Dependence of solar radiation transport in rice [Oryza sativa] canopies on developmental stage

Solar radiation in a rice canopy plays an important role in the energy balance on both the plant leaves and soil/water surface. A simple model was derived from the two-stream model to estimate two important factors characterizing radiation in a plant canopy, namely, transmissivity of the canopy (tausub(c)) and the albedo of the canopy (ref), from the absorption coefficient of leaves (alpha) and leaf inclination factor (F). To clarify the seasonal variation in alpha and F with growth, season-long observations were conducted in paddy fields during three different cropping seasons. Values of alpha were almost constant throughout the growing period; however, values of F tended to increase with growth. Values of F were larger than 0.5 (the theoretical value for random leaf distribution) in the late growth stage mainly due to the alterations in leaf geometry with the change in leaf inclination angle along a more horizontal axis after flowering, while in contrast, values of F were less than 0.5 in the early growth stage mainly due to the distribution bias of leaves after transplanting. Seasonal variation in F during different cropping seasons could commonly be expressed as a function of developmental stage (DVS). Using this function, tausub(c) and ref could be estimated with more accuracy. The proposed radiation model and function is expected to be applicable in more accurate evaluation of solar radiation, dry matter production and soil/water temperature in paddy fields