Solar radiation flux, irradiance, is a fundamental driver of almost all hydrological and biological processes. Ecological models of these processes often require data at the watershed scale. GIS-based solar models that predict insolation at the watershed scale take topographic shading into account, but do not account for vegetative shading. Most methods that quantify subcanopy insolation do so only at a single point. Further, subcanopy model calibration requires significant field effort and knowledge of characteristics (species composition, leaf area index & mean leaf angle for each species), and upscaling to watersheds is a significant source of uncertainty.We propose an approach to modeling insolation that uses airborne LiDAR data to estimate canopy openness as a Light Penetration Index (LPI). We couple LPI with the GRASS GIS r.sun solar model to produce the Subcanopy Solar Radiation model (SSR). SSR accounts for both topographic shading and vegetative shading at a landscape scale.After calibrating the r.sun model to a weather station at our study site, we compare SSR model predictions to black thermopile pyranometer field measurements and to hemispherical photographs using Gap Light Analyzer software, a standard method for point estimation of subcanopy radiation. Both SSR and hemispherical models exhibit a similar linear relationship with pyranometer data, and the models predict similar total solar radiation flux across the range of canopy openness. This approach allows prediction of light regimes at watershed scales with resolution that was previously possible only for local point measurements.
Bibliographical noteFunding Information:
This research was funded by the National Science Foundation's National Center for Earth-surface Dynamics ( EAR-0120914 ), and the Eel River Critical Zone Observatory ( EAR-1331940 ). LiDAR was flown by the NSF National Center for Airborne Laser Mapping. We would like to thank Graham Law and Charles Post for technical assistance with photography; the University of California Natural Reserve System and the Heath and Marjorie Angelo Coast Range Reserve and its steward, Peter Steel for providing a protected research site for this work; and the Saint Anthony Falls Laboratory (SAFL) at the University of Minnesota for use of their compute-workstations.
- Angelo Reserve
- Radiative transfer
- Solar model
- Vegetation shading