Monitoring the Spring Breakup Flood in the Lena River Delta with TerraSAR-X Imagery and the TanDEM-X DEM

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 rivers. 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 the few weeks lasting spring flood. Studies of the spring flood process mainly rely on hydrological modelling and sparsely available in situ gauge measurements. We present a new approach that combines the evaluation of high resolution SAR data provided by the TerraSAR-X and TanDEM-X satellites with a hydrologic modelling. A comprehensive collection of remote sensing data is available for the Lena Delta. The monitoring of Polar Regions with optical data suffers from the reduced visibility of the ground due to cloud cover and a low solar altitude for most of the time of the year. Here the all-weather and day and night observation capability of radar (SAR) systems can be utilized. A time series of TerraSAR-X imagery enables the assessment of the spatial and temporal evolution of the spring flood event. Furthermore the backscatter of SAR data is also sensitive to the surface roughness which is in turn an input parameter for the hydrological modelling. The TanDEM-X Digital Elevation Model provides a very detailed description of the land surface. As the water bodies were frozen at the time of the acquisitions also reliable information is provided for the corresponding water surfaces. 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.