Numerical modeling of vertical stratification of Lake Shira in summer
A one-dimensional numerical model and a two-dimensional numerical model of the hydrodynamic and thermal structure of Lake Shira during summer have been developed, with several original physical and numerical features. These models are well suited to simulate the formation and dynamics of vertical stratification and provide a basis for an ecological water-quality model of the lake. They allow for the quantification of the vertical mixing processes that govern not only the thermal structure but also the nutrient exchange, and more generally, the exchange of dissolved and particulate matter between different parts of the lake. The outcome of the calculations has been compared with the field data on vertical temperature and salinity distributions in Lake Shira. Lake Shira is meromictic and exhibits very stable annual stratification. The stratification is so stable because of the high salinity of the water. If the water in Lake Shira were fresh and other parameters (depth, volume, and meteorology) were the same, as now, the lake would be mixed in autumn. Using the newly developed models and using common meteorological parameters, we conclude that Lake Shira will remain stratified in autumn as long as the average salinity is higher than 3‰.
KeywordsStratification Simulation Salinity
In a stratified reservoir, wind-induced currents and the structure of the thermocline mainly control the vertical distribution of heat, dissolved substances, and nutrients in the water column (Belolipetskii et al. 2002; Bonnet et al. 2000; Elci 2008; Holloway 1984; Gargett 1991; Patterson 1991; Belyaev 1992; MacIntyre 1993). Stratification in summer acts as a barrier restraining the mixing of the water column. The warm water in the epilimnion is unable to mix with the cold, dense water of the hypolimnion. As a result of the incomplete mixing of the water column and lack of light for the photosynthesis at the hypolimnion, the water column can become anoxic. It is shown (Imberger and Patterson 1990) that when stratification is established in small to medium lakes, transverse and longitudinal heterogeneities are negligible compared to those in the vertical direction. The need to manage the quality of the water stored in lakes and reservoirs has led to the continuing development of numerical models capable of predicting the dynamics of vertical stratification and mixing. Many such models have been constructed. To mention a few: SEEMOD (Zamboni et al. 1992), Minlake (Riley and Stefan 1987), LIMNMOD (Karagounis and Zamboni 1993), DYRESM (Imberger and Patterson 1981), PROBE (Svensson 1998; Elo et al. 1998), GOTM (Burchard et al. 1999), and others (Belolipetskii et al. 2002; Bonnet et al. 2000).
Meromixis is a condition of persistent chemical stratification with incomplete mixing over the course of a year; it usually results in anoxia and the accumulation of nutrients in the monimolimnion and reduced vertical mixing. Numerous saline lakes worldwide are known to be meromictic (Hammer 1986). The water in the upper mixed layer (the epilimnion) is characterized by strong turbulence, which usually leads to relatively constant temperature distribution. The mixed layer is separated from the deeper hypolimnetic waters by a region of increasing density. One of the main factors that determine the mixing status of Lake Shira is salinity. It is therefore important to note that from 1980 the average salinity has decreased approximately from 20 to 13‰ (Kalacheva et al. 2002). A gradient of salinity is formed in spring because of introducing fresh water due to melting of ice and snow. Later, this salinity gradient prevents the lake from being mixed by wind. In summer, the water is stratified because of both the temperature and the salinity gradients. In autumn, only the salinity gradient maintains the stratification. Therefore, if the water of Lake Shira were fresh, it would be mixed in autumn. Using the models documented in this paper, we have calculated the minimum value of salinity necessary to prevent Lake Shira from mixing by wind.
Earlier studies investigated the mathematical modeling of hydro-thermodynamic processes in Lake Shira (Belolipetskii et al. 2002). In this earlier study, the vertical distribution of salinity was not calculated and, mainly, summer stratification was investigated. The present study deals with the mathematical simulation of the hydrophysical processes in Lake Shira, including the lake thermal structure, wind and wind-induced currents, and the conditions for steady stratification in autumn have been investigated. Therefore, much attention has been given to the vertical salinity gradients.
Summarizing, there are two aims of this study. The first one is to develop, parameterize and compare a one-dimensional model and a two-dimensional model for Lake Shira, which can serve as the basis for an ecological water-quality model of the lake (Prokopkin et al. 2010). The second aim is to analyze, using both models, at which level of salinity meromixis is predicted to disappear.
The lake is located in Siberia (54°29′N and 90°14′E) and is an inland lake, without islands. Lake Shira features unique therapeutic properties, which are largely determined by the specific chemical composition of the water (Lobova et al. 2002). The Son River flows into the lake from the south. As the inflowing volume of water is very small, the river only impacts the lake near its mouth. The lake has an oval shape and is 9.4 km long and about 5 km wide. The surface area of the lake is 34.7 km2, the maximal depth is 24 m, and the average depth of the middle part is 21 m. The underground water input constitutes about 9% of the total water input of the lake. The water temperatures are much higher in the epilimnion (15–20°C) than in the hypolimnion (2–3°C) in summer. The salinity is higher in the hypolimnion. The summer stratification is very stable because of the temperature stratification and the high salinity of the lake. The distinct density stratification is defined by both the decrease of the water temperature and increase of salinity with the lake depth. So mixing between the top and bottom layers is very low—this fact was repeatedly proved by measurements and was confirmed during simulations.
Equations for one-dimensional model
There are various methods of calculation of short-wave radiation. In some of papers, penetrating radiation is taken into account only in the boundary conditions: α = 0 (see, for example Odrova 1979), in the other ones—in the right part of Eq. 1: α = 1 (see, for example Henderson-Sellers 1987).
