Analysis of Influencing Factors in Wheat/Maize Canopy RVI and NDVI Acquisition Using Ground-Based Remote Sensing System
ZHENG Yu-dong1, XU Yun-cheng1, YAN Hai-jun1*, ZHENG Yong-jun2
1. College of Water Resources & Civil Engineering, China Agricultural University, Beijing 100083, China
2. College of Engineering, China Agricultural University, Beijing 100083, China
Abstract：Implement ground-based remote sensing technology in a large-scale sprinkler irrigation system can monitor crop growth status and has played an important role in detecting of crop water and nutrient demands and field management of crop production. Crop canopy has bidirectional reflectance characteristics, so different observation methods used in ground-based remote sensing will affect the accuracy of canopy spectral reflectance measurement. This study used a self-built ground-based remote sensing system to simulate the field observation conditions of large sprinkler irrigation machinery, obtained spectral reflectance information of wheat and maize canopy through multi-spectral optical sensors, and quantified the variation of the crop canopy information data caused by the canopy bidirectional reflection characteristics by the coefficient of variation CV, and analyzed the effect of the observation conditions on the crop canopy spectral reflectance measurements through influence factor weight W. Reflectance in the red band (650 nm) and reflectance in the near-infrared band (810 nm) of winter wheat at regreening to filling growth stages and summer maize at V7 to V14 growth stages were measured and recorded in 2019 season. Effect of various observation factors on the ratio of vegetation index (RVI) and the normalized index of vegetation (NDVI) was analyzed. Results showed that the correlation of observation height in 0.5~2.5 m, observation frequency in 2~60 min-1 and moving speed in 0~4 m·min-1 with the measurements of the canopy spectral reflectance characteristics were not significant (p>0.05); observation time in 8:00—18:00, observation zenith angle at -60°~60°, and observation azimuth angle from 0° to 180° had an extremely significant correlation with the measurements (p＜0.01). Results of canopy spectral reflectance measurement for wheat and maize depended mainly on the degree of the canopy coverage. Under the same leaf area index (LAI), the canopy spectral reflectance would also be affected by the observation time, observation azimuth and observation zenith angle: canopy spectral reflectance had significant bidirectional reflection characteristics. In wheat crop, the coefficients of RVI and NDVI variation were 15%~50% and 2%~50%，respectively, while in maize, they were 10%~33% and 18%~39%, respectively. When measuring RVI and NDVI with wheat and maize crops, the desired time for the measurement could be 12:00—14:00 because the solar zenith angle is relatively stable. The observation angle should be in a fixed angle, and also the influence of shadow effect and hot spot effect should be noticed. When measuring RVI and NDVI of wheat during regreening to jointing stage and heading to the flowering stage, close attention should be paid to the effects of observation zenith angle and observation time, respectively. This study performed a quantitative analysis of measuring canopy spectral reflectance with wheat and maize crops. The results obtained in the study could provide technical support for accurate and effective measurement of the crop canopy spectral reflectance.
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