Lake studies from satellite radar altimetry

For more than 10 years, satellite radar altimetry has been a successful technique for monitoring the variation in elevation of continental surface water, such as inland seas, lakes, rivers, and more recently wetland zones. The surface water level is measured within a terrestrial reference frame with a repeatability varying from 10 to 35 days depending on the orbit cycle of the satellite. With several decades of technique refinement, current data processing can be fairly simple or complex depending on the mission and the instrument tracking methods. Data acquisition is not affected by weather conditions, but the technique can have a number of limitations. Rapidly varying topography or complex terrain may inhibit the retrieval of good elevation data. The instruments can only operate in a profiling mode and do not have a true global view. Stage accuracies are also dependent on target size and surface roughness, which will limit worldwide surveying and limnological applications. However, there is the scope for systematic continental scale monitoring and the provision of new stage information where gauge data is absent. The technique is sufficiently advanced to have enabled a number of inland water case studies. Focussing on the large lakes, the links between lake evolution and the local climate cycle on seasonal to interannual timescales have been explored, and water storage balance for water management has also been brought into focus. Typically altimetric stage measurements can range in accuracy from a few centimetres (e.g., Great Lakes, USA) to tens of centimetres (e.g., Lake Chad, Africa), depending on size and wind conditions. In the case of the Aral Sea, contributing E-P (1.5 km 3 yr −1 ), river run-off (3.0 km 3 yr −1 ) and altimetry (1.5 km 3 yr −1 ) errors combine to give a ∼3.5 km 3 yr −1 water mass balance error. Of particular interest are those lakes in arid and semi-arid regions where water is an essential economical resource. Anthropogenic influences (irrigation and the construction of dams) may strongly affect lake and inland sea evolution or affect the global water cycle in general, noting the worldwide development of large reservoirs over the last 50 years. With regards to societal and economic issues then, this article reviews several of the case studies focussing on the Aral and Caspian Seas, and Lakes Issykkul and Chad.