Evidence for flash floods over deserts from loss of coherence in InSAR imagery

Flash floods in complex terrain play an important role for sediment transport in arid regions and thus potentially for dust production, but observations of these phenomena are scarce over most of the world's deserts. Here, methods from radar interferometry, applied to 8 years (2003-2010) of ENVISAT Advanced Synthetic Aperture Radar data over the northwestern Sahara, are used to demonstrate the potential of these data to identify significant flash floods. Loss of coherence between two consecutive images is indicative of changes in surface characteristics. Tropical Rainfall Measuring Mission daily precipitation estimates, together with the digital elevation model from the Shuttle Radar Topography Mission, are analyzed to assess the likelihood of these changes to be related to flash floods. Four interferometric pairs representing periods with different rainfall amounts are examined with regard to changes in surface characteristics caused by precipitation. To do this, ratios of coherences are calculated, highlighting the changes in soil texture through loss of coherence in particular for desert valleys. Many pixels within wadis show large coherence during dry periods and a significant loss during wet periods, while others show low coherence irrespective of rainfall, possibly due to Aeolian processes. In the long term, findings from this study will be used to investigate the relation between flash floods and interannual variability of local dust emission fluxes.