Modeling Photosynthetically Active Radiation using a Spectrally Weighted Raytracing Approach

Abstract

Green infrastructure in cities is associated with numerous benefits, such as decreasing temperatures, mitigating urban heat islands, improving air quality and thermal comfort, reducing energy consumption, and improving quality of life. Although including urban green areas in cities is often presented as a viable design strategy, the feasibility of establishing specific vegetation types within geometrically complex microclimates resulting from solar exposure differences is often disregarded. This article presents the development of a methodology to generate annual estimates of photosynthetically active radiation (PAR) in outdoor spaces. The methodology employs the Radiance raytracing system and custom scripts to calculate hourly averages of PAR in terms of photosynthetic photon flux density (PPFD) using a novel wavelength-based calculation. A computational tool was developed which can provide light availability in PPFD and be converted to daily light integral (DLI), a variable commonly used to assess plant growth potential. A case study demonstrates the importance of the geographic location, the sky conditions, and the definition of the growing season for optimum utilization of the tool. Urban farmers, city planners, and landscape designers can benefit from this computational method that provides modeled light conditions to inform plant selection, especially within a dense urban environment.