Response of the water level of the Balkash Lake to the distribution of meteorological and hydrological droughts under the conditions of climate change

Droughts are among the most common and severe natural disasters worldwide due to their frequency, wide range of impacts, and long duration. Most often, they arise due to water shortage, which can be the result of the interaction of various factors, such as weather conditions (precipitation deficit), hydrological, hydrogeological, or water management factors (Wong et al. 2013; Jiang et al. 2020; Yerdelen et al. 2021).

According to the classification (Wilhite 2000; Dai 2011) droughts can be divided into meteorological, hydrological, agricultural, and social-economic, as they indicate water shortage, which is caused by an imbalance between the demand and supply of water for various activities. Meteorological droughts caused by deficient precipitation over a long period of time are the driving force behind other types of droughts. For example, lack of rainfall leads to decreased river flow and insufficient groundwater recharge.

Hydrological droughts occur when river flow and water reserves in aquifers, lakes and reservoirs are below long-term average values. Hydrological drought usually occurs with a delay compared to meteorological drought. Regions are interconnected by hydrological systems; therefore, the area of distribution of hydrological drought could be greater in extent than the area of meteorological drought that caused it. Identifying the relationship between hydrological droughts and climate-related precipitation deficits is often complicated by the simultaneous impact of other factors on the hydrological characteristics of the basin, such as ineffective water resource management, changes in land use (land degradation, soil erosion), as well as the construction of reservoirs and dams (Salnikov 2020).

As a result of climate change, extreme droughts have become more frequent worldwide (Trenberth et al. 2013; Mukherjee et al. 2018; Ault 2020), especially in Central Asia (Guo et al. 2018; Chen et al. 2019; Salnikov 2020), which is one of the driest regions of the world. The increasing intensity and frequency of droughts are one of the key factors that affect sustainable economic development and environmental security. Thus, the occurrence and impact of drought in the context of climate change is an important research topic that has received additional attention from scientists (Nam et al. 2015; Ukkola et al. 2020; Haile et al. 2020).

The increase in drought due to climate change and anthropogenic factors increases the impact on the hydrological cycle in countries with a water crisis, especially in countries with drainless basins and lakes that do not have access to the oceans. Drought monitoring, i.e. determining the time of its onset and end, and its types, is of great importance in planning and managing water resources (Salnikov 2020). Most of Central Asia is a giant drainless region (more than 6,000 lakes with a total area of 12,300 km²). The drainless lakes of Central Asia include the Aral Sea and the lakes Alakol, Ebinor, Lobnor, etc., including the Ile River basin and Lake Balkash (Savvaitova & Petr 1992; Che 2019).

Ile-Balkhash watershed originates in the high mountains and is almost entirely fed by glaciers, snow cover and precipitation and its lower reaches contrast sharply with hyper-arid (extra-arid) deserts. The problem of drought is relevant in this region and according to the authors (Yegizbayeva et al. 2024), drought has affected about 44% of the Balkash Lake basin (including the territory of Kazakhstan and China). According to the authors, severe agricultural droughts occurred in 2000, 2008, 2014 and 2015, highlighting the region's vulnerability. Any type of drought in this region has significant impacts on water resources, agriculture and food security. In addition, changes in the river flow regime, rising temperatures, changes in the nature of precipitation and increased anthropogenic factors have a complex impact on the optimal level of Balkhash Lake, which is very important for the country.

Understanding how meteorological droughts transform into hydrological droughts is critical for managing hydrological droughts. However, most studies have focused on aspects of the distribution of meteorological and hydrological droughts in river basins and much less attention is paid to how these droughts affect the level and area of the lake, its flora and fauna.

As long as the lakes are an important part of the hydrological cycle, especially in arid, drought-subjected regions and store large amounts of freshwater resources, they are of great value to aquatic ecosystems. Long drought in the lake area leads to the death of many aquatic plants, to decrease in the self-purifying ability of the lake and deterioration in water quality and could also lead to the drying out and destruction of a significant part of the wetlands (Havens et al. 2007; Sobhani et al. 2019; Perales et al. 2020).

Wetlands play a vital role in maintaining ecological balance, reducing droughts, preserving water sources, and water and soil loss and providing environmental, economic and social benefits for human survival and development (Zang et al. 2016). Since 2012, the southern part of Balkash Lake and the Ile River delta have been included in the list of wetlands of global importance under the Ramsar Convention (Tesch & Thevs 2020). According to UNEP (United Nations Environment Programme), the number of migratory birds has almost halved over the past 30 years, as they are at greatest risk during the migration season, so the survival rate of their nestling is rapidly decreasing from year to year. Kazakhstan is located on the migration and nesting route of a dozen species of birds, such as pink flamingos, pelicans, cranes and others. Flying across dozens of countries, the birds are affected by the triple planetary crisis associated with climate change, loss of biodiversity and environmental pollution (Imentai et al. 2015).

