Estimation of Photosynthetic Photon Flux Density from 368-nm Spectral Irradiance

The estimation of photosynthetically active radiation across the earthʹs surface is needed to model plant productivity and future impacts of ultraviolet-B (UV-B) radiation on plant productivity. While radiation in the ultraviolet and visible wavelengths is routinely measured using broadband and multifilter rotating shadowband radiometers in the United States Department of Agriculture (USDA) UV-B Climate Monitoring Network, measurements of photosynthetically active radiation have not been made prior to 2002. Three models were developed to estimate the photosynthetic photon flux density (PPFD) from measurements of the diffuse and global spectral irradiance at 368 nm made by multifilter rotating shadowband radiometers. The models were developed from measurements made at half-hour intervals between March and November 2002 at five USDA UV-B Climate Monitoring Network locations in the United States between 34° and 43°N latitude and 77° and 112°W longitude. The model that best estimated the PPFD included global and diffuse 368-nm irradiance, solar zenith angle, sky brightness, and aerosol transmittance and had a residual error of 79 (mu)mol m^2 s^-1 PPFD. The best two-variable model for estimating PPFD utilized measured global and diffuse 368-nm irradiance, with a root-mean-squared error of 85 (mu)mol m^2 s^-1. Since errors of the order of 80 (mu)mol m^2 s^-1 represent biologically significant amounts of dry matter production in crops and the 9% model error is approximately twice that of the nominal measurement error for the PPFD sensor, PPFD measurements should be made and not modeled wherever possible.