Monitoring the Lena River Spring Breakup Flood 2013 with TerraSAR-X Imagery and the TanDEM-X Digital Elevation Model

Kurzfassung

River runoff is the single largest contributor to the Arctic Ocean's freshwater budget.. The Lena River in Eastern Siberia is one of the major rivers flowing into the Arctic Ocean. More than 50% of the total runoff from the mainland originates from the four large rivers Ob, Yenisei, Lena and Mackenzie. Global warming is expected to significantly influence the amount and temporal dynamic of river. The average annual discharge of fresh water from the six largest Eurasian rivers into the Arctic Ocean has already increased by 7% over the last century. It has been demonstrated, that the timing and magnitude of discharge of Arctic rivers is strongly linked to snow mass storage during the winter and the subsequent melt. The melting of the accumulated snow during spring leads to extreme flooding that represents the major hydrologic event of the year. About 40% of the Lena River's annual discharge is released during this few weeks lasting spring flood. Studies of the spring flood process mainly rely on hydrological modelling and sparsely available in situ gauge measurements. Also remote sensing techniques are applied to map and monitor the spatial and temporal evolution of the event. We present a new approach to delineate water levels during spring flood events that solely relies on the evaluation of high resolution SAR data provided by the TerraSAR-X and TanDEM-X satellites. The German Earth Observation satellite TerraSAR-X was launched in 2007. It provides SAR data in different modes and resolutions. Operationally single and dual polarization is possible. Since 2010 it is flying together with its twin satellite TanDEM-X in close formation enabling single pass interferometry. Here, the primary mission goal is the generation of a global digital elevation model (GDEM) in outstanding quality and resolution that allows classifying the shoreline and land’s topography at an unprecedented level of detail. The spatial resolution is 12.5 meters with a vertical accuracy of 2.5 meters. Our test site covers the central part of the Lena River Delta where the main channel diverges into its major distributaries, the Trofimowskaya, Bykowskaya and Olenekskaya channels. The spatial extent of the flooding is derived from 22 TerraSAR-X images acquired between end of May and mid of July 2013. The satellite's footprint pattern allows observations on three consecutive days, followed by a three days gap. The GDEM product of the TanDEM-X mission is utilized to refine the water mask and to derive the bathymetric 3d information of the flooding. The high spatial resolution of three meters and the dual polarization of the TerraSAR-X data acquired in the StripMap mode, allow for a fine delineation of the water line and an improved separation of open water, ice cover and land, respectively.