Wetlands in arid landscapes provide vital habitat for millions of migratory waterbirds worldwide. Migratory waterbirds in arid and semi-arid mid-latitudes rely on a limited number of important wetlands to connect continental movements that support annual life-history events (Zang et al. 2016). To preserve them, the Ile-Balkash State Nature Reserve was created in June 2018. The Ile-Balkash natural reserve is represented by 32 species of fish, 3 species of amphibians, 19 species of reptiles, 284 species of birds, and 39 species of mammals (Imentai et al. 2015).

Since 2004, the United Nations Development Program (UNDP), together with the Government of Kazakhstan with the support of the Global Environment Facility (GEF), has implemented a number of initiatives aimed at the comprehensive conservation of priority globally significant wetlands as nesting sites and habitats for migratory birds in Kazakhstan. In this regard, it is very important to identify hydrological and meteorological droughts in the region of river basins and lakes, since it is not surprising that drought is one of the factors causing changes in the level and area of lakes, which in turn plays an important role in maintaining the ecological system of this region. The following drought indices are used to determine hydrological drought in the lake basin: Standardized Precipitation Index (SPI) and Streamflow Drought Index (SDI) (McKee et al. 1993; Shukla & Wood 2008; Li et al. 2016).

The purpose of this scientific research is to consider the phenomenon of hydrological drought in the Balkhash Lake basin, to study meteorological and hydrological patterns of drought distribution and to determine their relationship with the lake level changes.

The lake's watershed covers an area of 413,000 km², of which about 15–20% is located in the territory of People's Republic of China. Its length is approximately 600 km, its width varies from 9–19 km in the eastern part to 74 km in the western part. The significant size of the territory, inland location, and orographic and climatic heterogeneity determine a wide variety of natural conditions. According to the nature of the orography, the basin is divided into three parts (Dostai 2009; Dostai et al. 2012).

The northern and northwestern part which occupies the main region of the Northern and Northwestern Balkash region is located in the area of Kazakh hummocky terrain. The relief of this part of the basin is characterized by a combination of plains with hills, the slopes of which are dissected by numerous hollows. The tops of the hills rise 30–40 m, sometimes 100–200 m above the surrounding area.

The central part is the Balkash depression. It is an accumulative sandy-desert plain stretching from the southern border of Saryarka to the mountain belt in the south and southeast. In the northern part of the Balkash depression, there is a flat plateau. The prevailing heights here are 400–450 m. The plateau is dissected by hills, knolls, valleys of dry ravines and drying-up rivers. In the southern part of the depression, there are mainly sandy deserts: Taukum, Moiynkum, and Saryyesik Atyrau. The slope of the plain toward the lake from all sides is very insignificant, from the south it is only about 0.003°.

The southeastern and southern parts of the basin are occupied by the mountainous region of Zhetysu Alatau and the northern ranges of the Tien Shan mountain system. The mountain region of Zhetysu Alatau (in the limits of Kazakhstan) includes the Tarbagatai ridge, Urkashar and Zhaiyr hills, Barlyk and Maily ridges, Zhetysu Alatau ridges with altitudes of 2,000–4,000 m.

The Ile depression, dividing Zhetysu Alatau and Borokhoro mountain systems from the eastern part of the Northern Tien Shan, stretches for hundreds of kilometers from the Kulzha oasis in Xinjiang to the Kapshagai gorge. Its absolute heights vary from 600 m near the Kazakh-Chinese border to 500 m in the area of the Kapshagai hydroelectric power station dam site. Approximately in the middle, this section of the Ile depression is divided by Katu and Kalgan hills on the right bank and Bugyty mountains on the left bank and is divided into two independent depressions. In the western part, the Ile depression is closed by the Karaoy and Itzhon plateaus (Sokolova 1989; Dostai et al. 2012).

The sources of surface watercourses of the considered territory are mainly located in the mountainous areas surrounding the region – the mountain ranges of Tien Shan, Jungarian Alatau and Tarbagatai. From the eastern and southern parts of Balkash Lake the largest tributaries originate from the mountain formations of the northern part of the Ile Alatau, Jungarian Alatau and Tarbagatai. Ile River provides the biggest part of the total flow; there are also smaller rivers – Karatal, Lepsi, Aksu and Ayagoz. With the exception of Ayagoz, all of them are full-flowing arteries carrying large amounts of water and are especially high-water in the summer due to the intensive melting of glaciers in the mountains. From the west and north, the Mointy and Tokraг rivers flow into the lake, having active flow only in the upper reaches and having huge dry valleys which are completely waterless throughout the year with the exception of a short period of spring snowmelt. The unevenness of river flow is noticeably reflected in the level of Balkash Lake, which is subject to significant fluctuations.

