DEM Generation Using ASAR (ENVISAT) for Addressing Hydrological Characterization of Santa Cruz Island, Galapagos
Violette, S1; d'Ozouville, N2; Deffontaines, B.3; Benveniste, J.4; Wegmuller, U.5; de Marsily, G.6
1Paris VI University; 2Paris VI University, UMR.7619-Sisyphe; 3Marne La-Vallée University - LGGI; 4ESA - ESRIN; 5Gamma Remote Sensing; 6Paris VI University - UMR.7619-Sisyphe

Low topographic oceanic islands often suffer from scarcity of freshwater resources and are poorly characterized due to their complex internal structure and challenging access. Remote sensing has proved to be an effective tool to generate valuable data for hydrological analysis. However, these are usually tested over areas with existing validation databases and not always where the need is greatest. Here we address the need for topographical data for hydrological understanding of Santa Cruz Island (Galapagos Archipelago) where no high resolution, no high accuracy topographical data exists. 97% of Galapagos territory consists of inaccessible National Park land which makes the use of indirect methods indispensable. We used new ASAR data (ENVISAT) for Digital Elevation Model generation, in order to extract drainage network, watersheds, and flow characteristics from a morpho-structural analysis. Results show the high potential of this data for both interferometric and radargrammetric generation methods. If interferometry suffered from low coherence over highly vegetated areas, it showed high precision over the rest of the island. Radargrammetry gave consistent results over the entire island, and detail was enhanced by integrating the SRTM as an external DEM. Validation of the extracted river networks and watersheds was carried out using ground-based field observations, comparison to drainage network extracted from aerial photographs and to high resolution (1 m) satellite imagery. For the first time watershed characteristics and flow paths are made available for an island of the Galapagos archipelago. Drainage networks and underground percolation are strongly influenced by fractures. The case study of Pelican Bay watershed, where urban areas, agricultural land and Galapagos National Park land are concomitant, was chosen to carry out quantification of water balance.


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