An experimental approach to the measurement of the effects of water depth and substrate on optical and near infra-red reflectance: a field-based assessment of the feasibility of mapping submerged instream habitat

This study represent an assessment of the feasibility of using optical and near infra-red wavelengths to map water depth and substrate type which are two of the primary components of river physical habitat. The objective was met by measuring reflectance using field spectroscopy of exposed and then progressively submerged (0-1 m) artificial substrates during rising tides on a tidal reach of the River Forth; this approach provided absolute control for substrate type. The field data were used to simulate multi-spectral response as measured using Airborne Thematic Mapper (bands 1 to 7; 420nm to 900nm) since airborne remote sensing provides the way forward for synoptic mapping of small streams (<20m), such as those typically found in Scotland. In the presence of sunlit conditions significant differences in reflectance over a broad range of wavelengths was evident with strong correlations with water depth and substrate. Differences in reflectance were most marked under shallow conditions (<0.75 m) and then progressively diminished. Application of simple linear correlation, band ratios, the Lyzenga algorithm and multiple discriminant analysis all suggested the possibility of mapping shallow river channel water depths. The results also showed that substrate type and the analysis technique selected also affected the optimum spectral wavelength for detection of channel substrate type and water depth. Overall the work suggests mapping channel depth and substrate type is possible using remotely sensed data in optical and near infra-red wavelengths, using high spatial and spectral resolution multi-spectral imagery. However it may be restricted to depths of less than I m where there is high organic colour and/or a significant periphyton cover.