The river's water regime is mainly determined by climatic conditions, namely the regime and amount of precipitation, distribution of air temperature, evaporation, share of river feeding sources, nature of the relief as well as hydrogeological and other features of river basins. One of the main factors of runoff is the topography of the basins and, above all, the absolute height. With the change in the absolute height of the catchment, climate factors and underlying surface change consequently change the feeding conditions of the rivers. While glaciers and snow play a significant role in feeding rivers in high mountain regions, in the mid-mountain and foothill zones the importance of seasonal snow cover, liquid precipitation and groundwater increases significantly (Sokolova 1989; Dostai 2009).

To study meteorological and hydrological droughts in the area under consideration, the data on atmospheric precipitation and river flow were used. Meteorological and hydrological data were atmospheric precipitation series from 21 meteorological stations (MSs) and runoff series from 8 hydrological stations (HSs) located in the Balkash Lake basin. Systematic observation data were taken from the materials of RSE ‘Kazhydromet’ (from the beginning of instrumental observations until 2020 inclusive).

Positive SPI values indicate humid conditions. Negative SPI values represent dry conditions; the lower the SPI, the drier the time period under study. Precipitation time series for calculating SPI must be sufficiently long, at least 30 years. Since the SPI is associated with probabilities, for each specific location the probability of drought of a certain intensity can be determined. The beginning of drought can be determined when the SPI value drops below −1.0, and the end of the drought is determined by the time the index becomes positive. Drought intensity should be calculated as the sum of all SPI values during the drought period.

Table 1 shows the different drought categories with extreme dry (below −2) and extreme wet (above 2). The number of drought classifications used by SPI and SDI ranges from seven to nine, with some classifications combining moderate wet and moderate drought into one value.

According to Svoboda & Fuchs (2017), drought occurs whenever the indices drop to −1 or below. The drought ends when the indices reach positive values. Thus, to determine the beginning of different types of droughts, precipitation accumulations are used on the following scales: 1–2 months – for meteorological drought; 1–6 months – for agricultural drought; 6–24 months or more – for hydrological drought. However, a 9-month SPI associates seasonal drought with long-term droughts that can become hydrological, and a 12-month or more SPI is associated with a significant decrease in river waters, reservoir levels and groundwater levels.

SPI and SDI values were calculated for 12- and 24-month periods to determine drought conditions for each station. SDI values were calculated using DrinC (http://drinc.ewra.net/) (Tigkas et al. 2015, 2022) and SPI values were calculated using SPI Generator Application software (McKee et al. 1993; Edwards & McKee 1997; Ali et al. 2020).

The water level of the Balkash Lake is characterized by significant cyclic fluctuations caused by fluctuations in the humidity of the territory and, accordingly, by the water balance of reservoirs. The main factor of the lake formation is inflowing rivers. The inflow into the western part of the lake is carried out by Ile River and to the eastern part by the rivers Karatal, Lepsy, Aksu and others. The change in the long-term fluctuations of Balkhash Lake mainly depends on the flow of Ile River (about 80%), since this river belongs to most of the water flowing into the lake.

In modern conditions, changes in reservoir level depend not only on natural factors determining the amount of river flow but also on the operating mode of the Kapshagai hydroelectric power station, irreversible water consumption and construction of new reservoirs in the upper part of Ile River basin in the territory of the People's Republic of China, as well as in the middle part of the basin in the territory of the Republic of Kazakhstan. The second factor in the incoming part of the lake's balance is atmospheric precipitation, which accounts for about 20%. It is very important to take into account long-term changes in these two main factors of the lake's water balance since the moisture content of the territory and drought conditions in the considered area depend on them.

Analysis of the drought characteristics in the Ile-Balkhash basin based on the used aridity indices shows that the spatial distribution of severity, intensity and duration was relatively similar across all hydrometeorological stations, but differed in drought duration. According to the figures, the time course of dry and wet periods based on precipitation and river flow data are synchronous, but the dry period calculated based on precipitation (SPI index) occurs a little earlier compared to the SDI index calculated using river flow data.

In this study, drought is defined as a period of time when the index value is persistently negative and reaches a threshold of −1.0 or lower. In this regard, according to SPI data, the driest period in the Ile River basin and in Zhetysu Alatau (Lepsi, Karatal, Koksu) river basins was the period from 1943 to 1947, when extreme hydrological drought was observed (Figures 3–5), especially in the Southern Balkash region: MS Lepsi – 1944 with an index of −3.51, MS Matai – 1945 with an index of −3.34, MS Sarkand – 1945 with an index of −3.32 (Figure 4).

During the period from 1974 to 1978, the Ile-Balkhash basin was regularly subjected to severe and extreme meteorological and hydrological droughts. The results of scientific research also show that in the time period from 1980 to 1990, there was a dry period compared to other decades, which is confirmed by research (Alimkulov et al. 2019); periods of low water were observed in all rivers of the considered basin.

