Erosion Prediction in Ungauged Basins: Integrating Methods and Techniques (Proceedings of symposium ^A I HS01 held during IUGG2003 at Sapporo, July 2003). IAHS Publ. no. 279. 2003. Z * L Spaceborne radar interferometry: a promising tool for hydrological analysis in mountain alluvial fan environments F. CATANI, P. FARINA, S. MORETTI University of Firenze, Earth Sciences Department, Via La Pira, 4,1-50121 Firenze, Italy fcatani@.geo, unifi.it G. NICO National Research Council, Institute of Radio Astronomy, c/o ASI/CGS, 1-75100 Matera, Italy Abstract Morphometric attributes derived from digital terrain models are frequently substituted for hydrological parameters in ungauged basins. In many cases, however, the reliability of such derived data is severely hampered, both by errors inherent in the original data, due to map scale and map age, and by errors derived from interpolation and computation procedures. This is especially true in the case of alpine basins, where sedimentation and flooding typically occur in populated downstream regions on alluvial fans. Data obtained by interferometric SAR (InSAR) techniques can be successfully substituted for DTM-derived data, allowing the direct computation of fundamental, secondary DTM attributes such as slope and aspect. A set of ERS 1/2 SAR images of the Cellina alluvial fan in northeastern Italy has been acquired and interferometrically processed. Experimental results show that, compared to traditional DTM-based measures, SAR-based attributes appear to be more stable and should offer better characterization of the slope surfaces. Furthermore, the present availability of such images allows fast and inexpensive updating of local hydrological characteristics of an area after major flooding events. Key words alluvial fans; Eastern Italian Alps; Italy; mountain streams; Northern Apennines; radar interferometry; sediment transfer INTRODUCTION Among the global environments subjected to hydrological and geological disasters, the areas in which relatively young mountain ranges deposit water and sediment occupy an important place. Along these boundaries, mountain drainage systems usually enter the wide alluvial plain of a major river as gravel-bed braided streams, giving rise to large alluvial fans that are often coalescent and that represent the result of the loss of stream power due to the sudden decrease in gradient. There is general agreement about the processes dominating fan environments (Harvey, 1988). In particular, alluvial fans in Alpine areas are thought of as the result of a succession of glacial cycles (Weissman et al, 2002). Such areas are strongly controlled by episodes of fluvial erosion on the fan, alternating with episodes of high rates of erosion in the source basin, with the consequent increase of sediment discharge to the fan. It is well understood that in the last 1000 years the principal causes of such changes have been human-induced climate change and human activities such as water control structures, draining of wetlands in the lower valleys and streambed quarrying (Guzzetti et al, 1997; Spaliviero, 2O02).