Prediction of solar irradiance distribution in a wheat canopy using a laser technique

A new measurement technique, the "laser technique", is proposed for providing timely information concerning the solar irradiance distribution in a vegetative canopy. The method is conceptually similar to the point quadrat method and involves measuring the height where a collimated light beam of very small cross section, aimed at the canopy, first hits foliage. To demonstrate its use, the technique is used to determine power and energy budgets of radiation intercepted by the various foliage components of a wheat (Triticum turgidum L.) canopy. It was found for the wheat canopy in the dough maturity stage that the middle and upper canopy layers intercept the bulk of the solar irradiance during the middle hours of the day, and that the soil and leaves intercept no sunlight for low sun angles. In comparison with other available measuring methods the laser technique is shown to have two major advantages. First, it is amenable to automation, and, using such an automated system, to rapid acquisition of large data sets. Second, it is applicable to canopies of various heights to about 30 m. Several potential sources of error in application of the technique, methods of error minimization, and improvements to the technique are discussed.