Table 2 shows the parameters of extreme droughts identified by SPI with a time scale of 24 months with values below −2. Analysis of Table 2 shows that in the considered region, the duration of drought varies widely from 141 months (MS Narynkol, 1976–1987) to 7 months (MS Kogaly, 1958), and the minimum value of the SPI index is −3.91 (MS Zhalanash, 1977–1981).

Table 2

Parameters of extreme droughts identified by SPI with a time scale of 24 months and with values below − 2

Identification of groupings of years of hydrological drought that influenced the lake level yellow (first critical level) and green (second critical level).

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In the modern period, a large-scale hydrological drought in the considered basin was recorded in the period 1996–1998, lasting 34 months (MS Kuigan). It is advisable to note that in a conditionally natural period, the duration of hydrological drought ranges from 7 months (1930s, MS Almaty) to 36 months (1,040 s, MS Lepsi). The duration of hydrological drought in the modern period has increased for the longer period by 3 times (from 34 months to 141 months).

In 1943–1946 in spatial terms, the SPI values show that at weather stations located near the lake, moderate drought is observed and in the areas where river flow is formed, the drought values reached extreme values. Consequently, the intensity of drought increases from the branches of the southern part of Kazakh hummocky terrain through Balkash Lake to the northeastern ridges of the Zhetysu and Ile Alatau. In the period 1973–1975, the spatial distribution of drought had a different character; the areas of the most severe droughts were concentrated in the Northern Balkhash region. Extreme droughts were recorded in the areas of the southwestern part of Tarbagatai Ridge and on the western coast of Balkash Lake (Figure 6).

Regarding the level of Balkash Lake, hydrological drought clearly has a tendency to influence, since the first critical drop in the water level in the lake occurred in 1946 (Figure 2), which is confirmed by calculations using the SPI drought index in almost all MS of the considered region. From 1944 to 1947 (highlighted in yellow in Table 2), a drought was recorded; its maximum duration at MS Lepsi was 36 months.

The second critical drop in the lake water level was observed in 1987 at the level of 340.65 m BS, the occurrence of hydrological drought during this period is confirmed by calculations using the SPI drought index at the Sarkand and Narynkol MSs (highlighted in green in Table 2), the maximum duration at the Narynkol MS was 141 months. Table 3 shows the parameters of severe hydrological droughts identified by SDI with a time scale of 12 months and with values below −1.0.

Table 3

Parameters of severe hydrological droughts identified by SDI with a time scale of 12 months and with values below −1.0

Identification of groupings of years of hydrological drought that influenced the lake level yellow (first critical level) and green (second critical level).

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The results of Table 3 analysis showed that during the first critical decrease in the lake's water level, the parameters of severe hydrological droughts according to the SDI flow index were identified from 1943 to 1946 with a drought duration from 11 months along Ile River – 37 km to 36 months along Lepsi River – Lepsy (highlighted in yellow in Table 3).

It should be noted that, according to the SDI flow index, during the second critical decrease in Balkash Lake water level, the parameters of severe hydrological droughts were identified from 1982 to 1987 with drought duration from 12 months along Koksu River to 52 months along Ile River (highlighted in green in Table 3). In the Ile-Balkhash water economy basin, the average duration of hydrological drought for the period before 1973 was 13 months; for the modern period, the duration of hydrological drought is 22 months.

Analysis of Figure 6 shows that the occurrence of extreme hydrological droughts along the Ayagoz River is 2%, along the Tekes River it is 3%, and along the Koksu River it is 7%. The frequency of severe hydrological droughts on the main rivers of the considered region varies from 8% to 12%. It appears that with the probability of serious problems associated with hydrological drought, there will be irreversible consequences for the ecological system of Balkhash Lake and the environment, leading to intense water shortage problems. This suggests the requirement to develop strategic plans for water management and to ensure preparedness in drought-subjected areas to combat the increased risk of water scarcity.

This scientific article conducted a study aimed at assessing hydrological drought using two drought indices: SPI and SDI. The importance of such analysis is considered in the context of hydrological systems for monitoring, forecasting and effective management of water resources. Based on the research results it was concluded that SPI and SDI indices are highly effective in diagnosing and monitoring hydrological drought. It is shown that these indices provide an objective and quantitative assessment of river flow deficit, which is important for the rapid response to drought conditions. The performed analysis has practical importance for planning the sustainable use of water resources, taking measures to adapt to dry periods and ensuring the sustainability of water supply in various sectors of the economy, such as agriculture and industry. Thus, the study results highlight the importance of using SPI and SDI indices to assess hydrological drought, providing valuable information for developing water resource management strategies and mitigating the negative impacts of drought conditions.

This research was funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP19677869 ‘Hydrological bases for managing the level regime of Balkash Lake’).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.