Abstract
Lake Urmia, a hypersaline lake in northwestern Iran, is under threat of drying up. The high importance of the lake's watershed for human life demands a comprehensive monitoring of the watershed's behavior. Spaceborne sensors provide a number of novel ways to monitor the hydrological cycle and its interannual changes. The use of GRACE gravity data makes it possible to determine continental water storage changes and to assess the water budget on monthly to multi-annual time scales. We use satellite altimetry data from ENVISAT and CryoSat-2 to monitor the lake water level. Moreover, we employ optical satellite imagery to determine the surface water extent of the lake repeatedly and at an appropriate time interval.Our altimetry results indicate that, on average, the lake has lost 34±1cm of its water level every year from 2002 to 2014. The results from satellite imagery reveal a loss of water extent at an average rate of 220±6km2/yr, which indicates that the lake has lost about 70% of its surface area over the last 14years. By combining water level from altimetry, surface water extent from satellite imagery and local bathymetry, we ascertain the changes in lake volume. Results indicate that the lake volume has been decreasing at an alarming rate of 1.03±0.02km3/yr. The water volume of the lake behaves differently from the water storage of the whole basin captured by GRACE. Our results show that the onset of a drought in 2007 over this region together with an increase in the rate of groundwater depletion caused a new equilibrium level for water storage of the whole basin. Comparing the results from GRACE and the obtained water volume in the lake with in situ groundwater level data reveals the anthropogenic influences on an accelerated lake desiccation. In fact, our monitoring approach raises critical issues regarding water use in the basin and highlights the important role of spaceborne sensors for any urgent or long-term treatment plan.
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