Abstract
The fraction of absorbed photosynthetically active radiation (FPAR) is a key parameter in describing the exchange of fluxes of energy, mass and momentum between the surface and atmosphere. In this study, we present a method to measure FPAR using a digital camera and a reference panel. A digital camera was used to capture color images of low canopy vegetation, which contained a reference panel in one corner of the field of view (FOV). The digital image was classified into photosynthetically active vegetation, ground litter, sunlit soil, shadow soil, and the reference panel. The relative intensity of the incident photosynthetically active radiation (PAR), scene-reflected PAR, exposed background absorbed PAR and the green vegetation-covered ground absorbed PAR were derived from the digital camera image, and then FPAR was calculated. This method was validated on eight plots with four vegetation species using FPAR measured by a SunScan instrument. A linear correlation with a coefficient of determination (R2) of 0.942 and mean absolute error (MAE) of 0.031 was observed between FPAR values derived from the digital camera and measurement using the SunScan instrument. The result suggests that the present method can be used to accurately measure the FPAR of low canopy vegetation.
Highlights
The fraction of absorbed photosynthetically active radiation (FPAR) characterizes the function of vegetation canopy and its energy absorption capacity
To demonstrate and test the approach, low canopies suitable for SunScan were selected, and their photos were taken by the digital camera vertically, with a low reflectance panel fixed at one corner of field of view (FOV), and the four components for absorbed photosynthetically active radiation (APAR) as shown in Equation (1) are derived from the digital photo
We presented a novel ground-based green vegetation FPAR measurement method for low and potentially very-low canopy by integrating a digital camera and reference panel
Summary
The fraction of absorbed photosynthetically active radiation (FPAR) characterizes the function of vegetation canopy and its energy absorption capacity. FPAR is generally defined as the fraction of photosynthetically active radiation (PAR) absorbed by vegetation in the 400–700 nm spectral range. For very-low canopy vegetation, such as the grassland with a height less than 15 cm, which is the case in many arid and semi-arid regions in the world, it is impossible to put the SunScan probe below the canopy to measure canopy-transmitted and background-reflected PAR to calculate FPAR. We present a novel method to measure FPAR using a digital camera and reference panel, and evaluate the method by traditional radiation transmittance measurements in eight independent plots with the low canopy of approximately 30–50 cm
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