Transmitted photo-synthetically active radiation (
TPAR) is a key variable for biomass accumulation of the green plants. Accurate observation of TPAR can improve accuracy of biomass simulation. There are few studies on
measurement methods of TPAR for different
row crops. The purpose of this study is to compare and analyze the differences of three measurement methods for TPAR under crops canopies and to suggest a better method. The study was done on the experimental fields of the winter wheat and the summer maize during 2001 (11 April and 22 September) and 2002 (8 April and 22 September). The experiment had four densities with four replicates each, and 16 completely block designed plots distributed randomly. The PAR/SUNFLERK (Ceptometer light bar) was used to measure TPAR. There were three measurement methods. One is to let measurement instrument
perpendicular to the row
direction. The other is across to the row direction. The third is parallel to the row direction. The results showed that for the winter wheat, the perpendicular method had the lowest relative
error of 1.3%, but the across method had the highest relative error of 9.4% during the whole growing stage. For the summer maize, the across method had the lowest relative error of 2.1%, and the highest relative error of 7.4% for the paralleling method. T-test indicated that three measurement methods were not significantly different at the 0.001 probability level though. The recommended measurement method of TPAR is perpendicular to the row direction for the winter wheat, and across to the row for the summer maize.
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