Monitoring Mountain Rivers From Space: A Pilot Study of Bedrock River Flow Measurement Using Ultra-High Resolution Optical Satellite Imagery

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The measurement of flood-stage discharge in mountain rivers is an enormous logistical challenge, particularly on large space and time scales, and the inevitable paucity of such data has impeded progressin our understanding of mountain landscape processes. We propose a novel solution that exploits newhigh resolution satellite imagery (2m, FORMOSAT-2) with unusually high revisit frequency (daily) overa region of very broad interest to the earth science community (Taiwan). Our proposed method entailsthe collation of a time series (2004-2005) of orthorecti*ed, georeferenced images of selected mountainriver channels, in tandem with contemporaneous stage measurements of discharge at the nearest gaugingstation downstream. Patterns of inundation along selected channel reaches will be mapped in eachimage. With the aid of cross-channel profiles mapped in the field at low stage, we will make estimatesof low width, depth, mean bed shear stress and mean low velocity. Once a broad range of dischargeshas been processed, we expect to be able to construct rating curves for these low measures as a functionof the directly observed, at-station stage data. Combination of the rating models with the multi-decadalarchive of station data in Taiwan will permit an assessment of the long-term probability distributions oflow widths, depths and shear stresses along and across the selected bedrock channels. Measurements ofthis kind, albeit indirect, will be of enormous interest to the geomorphology community. However, themethod is by no means guaranteed, because remote river gauging has never been attempted in mountaincatchments; it remains to be seen whether the precision of the anticipated rating curves will suffice forrivers akin to those of the Taiwan Central Range. The main task of the pilot project will be to addressthis issue and to lay the foundation for a more comprehensive, longer-term assessment of Taiwanesemountain river discharge.Broader outcomes: While low measurements in steep mountain rivers are particularly sought after bythose interested in bedrock channel processes, they are also of great interest beyond: from modelersof landscape evolution and the interaction between climate and tectonics, to hydrologists preoccupiedwith the assessment of flood hazard in populated mountain environments. Moreover, if the pilot projectproves successful it will provide the Taiwanese space agency with an excellent example of the value ofFORMOSAT-2 in natural hazard assessment, one of its principal missions. As a result, more attentionwill be paid to the regular acquisition of FORMOSAT-2 imagery in tropical mountain areas, an essentialstep if the elusive goal of a high-resolution, multitemporal image archive is to be achieved - at present,images of highland areas are a relatively low prioity (for FORMOSAT-2 and other equivalent satellites)and are only acquired daily during emergency periods (e.g. typhoons). Such an archive would be widelyappreciated across the discplines of earth sciences, hydrology, ecology, not to mention in remote-sensingR&D itself.A-1