Master Thesis in Nuclear Engineering
A radiative transfer model for homogeneous parallel plane vegetative canopies is presented, and a method to decompose and calculate the uncollided, once-scattered, and multiply-scattered radiation components is given. Three realistic models for leaf scattering are considered: generalized bi-Lambertian scattering, specular reflection model, and combined specular/bi-Lambertian. model. The canopy hot spot is included, albeit approximately, in the turbid medium radiative transfer model by adjusting the canopy extinction coefficient (or more specifically, the geometry factor) for the once-scattered radiation to include weakened extinction of photons scattered into the retro-sun direction. A simple model for this modified geometry factor is proposed and incorporated into the radiative transfer equations for the once-scattered intensity.
Based on the proposed canopy model, a computer code, formulated on a variant of the discrete-ordinates numerical solution method, is developed to calculate the angularly-dependent radiation field reflected from a plant canopy. Three studies have been performed with this computer code. First, the radiation reflected from both highly-absorbing and highly-scattering canopy media is examined for the generalized bi-Lambertian, specular, and combined scattering models. In this study the effects of internal (bi-Lambertian) and surface (specular) scattering mechanisms are compared. The results indicate that surface scattering is generally important for highly absorbing canopy media, but not as important for a highly scattering medium.
The second study uses the canopy model to examine canopy hot spot effects under different illumination conditions and for different canopy scattering properties. The effect of the canopy hot spot was found to be most pronounced for radiation incident obliquely on a highly-absorbing thick canopy and was least important for radiation incident normally on a highly-scattering thin canopy.
The final study compares calculated values to recently measured data for canopy reflectance fractions for a grass canopy. Results indicate that, in spite of the idealizations and simplifications of the proposed canopy radiative transfer model, realistic predictions of the canopy reflectances can be obtained.
Download MS thesis in Adobe PDF format [zip file or self-extracting zip file]. Both files are approximately 11 Mbyte in size.
A copy of the canopy code, called CANTEQ (formerly called TEQ), used in the reported studies can be obtained by sending an email message to Rob Stewart at trebor@purdue.edu