SATELLITE GAGING REACHES: A TECHNIQUE FOR STREAMFLOW MEASUREMENTS FROM SPACE

Satellite gaging reaches are 4-sided polygonal land parcels approximately 20 km in length and 5-15 km in width. The longer sides border a discrete floodplain and are parallel to the contained river or stream. The shorter sides are approximately orthogonal to the flow direction and define an upstream and downstream boundary. Each reach provides adequate surface water area for accurate measurement of discharge changes from space using frequent repeat, wide area coverage sensors. Thus, for a river .5 km in width, a 20 km gaging reach includes 10 sq km of water area or 160 250 m MODIS pixels. River conveyance channels, including floodplains, are complex in morphology when the full range of discharge is considered: flow variations induce observable width and surface area as well as stage changes. Along the gaging reaches in particular, different-size flood discharges are accompanied by easily discriminated surface water areas. The Dartmouth Flood Observatory has now characterized nearly 200 satellite gaging reaches worldwide for MODIS-based flood monitoring: http://www.dartmouth.edu/artsci/geog/floods/reaches/index.html. It is also measuring rivers under normal or reduced flow circumstances as new MODIS data are assimilated. Surface water areas are transformed into characteristic flow widths (total surface area divided by measured flow length). Work within the U.S. and in Romania is aimed at calibration of flow widths to discharge using in-situ gaging stations. The rating equations which result can be used to infer discharge at other locations lacking in situ data and with similar channel and floodplain morphology.