Characterizing surface drainage systems in selected areas of central Texas using spaceborne imaging radar

This study involves an examination of certain hydrologic features within five scenes of SIR-B (Shuttle Imaging Radar B) sensor digital data of Central Texas. The imagery, a subset of Datatake 81.2 (a 40km. wide swath extending from northwest of San Antonio to southeast of Dallas), provides an opportunity for synoptic planimetric mapping and studies of river morphology, surface limnology, and drainage basins. The analysis centers on the feature extraction potential of the imagery by highlighting the planimetric geometry and extent of surface drainage systems. The use of spaceborne imaging radar data for drainage analysis, like that of SIRB, provides land-use planners, and flood control specialists with an additional tool for doing synoptic mapping.

Previous work in hydrologic analysis employing radar has been done primarily with Side Looking Airborne Radar (SLAR) from airplane platforms. However, the advantages for using spaceborne radar include the potential for multitemporal and multispatial data acquisition as well as remote sensing from higher altitudes, thus providing less distortion. Radar's capability of illuminating targets under most weather conditions, day or night, is an obvious advantage over other spaceborne sensors. The unique characteristics of spaceborne imaging radar, like SIR-B, that make it an ideal sensor for mapping hydrologic catchment systems, includes the ability to vary direction of illumination at various incidence angles and look directions at specified radar frequencies and polarization. The SIR-B radar is also sensitive to soil moisture and vegetation, as well as geologic boundaries. The resulting imagery provides a marked contrast between smooth and rough terrain, and variations which depend on target dielectric.

It was theorized that maps derived from the SIR-B data could be used for drainage studies, such as: (1) calculations of river morphology, like stream length, meander wavelength and belt width, sinuosity index, wavelength radius-to-river-width ratio, cluster analysis of river curvature, and stream confluence; (2) areal limnological calculations, like lake and reservoir shoreline length, area, and shoreline development index; and (3) drainage basin calculations, like estimates of linear distributions of water impoundments, and basin geometry. Feature extraction from the SIR-B imagery for a number of hydrologic features involving drainage basin characteristics was possible in most instances, but difficult in others. Probable explanations for interpretation difficulty include misspecification of incidence angle and illumination direction (Radar incidence angle needs to be specified in accordance with the local incidence angle, and illumination direction needs to correspond with terrain slope). Suggestions for future coverage of this geographical area include optimizing radar system configuration and flight path.