USING ASTER DATA TO QUANTIFY RIPARIAN CANOPY AREA FOR A RANGE OF STREAM ORDERS IN EASTERN PENNSYLVANIA, USA

The riparian canopy can play a major role in controlling aquatic habitat characteristics. Quantifying canopy cover is therefore of importance to the study of stream ecosystems. Lehigh University’s ongoing study of ultraviolet radiation in the Lehigh River watershed uses ground-based measurements from a hemispherical camera and irradiance sensors in order to quantify optical transmittance of the canopy. The percent of sky visible in each photograph can be quantified in a semi-supervised procedure. We are currently working to calibrate this technique, however the procedure is difficult to scale up to a watershed. Here, we propose a remote method for estimating canopy cover using GIS files, aerial photography and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite imagery. This method could be coordinated with the camera approach and could be applied at stream reach and watershed scales.

Six forested stream reaches in the Lehigh River watershed were chosen as examples, each representing a different stream order as defined by the Strahler method. Preliminary results from our camera efforts indicate that percent canopy cover is inversely related to stream order. Profiles of normalized difference vegetation index (NDVI) values, calculated from ASTER data, were made along transects perpendicular to the stream reaches. These profiles show canopy-like NDVI values on the river banks, water-like values at the channel center, and intermediate values where canopy overhangs the margins of the stream. Since each point on a profile represents a known area, it should be possible to relate the number of intermediate-reflectance pixels to the sum of water-like and intermediate-reflectance pixels to calculate the percent cover over the stream channel. Profiles will be partially calibrated using field measurements of stream width; complete calibration will be done using hemispherical canopy photographs over the course of a growing season. From consistent, quantified canopy measurements we hope to model the impact of solar radiation on the aquatic stream environment.