The observation of spectrum reflectivity during the growth of rice paddy.

Zhu Yonghao Deng, Renda Tang Yonggui Cui, Jue Wang Yifang
(Anhui Institute of Optics and Fine Mechanics, Academia Sinica)

In order to research how to use remote sensing method in monitoring in growing condition of rice paddy and estimating the production, we measured the spectrum reflectivity of seven pieces of paddy field, from Jun. to Sept. in 1980. They are all of same species of rice but their time of transplanting the seedling was different from each other. Besides, we also did the same measurement for another badly-grown paddy field.

We used two sets of hand filter-radiometer in the open field each with four filters of MSS 4,5,6,7 respectively. We measured once per 10-20 days. With 3° visual field radiometer, we measured the average reflectivity by way of multiple sampling method. With 10° visual field radiometer we got the average reflectivity by the way of scanning integral method. Later we measured the spectrum reflectivity of the leaves and ears of the paddy with the grating and prism monochromators in Laboratory. The results of measurement show:

1. During 40 days after transplanting the change of average reflectivity lies in the dropping-down of MSS 4,5 and the rapidly rising-up of MSS 6,7. During this time, the leaf area index and coverage rate of the paddy increased quickly.
2. For about 1-2 months after first 40 days, the rice leaf maintained green. The worse rice grew, the shorter they maintained green. During this time, reflectivity almost maintained constant, the leaf area index and coverage rate were almost unchanged too.
3. About 80-100 days after transplanting, the leaves and ears of paddy turned to yellow, the rice being ripe. During this time the change of average reflectivity showed that MSS4, MSS5 rose up and MSS 6, MSS7 remained constant. The reflectivity of leaf and ear in visible spectrum increased. Moreover, the absorption band of 680 nm of chlorophyll increased obviously. The reflectivity of infrared 1.1-2.2µ increased too, which indicated that the water contained in the leaf and ear decreased.

To sum up, we can say that the change of average reflectivity during 40-50 days after transplanting mainly indicates the characteristics of group-unit change which lies in the increasing of leaf area index and coverage rate. And during the time of ripening, because the leaf and ear are turning yellow, the average reflectivity change too. At the time of measuring the spectrum reflectivity, we measured the leaf area index and the dry weight of the crop. The statistical and analytical work is being under way and not complete yet.