Analyzing the characteristics of FPAR from maize canopies measured in Northwest China

The Fraction of Absorbed Photosynthetically Active Radiation (FPAR) of crop canopies directly measured in the field is affected by many factors, i.e. the structure of canopies, the solar incident angles, and the weather. Therefore, it is difficult to apply the measured data to validate the FPAR products of remote sensing directly. In this paper, FPARs of maize canopies are measured from May to July, 2012, in Zhangye City, Gansu province, which is located at Heihe River Basin, Northwest China. The relationships between FPAR and the structures of canopies (i.e. leaf area index-LAI, fraction vegetation cover-FVC and row directions) are analyzed based on the experimental data. The impacts of the solar incident angles and the ratios of skylight on FPAR of maize canopies are analyzed based on the data simulated by RGM (Radiosity-Graphics combined Model). It is found that (1) FPAR change little with the ratios of skylight; (2) It is acceptable that FPAR of maize canopies are measured and calculated by a simple algorithms based on the transmittance of canopies; (3) FPAR of maize canopies with different row directions change with the solar incident angles of one daytime, which shows obvious diurnal variation character; (4) FPAR is the exponential relationship with LAI, and the linear relationship with FVC.