Changes in area and water volume of the Aral Sea in the arid Central Asia over the period of 1960–2018 and their causes

Highlights

• The area of the Aral Sea shrank by 60,156.50 km² (about 87.85%).

• The total loss of water volume was approximately 1,000.51 km³.

• Glacier mass loss in the basin was much larger than lake volume variation.

• Human activities especially damming and irrigation are the driving factors.

Abstract

The Aral Sea (68,478 km² in 1960) was the world’s fourth largest inland lake in 1960s. However, it shrank sharply over the past six decades, and its changes caused a series of severe environmental issues. In this paper, we reconstructed its variations over the period of 1960 to 2018 by using observation data and remote sensing data, and analyzed their influencing factors. The results show that the area of the Aral Sea shrank dramatically by 60,156.50 km² (about 87.85%) and the total loss of water volume was approximately 1,000.51 km³ over the study period. In 1986, the Aral Sea broken up into the south and the north parts. Since then, the South Aral Sea has shrunk continuously, while both the area and the water volume of the North Aral Sea have had a little change and shown a very slightly increasing trend. Through comprehensive analysis, it was found that human activities, especially damming and irrigation, are the dominant factors influencing the long-term variation of the Aral Sea. The increased precipitation and glacier meltwater could not compensate for the water loss of the Aral Sea.

Introduction

Inland lakes are essential components of global hydrological and biogeochemical water cycles (Sheng et al., 2016). They play a crucial role in freshwater supply and river runoff regulation (Giardino et al., 2007), which help maintain oases and improve the regional eco-environment. Inland lakes are sensitive to the regional climate and environmental change, and human activities (Mason et al., 1994, Chaudhari et al., 2018). The arid Central Asia is a vulnerable eco-environment. It is thus of vital importance to study variations of inland lakes in this region, to better understand their responses to both natural climatic and anthropogenic factors, and the impacts on the regional water cycle, eco-environment, and human living conditions.

Remote sensing can provide long-term, frequent, and continuous observations at the global scale, offering a convenient tool for monitoring lake variations in the arid Central Asia (Singh et al., 2012, Shi et al., 2014). Recent studies indicate that the total area of inland lakes in Central Asia decreased by 45,352.83 km² (about 49.62%) during 1975–2007, but this overall decrease showed an obvious heterogeneity at different altitudes. The terminal lakes in plain areas shrank dramatically, while alpine lake areas were relatively stable (Bai et al., 2011). The trend for inland lakes also change with time. For instance, the water level of Bosten Lake in the Tien Shan Mountains experienced a declining-rising-declining trend during the periods of 1955–1986, 1986–2002, and 2002–2009, respectively (Li et al., 2011a, Shi et al., 2003).

In the mid-20th century, the Aral Sea was ranked as the world’s fourth largest inland lake (Izhitskiy et al., 2016). However, increasing water withdrawal since the 1960s, especially for agricultural irrigation, and drought since the 1970s have resulted in a sharp decline in water level and area (Zavialov et al., 2003, Cretaux et al., 2013). The area of the Aral Sea decreased by 55,369 km² (82.03%) from 1960 to 2011, with the lake splitting into two bodies of water in 1986 (Micklin, 2007). The water level of the North Aral Sea rose at first but then fell, while that of the South Aral Sea fell continuously, during the period of 2003–2009 (Li et al., 2011a). In recent years, the North Aral Sea has experienced a slight expanding trend. In contrast, the South Aral Sea continues to shrink, and its eastern part still has the possibility of disappearance because its water level is relatively low (Micklin, 2016, Barale and Gade, 2019).

The shrinking of the Aral Sea has had socio-economic consequences and led to severe ecological issues including the aggravation of drought, desertification, and soil salinization (Zavialov et al., 2009, Gaybullaev et al., 2012). Therefore, it is necessary to monitor the variations of the Aral Sea and to analyze their responses to climatic and anthropogenic factors, to provide a scientific basis for rational exploitation and protection of water resources, as well as regional sustainable development in the arid Central Asia.

Previous studies have focused primarily on recent hydrological changes of the Aral Sea based on quantitative analysis of remote sensing data (Shi et al., 2014, Sun and Ma, 2019). A comprehensive and systematic analysis of long-term continuous variations in area and water volume, and their potential driving factors, is still lacking. Therefore, the objectives of this study were as follows: (1) to extract lake surface area of the Aral Sea using the Normalized Difference Vegetation Index (NDVI) based on the MOD02QKM product during 2000–2018; (2) to quantify and characterize the inter-annual variations in area and water volume of the Aral Sea during 1960–2018 based on MODIS and long-term hydrological data; and (3) to comprehensively explore the dominant factors affecting fluctuations in area and water volume of the Aral Sea in terms of climate change, glacier meltwater, and human activities.