A new approach for remote sensing of canopy-absorbed photosynthetically active radiation. I: Total surface absorption

The canopy-absorbed photosynthetically active radiation APARCAN is the solar energy consumed in the canopy photosynthetic process. Due to the difficulty of acquiring extensive ground-based observations, increasing efforts are being devoted to estimate APARCAN from optical satellite measurements. So far, APARCAN has been obtained from the downwelling PAR at the surface (SFC), PARSFC↓, and the fraction of PAR absorbed by a canopy, FPAR. This study proposes a new approach which defines APARCAN as the product of APARSFC and RPAR. APARSFC is the total PAR absorbed by all surface materials including canopy, soil, litter, etc., while RPAR is the ratio of the PAR absorbed by the green canopy only, to APARSFC. The advantage of this approach is that APARSFC can be determined more accurately and readily than PARSFC↓, while the determination of RPAR is as accurate as that of FPAR with the same difficulties. The whole approach is introduced in two parts. Part I, as presented in this article, deals with the retrieval of APARSFC. Using a complex atmospheric radiative transfer model, APARSFC is related to the upwelling PAR reflected at the top of the atmosphere (TOA), PARTOA↑. The relationship is independent of cloud parameters and surface conditions, and moderately dependent on ozone amount and aerosol optical properties. A parameterization was developed to estimate APARSFC from PARTOA↑, which is inferred from satellite measurements in the visible bands. Error analyses were made using data from both model simulations and field observations. The parameterization is valid to within 5 W m−2 compared to the results of detailed radiation model simulations. A preliminary comparison against FIFE ground observations showed a bias error of −2.7 W m−2 and a standard error of 21.9 W m−2 for the instantaneous estimates of APARSFC.