impact of the hydraulic connectivity on the spatial-temporal salinity changes in Bosten Lake, three numerical experiments (
Table 1) were carried out for studying the impacts of the hydrological connectivity scenarios on the salinity distribution of the lake. They were A2—the increasing of the inflow of the Kaidu River into the lake, A3—the transferring of some freshwater to the Huangshuigou region, and A4—the changing of the outflow positon from the outlet of the Kongque River to the Caohu region. The simulation A1 was driven by actual observed data and was subsequently considered as the control run. In simulation A1, the model was initialized using average water temperature and salinity on 5 April, and driven by hourly mean surface heat flux and wind stress, and daily average flows of the rivers. In total there were four simulations. Experiments A2–A4 were hydraulic connectivity scenarios. From simulations A1–A4, the influence of three connectivity scenarios on the salinity distribution of Bosten Lake can be found.