1. Introduction
Due to the arid climate, water has become a priority in the socio-economic development of Central Asia. Along with the rapid economic growth and population rise in this region, the future demand for water resources will certainly continue to increase and the water shortage will likely become more and more serious [
1,
2]. A population growth of 20 million people in this region is expected by 2040 [
3], which will require additional irrigated areas in response to a larger food production. For example, the irrigated area in Uzbekistan is expected to rise by 5–11% by 2020 [
4,
5], eventually causing a 5–19% increase in the water demand [
6]. Also, water resource management will encounter additional difficulties from climate change, which will increase the vulnerability of the water supplies [
7]. On the one hand, the projected temperature augmentations will lead to a longer growing period and a higher actual evapotranspiration, causing a 5% rise in the irrigation water consumption by 2030 [
8,
9]. On the other hand, the glacier loss will augment the discharge at first and will have little effect on the annual discharge once the glaciers have disappeared. Therefore, the glacier loss in the Pamir, Tien-Shan, and Alay mountains has been accelerating because of the higher temperature, which will potentially cause the annual river discharges in Central Asia to become as low as 50% by 2050 [
10,
11]. Identifying the links between the water resources, human activities, and climate change in Central Asia could offer the potential to improve the water resource management for human well-being and environmental sustainability.
In addition, due to the transboundary water conflict, water resource allocation in Central Asia is a big and complicated problem [
12]. After the fall of the Soviet Union in 1991, the region’s major rivers (including the Syr Darya River and the Amu Darya River) became transboundary rivers, which triggered a series of water conflicts between upstream and downstream countries over the past few years [
13]. The Syr Darya River is the second longest river in Central Asia, which is a typical transboundary river and shared by Kyrgyzstan, Kazakhstan, Tajikistan, and Uzbekistan. It originates from the glacial meltwater and precipitation of the Tien Shan Mountains in Kyrgyzstan and eastern Uzbekistan (the upper streams are the Naryn River and the Kara Darya) and further meanders its way for about 2212 km west and northwest through Uzbekistan and southern Kazakhstan to the North Aral Sea. The river provides an abundance of water for agricultural production, industrial, and household activities and discharges huge amounts of sediments and freshwater into the North Aral Sea, thus adjusting the sea water level and the biogeochemical cycles [
14,
15].
With the rapid economic development and population explosion during recent years, the amount of water flowing from the Syr Darya River into the North Aral Sea has decreased over the past sixty years, directly causing a decrease in the sea water level [
16]. Typically, human activities including change in land use (especially the expansion of agricultural land) and dam construction (e.g., the Karkidon, Kasansai, and Andijan dams) have significantly affected the water supply and water demand patterns, while simultaneously being exacerbated by the increased pollutants [
17,
18]. Also, climate change is likely to exacerbate the water stress in the Syr Darya River Basin [
19]. As the water resources of the Syr Darya River mainly come from the glaciers and snowmelting in the high mountain ranges of the Tien Shan Mountains, the rising temperatures cause melting glaciers and ice sheets and disturb the hydrological dynamics as a result (e.g., the total and seasonal streamflow) [
20]. These changes eventually trigger a lower and lower runoff [
21], which causes negative implications for the water availability in the Syr Darya River Basin (and for the conflicting water demands between agriculture and hydropower).
Efforts in research have led to significant improvements in the evaluation of the water environment and resources in the Syr Darya River Basin [
22,
23,
24]. For example, using satellite data, Crétaux et al. [
25] estimated the total water storage of reservoirs in the Syr Darya River Basin; Wegerich et al. [
15] discussed the critical role of the water supply in achieving a sustainable water security and provided some measures for irrigated agriculture. Although all these studies focused on the changes in water resources in the Syr Darya River Basin, the majority of previous studies mainly discussed the water resources in a regional sub-basin and only a few researchers analyzed the water resources for the whole basin [
22]. Moreover, these studies only investigated the complete basin changes based on the limited hydrological and meteorological stations and remain unclear in some fields (e.g., the seasonal changes in the river discharges) [
17]. For example, Savoskul et al. [
14] comprehensively investigated the water modifications and other natural resources (climate, topography, land cover, and so on) and the related socio-economic aspects, but did not show the detailed seasonal changes between the upper, middle, and lower regions of the Syr Darya River.
