GROUND AND SATELLITE ESTIMATES OF THE PENETRATION OF ULTRAVIOLET AND VISIBLE RADIATION THROUGH FOREST CANOPY IN EASTERN PENNSYLVANIA, USA

Factors influencing the penetration of sunlight through forest canopy are of interest to both stream and forest ecologists. Exposure to solar radiation is beneficial to many organisms in aquatic and terrestrial environments, but ultraviolet radiation (UVR) and excessive heat from sunlight can be harmful. Radiation wavelengths and magnitudes transmitted through forest canopy are thus important for understanding habitat quality. Climate change and land-use practices tend to change forest density and the optical transmittance of the canopy. This research addresses the contribution of sub-basin forest canopy throughfall from precipitation as a source of dissolved organic carbon (DOC) in streams, and the effect of riparian forest canopy density on UVR reaching the surface of streams. This study is part of ongoing research at Lehigh University studying the effects of UVR on aquatic ecosystems within the Lehigh River watershed

Vegetative land cover was identified within selected sub-basins of the Lehigh River watershed using supervised classifications of Landsat Thematic Mapper (TM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images. Normalized Difference Vegetative Index (NDVI) was calculated for each image. Forest canopy was identified from the vegetative cover by comparison with high resolution gray-scale aerial photographs. From the amount of forest within the sub-basin, the relationship between the calculated tree canopy density from satellite images and ground-based measurements was explored. Ground-based measurements taken at points within the sub-basin measured percent transmittance of the canopy in visible and UVR wavelengths. We used hemispherical up-looking photographs to measure canopy density (Leaf Area Index and Sky View Factor) and UVR measurements of down-welling direct and diffuse radiation (UV and visible). Ground measurements were combined with the index of canopy cover to predict watershed-wide canopy density and light transmittance potential. Future work will involve more field measurements to quantify the spatial and seasonal variation in throughfall DOC concentration and the effects of riparian canopy density and stream order on penetration of direct and diffuse wavelengths of light to create a model that can be applied to other regions.