A four-band index for both liquid and solid water and its applications in the Aral Sea Basin

Various investigations have been conducted to analyze the water-coverage area of the Aral Sea and the Aral Sea Basin (ASB). However, the investigations incorporated considerable uncertainty and the used water indices had misclassification problem, which made different research groups present different results. Thus we first ascertain the boundaries of the ASB, the Syr and Amu river basins as well as their upper, middle and lower reaches. Then a four-band index for both liquid and solid water (ILSW) is proposed to address the misclassification problems of the classic water indices. ILSW is calculated by using the reflectance values of the green, red, near infrared, and thermal infrared bands, which combines the normalized difference water index (NDWI) and land surface temperature (LST) together. Validation results show that the ILSW water index has the highest accuracy by far in the Aral Sea Basin. Our results indicate that annual average decline of the water-coverage area was 963 km2 in the southern Aral Sea, whereas the northern Aral Sea has experienced little change. In the meanwhile, permanent ice and snow in upper reach of ASB has retreated considerably. Annual retreating rates of the permanent ice and snow were respectively 6233 and 3841 km2 in upper reaches of Amu river basin (UARB) and Syr river basin (USRB). One of major reasons is that climate has become warmer in ASB. The climate change has caused serious water deficit problem. The water deficit had an increasing trend since the 1990s and its increasing rates was 3.778 billion m3 yearly on average. The total water deficit was 76.967 billion m3 on average in the whole area of ASB in the 2010s. However, up reaches of Syr river basin (USRB), a component area of ASB, had water surplus of 25.461 billion m3. These conclusions are useful for setting out a sustainable development strategy in ASB.