Evaluation of techniques to estimate solar radiation and photosynthetically active radiation in the southeast US

Solar radiation is an essential atmospheric measurement for agricultural practices with direct impacts to plant monitoring and modeling, irrigation applications, and integrated pest management. Solar radiation in the visible part of the electromagnetic spectrum, from about 0.40 to 0.70 micrometers, is known as photosynthetically active radiation (PAR) since plants use this energy for photosynthesis (Oke, 1987).

Crop models, such as those included in the Decision Support System for Agrotechnology Transfer (DSSAT), have been developed to predict crop growth. These models require accurate local climate information including air temperature, precipitation, and incoming radiation. However, solar radiation and PAR observations are only available for a limited number of locations across the Southeast US. In the absence of observations, an estimation of these two parameters is required.

This study evaluates the accuracy of solar radiation estimation techniques based on Hargreaves (1985) and Stull (1988) as compared to observations from sensors in North Carolina. Modifications to the Hargreaves (1985) estimate have also been evaluated against observations to determine if local calibration of this estimate improves model accuracy. In addition, this study develops ratios of hourly PAR to incoming solar radiation from observations in North Carolina. These ratios can be utilized to estimate PAR from solar radiation estimates or observations at other monitoring sites across the Southeast US. Spatial and temporal variation in the ratio of PAR to solar radiation is analyzed in North Carolina.