Present State of the Aral Sea: Field Surveying and Modeling

Up to the early 1960s, the Aral Sea, a terminal lake situated in the Central Asian part of the former Soviet Union, was the fourth largest inland water body of the planet. Starting from 1961, the water budget of the Aral Sea that had been close to equilibrium for centuries became deficient because of large-scale diversions of river runoff for agriculture. Today, the overall level drop in the principal part of the lake is about 23 m. Salinity has attained values around 100 g/kg in the western basin and 135 g/kg in the eastern basin. In 2002, Russian Academy of Sciences in collaboration with a number of local institutions from Uzbekistan and Kazakhstan launched a long-term programme of field research and monitoring of the Aral Sea. The paper gives a summary of results obtained to date (2002-2006) within the programme, with special focus made on the hydrographic research. To date, 6 field surveys have been accomplished. Over the period of the observations, the surface salinity in the western, relatively deep basin has increased from about 82 g/kg in November 2002 to over 99 g/kg in March 2003. Because the lake surface level remained almost constant during this period, spanning only slightly between 30.1 and 30.7 m above the ocean level, this continuing salinity build-up in the western basin must be associated not with further shrinking of Aral's volume but, rather, with water exchanges with the shallow and saltier eastern basin. In the latter, salinity has decreased from up to 160 g/kg as previously reported for 2002 to only about 130 g/kg in 2006. As revealed from modeling (adapted Princeton Ocean Model), the denser eastern basin water sinking down the slope of the western trench is responsible for enhanced vertical stratification observed in 2002 and 2003. Such stratification greatly damped vertical mixing, which resulted in hypoxia and sulfide contamination in the bottom layer. However, the water column was ventilated in spring of 2004, presumably, following deep winter convection, thus suggesting that stratification and hypoxia are intermittent rather than permanent features. Modeling of today's water budget of the lake indicates that the present Aral Sea is close to dynamical equilibrium.