Aral sea hydrology from satellite remote sensing

Abstract

Space technologies have been widely used over the last 10 years for water surface monitoring worldwide and they have shown their capability to monitor components of the water cycle and water balance at regional scales and on time scales ranging from months to decades. We present here the applications of space data from radar altimetry and satellite imagery (Terra/MODIS) over the Aral Sea Basin (ASB). Radar altimetry, which has been designed to study the ocean, has opened a new era in monitoring lakes, rivers and reservoirs. The recent missions of satellite altimetry (Topex-Poseidon, Jason-1/2, Envisat, ERS-1 and ERS-2) have made it possible to measure with great precision inland sea level variations that can be used to determine water mass balances. Radar altimetry, coupled with complementary in situ data, has allowed quantifying precisely the water balance of the Aral Sea since 1992 as well as balances for large reservoir systems along the Syr Darya, in particular Chardarya and Toktogul, and for Lake Aydarkul. This approach has also made it possible to ascertain the water balances of lakes and wetlands in the deltas of the Syr Darya and Amu Darya. Satellite imagery, from low to high resolution (1 km to a few meters) offers a useful tool to monitor surface water area for lakes and floodplains. MODIS data for example provide every 8 days, the surface water area from 2000 to 2012, with a spatial resolution of 500 m. It has been used to create a spatial time series for the Aral Sea and the lakes and wetlands in the deltas of the Amu Darya and Syr Darya where water area has been precisely measured. Along with in situ observations and hydrological modelling, space observations have the potential to improve significantly our understanding of hydrological processes at work in large river basins, (including lakes, reservoirs and floodplains) and their influence on climate variability and socio-economic life. Unprecedented information can be expected coupling models and surface observations with data from space, which offer global geographical coverage, good spatial-temporal sampling, continuous monitoring over time, and the capability of measuring water mass change occurring at or below the surface. Based on these different techniques we have determined the surface area of water features within the Aral Sea Basin, as well as volume variations, which are the key parameters to the understanding of the hydrological regime in ungauged basins. A focus on the Aral Sea and the water bodies in the deltas of the Syr Darya and the Amu Darya rivers over the last 20 years from satellite data is presented in this chapter, with some implications for the water balance. We will also describe the specific behaviour of the Western and Eastern basins of the Large (South) Aral Sea over the last 5-6 years.