Therefore, the annual and seasonal changes in the river discharges, precipitation, and temperature and their correlations were investigated for the whole Syr Darya River Basin in this study. We also explored the potential for the land use impact on the water resources. Specifically, this study addresses: (1) The manner in which climate change is affecting the high mountains and plains of the Syr Darya River Basin; (2) the differences in the upstream and middle and lower regions of the Syr Darya River Basin under global warming and human activities; and (3) the manner in which the land use changes and their impact on the water resources could be evaluated. Previous research hardly addressed these issues. In particular, our paper adds insights by analyzing the changes in water resources in the Syr Darya River Basin based on 20 hydrological stations from 1930 to 2006 (the obtained results provide essential information for effective water resource management in this basin) and exploring the annual and seasonal changes in the river discharges and their influencing factors (e.g., precipitation, temperature, and land use changes). These were explored and discussed and could offer important information for effective water resource management in the Syr Darya River Basin.
This study is structured as follows: The study area, datasets, and methodology are briefly described in the next section. The annual and seasonal trend results and possible influencing factors will be presented in
Section 3, followed by the conclusions and future work (
Section 4).
4. Conclusions, Management Measures, and Future Work
The annual and seasonal changes of the water resources in the Syr Darya River Basin were evaluated based on 20 hydrological stations from 1930 to 2006. Also, the correlations between the river discharges, climate factors, land use changes, and reservoirs were characterized. Some interesting findings were gathered and summarized as follows: (1) The stations located in the upper streams of the Syr Darya Basin showed an increasing trend in the river discharges from 1930 to 2006, while a decreasing trend was visible in the middle and lower regions; (2) the increased precipitation and melting water led to a rise in the amount of water that flows from the upper to the middle and lower rivers, eventually leading to a rise of the annual river discharges from 1974 to 2006; (3) the expansion of the construction land (128.83 km2/year) and agricultural land (66.68 km2/year) from 1992 to 2015 increased the water consumption, exacerbating the stress of the water resources in the Syr Darya River Basin; and (4) the establishment of dams (e.g., Kasansai, Karkidon, Andijan, Kasansai, and Tortgul) and irrigation canals has significantly cut off the river discharge, especially from the 1970s onwards.
Results could offer useful information that will help to establish effective water resource management in the Syr Darya River Basin. The future progress in the sustainable water resource development in this basin will require (1) an updated, legal, and executable framework for managing the transboundary water resources between Kyrgyzstan, Kazakhstan, Tajikistan, and the Uzbekistan countries, (2) the development of an integrated water management system in the Syr Darya River Basin for the optimal management of hydropower and irrigated agriculture, and (3) the increased investments in national water sectors including agricultural techniques, irrigation networks, and technologies so as to increase the water use efficiency and productivity. Therefore, in future work, more detailed conditions should also be considered for a sustainable water resource development for the whole basin.
This study also has a few shortcomings and suggests several areas for future work. Firstly, due to the lack of ground observation data, the CRU dataset was applied to calculate the precipitation and temperature and its course resolution (0.5°) could not perfectly capture the real precipitation and temperature, probably causing some uncertainties. If possible, more gridded climate datasets will be combined with the existed observed data in order to obtain more reliable climate data. In addition, the outcomes from the Pearson’s correlation could not fully measure the effects of the river discharges because the Syr Darya River was affected by many factors, including the precipitation, temperature, evapotranspiration, and so on. Finally, more complicated hydrological cycles will be considered in order to fully assess the water resources, especially in society-relevant extreme events such as floods, droughts, and so on.