Dynamics of photosynthetic photon flux density (PPFD) and estimates in coastal northern California

Authors: GE, SHAOKUI - University Of California; SMITH, RICHARD - US Department Of Agriculture (USDA); KRAMER, MARC - University Of California; Carruthers, Raymond; JACOVIDES, CONSTANTINOS - University Of Athens

Submitted to: Journal of Theoretical and Applied Climatology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2010
Publication Date: 12/15/2010
Citation: Ge, S., Smith, R.G., Kramer, M.G., Carruthers, R.I., Jacovides, C.P. 2010. Dynamics of photosynthetic photon flux density (PPFD) and estimates in coastal northern California. Journal of Theoretical and Applied Climatology. 105(1-2):107-118. DOI: 10.1007/s00704-010-0368-6.

Interpretive Summary: Photosynthetically Active Radiation (PAR) is the portion of sunlight that plants use for conversion of CO2 and water into plant resources for growth and production. This study used a combination of field assessment and computer models to estimate the amount of incoming PAR across two growing seasons in northern California and to estimate hourly and seasonal changes in the amount of light available for photosynthesis. This work is important as it allows scientists and land managers to estimate plant growth potential for a wide range of important plant species such as food and fiber crops, bioenergy crops and invasive weeds that limit crop production. This study was actually conducted to aid in predicting weed growth and development in rangelands. Specifically, canopy light measurements and predictions were made for grass and yellow starthistle canopies in conjunction with a wider ranging effort to control this pest plant in several western states such as California, Idaho, Nevada, Oregon and Washington. Differential PAR affects the weeds ability to capture light and become invasive under different conditions.

Technical Abstract: The seasonal trends and diurnal patterns of Photosynthetically Active Radiation (PAR) were investigated in the San Francisco Bay Area of Northern California from March through August in 2007 and 2008. During these periods, the daily values of PAR flux density (PFD), energy loading with PAR (PARE), and ground-level broadband solar radiation (SR) averaged 48.51 mol/m2, 2938.88 watts/m2, and 6208.95 watts/m2, respectively. PFD and PARE had strong seasonal trends. The energy ratio of PAR to broadband solar radiation (fE) and the conversation efficiency of flux to an energy alternative (fFEC) were relatively conserved, but not constant. In this study from March through August, values of both PFD and PARE were low in March, with monthly averaged daily values of 30.28 mol/m2 and 1828.77 watts/m2, respectively. PFD and PARE approached their daily maximums near the summer solstice in June at 59.91 mol/m2 and 3638.29 watts/m2, then decreased back to relatively low levels in August. In parallel, the monthly averaged daily fE and fFEC changed from 43.60% and 2.01 micromol/J in March to 48.81% and 2.23 micromol/J in June, respectively. A new parameter, the loss ratio of PAR loading energy (LPR), is reported for the first time in a study of this type. LPR had an average daily value of 32.49%, and was highest in March (41.37%) and lowest in June (21.76%). While PFD, PARE and LPR all exhibited clear diurnal-patterns, there existed no significant differences in fE or fFEC among the morning hours. In contrast, differences between these parameters were observed among the afternoon hours. PFD and LPR were highly correlated with selected climatic and astro-geometric factors, including broadband solar radiation, temperature, relative humanity, and solar elevation. PFD and LPR were estimated more easily and precisely from these related factors than either fE or fFEC. In this study, two models were calibrated and validated to estimate PLR and PFD from broadband solar radiation and the atmospheric clearness index Kt, and calculated solar elevation.