Water Storage of the Central Amazon Floodplain Measured with GIS and Remote Sensing Imagery

Abstract

Interferometric processing of a Space Shuttle–based swath of synthetic aperture radar (SAR) data collected over the central Amazon floodplain reveals one-day decreases in water levels ranging from 1 to 11 cm in mid-October 1994. The decreases, plotted in a geographic information system (GIS), do not appear to be related to water-body size. Instead, the drops correlate with distance between measured location and main river channel. Because the flow routes connecting observed water bodies to a river channel are convoluted, stream network extraction algorithms operating within a GIS are ideally suited to identify flow-path distances. High-resolution digital elevation models (DEMs) are not available for the Amazon Basin; instead, extraction algorithms are applied to a basin-wide mosaic of raster-based SAR amplitude data. SAR amplitudes mimic topography with weak radar returns over open water lying in low elevations and strong returns over non-flooded, higher elevations. As flow-path distance increases, there is a general decrease in the floodplain water level drops. This functional relationship is used to extrapolate the swath of interferometrically based observations beyond their ∼15-percent coverage of the central Amazon floodplain. If the extrapolation is correct, the decreases in water levels sum to a storage change of 4,600 m3/s from an area of 11,800 km2 across the floodplain during midrecessional flow. This value is about 30 percent less than the storage change estimates previously derived from flood routing. The difference likely results from the model-based assumption that water-level fluctuations across the floodplain are equivalent to those in the main river channel. The interferometric observations suggest that this equivalence does not appear to be correct for mid-October 1994.