Equations for two-dimensional model
The factors of horizontal turbulent exchange K x , K xT , K xS are constants.
Numerical algorithm for one-dimensional model
Numerical algorithm for two-dimensional model
In the two-dimensional case, we solve the model by the splitting method (Belotserkovskii 1984; Fletcher 1988), which was realized in (Belolipetsky 2005; Belolipetskii and Belolipetsky 2006). The numerical algorithm consists of several stages:
Parameterization of vertical turbulent mixing
In the described procedure, the intensity of the vertical turbulent exchange is defined by the velocity gradient and stratification.
Parameterization of density
Parameterization of heat flow
Parametrization of wind stress
Implementation of the model and sources of meteorological data
The most important meteorological parameters affecting the variations in the water-quality parameters are the air temperature, lagged wind speed and humidity (Elci 2008). The meteorological data input into the model includes the cloud cover, air temperature, humidity, wind speed and direction. The meteorological data for the years 2002–2008 were taken from the server “Weather of Russia—weather archives” (http://meteo.infospace.ru/). These data were collected at a meteostation located 10 km west of Lake Shira.
Comparison between the model results and the field data
Analysis of salinity level influence on stability
Both the one-dimensional and two-dimensional numerical models allow for simulating dynamical changes in vertical stratification of Lake Shira. Although only the distributions for 2004 are presented (Figs. 1 and 2), they are illustrative for all years, because differences between years are small. The vertical gradients of salinity and temperature in the lake are formed in spring. The salinity profile is created because the inflow of fresh water from melting ice and snow onto the surface. The temperature profile is created by the heating of the surface water. Together these processes cause the formation of a thin upper layer (approximately 1 m) with low salinity (Fig. 2a) and high temperature (Fig. 1a). The density gradient is very large so the turbulent mixing and wind-forced currents cannot mix this thin upper with the lower layers. Subsequently, the energy of wind causes a deepening of the upper layer. Thus, the depth of the upper layer keeps growing until ice formation in late autumn. Also, the heating of the upper layer continues until midsummer while the temperature of the lower layer stays the same or changes only very slowly. So, at midsummer there are two processes contributing to stable stratification: the gradients of temperature (Fig. 1b) and salinity (Fig. 2b). Afterward, the water temperature does not change significantly for about a month. Then, in autumn, the upper layer is cooled and before ice formation, the temperatures of both layers become approximately equal (Fig. 1c). During these temperature transformations, the gradient of salinity is observed on the same depth as the gradient of temperature, and this depth is growing. Consequently, saltwater is added to the upper less saline mixed layer. Therefore, the salinity of the upper layer grows and that of the lower layer stays the same, and the gradient of salinity decreases but stays strong enough for preventing Lake Shira from mixing (Fig. 2c).
As it is mentioned, from 1980, the average salinity has decreased approximately from 20 to 13‰ (Kalacheva et al. 2002). What changes may cause the continuing decreasing salinity level? Is it possible that this may result in changing the mixing regime? An attempt was made to investigate this problem using our numerical models. If the water of Lake Shira were fresh, the summer stratification would not be very different (Fig. 3a). But in autumn, the lake would be fully mixed (Fig. 4a). From the salinities now observed in Shira (12–14‰) to the salinities in the range of 2–3‰, both the one-dimensional and two-dimensional models predict the stability of stratification (Figs. 3c and 4c). For the salinities in the range of 1–2‰, the two-dimensional model predicts the possibility of mixing while the one-dimensional model is still predicts the lake to be stratified (Figs. 3b and 4b). Using meteorological parameters near the common values, on basis of the two-dimensional model, it is concluded that Lake Shira will be stably stratified as long as the average salinity is higher than 3‰. The one-dimensional model predicts the possibility of mixing, again using the common meteorological parameters, only when the salinity is less than 1‰. We therefore conclude that the salinity in Lake Shira can be decreased greatly—from the average salinity of 13‰ to the average salinity of 3‰, and the lake will still stay meromictic. The reason is that the density changes associated with the salinity are very large in comparison with the changes in density associated with the temperature near 5°C. For example, the density difference between 4 and 5°C is approximately 100 times smaller than the density difference between 2 and 3‰ salinity.
The results of the one-dimensional model and two-dimensional model are in good agreement for the salinities higher than 3‰. When the salinity is smaller, the two-dimensional model predicts the possibility of mixing of Lake Shira, and in the one-dimensional model the lake is still stratified. The reason for the discrepancy in results between the one-dimensional and two-dimensional model at an average salinity of 3‰ is that the two-dimensional model, in addition to turbulent mixing, simulates transport processes caused by the wind-forced vertical circulation. When the density gradient is large, these wind-forced currents are not essential. But when the gradient is small, they become important and cause the observed differences between the results of the models. So, it is possible to use the simpler one-dimensional model for simulating the vertical dynamics in Lake Shira during the period free of ice only when the average salinity is higher than 3‰.
Two numerical models for the investigation of the vertical structure of a closed stratified lake were developed and analyzed. The model verification was carried out on the basis of the measured vertical temperature and salinity distributions in Lake Shira. The models show that for common meteorological parameters, Lake Shira will be stratified in autumn as long as the average salinity is higher than 3‰. The simpler one-dimensional model can be used for describing hydrophysical processes from spring until autumn as long as the average salinities in the lake are higher than 3‰.
The study was financially supported by the Netherlands Organization for Scientific Research (NWO), Grant 047.011.2004.030, RFBR, Grant 05-05-89002; RFBR, Grant 07-01-00153; Multidisciplinary integration project of SB RAS No. 95.