Estimating environmental water requirements for terminal lakes under varying meteorological conditions: A case study of the Big Aral Sea, Central Asia

Preserving terminal lakes in arid inland basins by satisfying environmental water requirements is critical for sustainable watershed management. A framework was proposed to assess environmental water requirements under varying meteorological conditions. The framework incorporated water balance model construction, time series analysis, scenario simulation, and dual-objective optimization, providing dynamic environmental management objectives for terminal lakes. The dynamic environmental management objectives were more flexible and practical than the static ones because they adjusted optimal solutions according to meteorological conditions. The framework was applied in the Big Aral Sea in Central Asia, a representative terminal lake suffering from a shortage of environmental water supply. As a result of the compounding effect of reduced runoff and enhanced evaporation, the lake shrank dramatically from 1950 to 2001, with the water level decreasing from 52.90 m to 32.67 m. The reduced runoff dominated the lake's shrinkage, dropping the water level by 19.11 m. The enhanced evaporation aggravated this process, lowering the water level by 1.12 m. The lake management objectives and associated environmental water requirements were obtained for wet, normal, and dry years, given the impact of meteorological conditions on lake water budgets. The environmental water requirement was indicated by river inflow into the lake. In a wet, normal, and dry year, the objective of preserving the water level to 33.96, 32.34, and 30.82 m and the lake area to 21.94, 17.53, and 13.37 thousand km2 required an environmental water supply of 13, 10, and 7 km3, respectively. Results from this case study showed the rationality and practicality of the framework, which would be an effective tool for environmental water assessment.