Researches of spectral feature and
growing monitoring of rice
Cha Zhongxing
Institute of Land survey and Planning of China
Wang Yanyi
Jiangsu Academy of Agrcultural Sciences
Qu Boolin
Institute of Land survey and Planning of China
Abstract
Using the method of power regression analysis, we find correlations between the Ratio Vegetation Index D/C (infrered to red, obtained from paddy spectral data) and LAI or TDM are very high. Before rice heading , the correlation coefficients (c.c.) are 0.96 and 0.93 respectively; after rice heading, the c.c. between D/C and TDM is -0.90 (5 varieties, 173 samples). Using the regression model obtained from one year to forecasting the LAI and TDM of another year, the precisions are: 96.4% and 94.6% respectively before rice handing, after rice heading it is 95.4% for forecasting TDM.
Introduction
Rice is one of the major crops in China. There are a great number of rice varieties and their growing periods and plant types are very different. So it is necessary to create a method to monitor the rice growth for the paddy management and yield estimation. In order to predict rice yield, agronomists usually measuse LAI and TDM during the rice growth period, but their methods are laborious and time-consuming.
With the development of remote sensing technology, it has been recognized that the crop spectral feature (such as light absorption, transmission and reflection) are closely determined by crop physiological characters which can express growing vigour and yield components of rice. From this point of view, the research was carried out the explore the relationship between rice spectral parameter and agronomic parameters during rice growing period and to monitor dynamically the rice growth.
Experiment Method
Experiments were carried out in Jiangsu Academy of Agricultural Sciences, Nanjing, 1987-1989,with 4 rice varieties (Yangan No.2-mediumjaponica, Xiu Shui No.4-late japonica, Nanjing No.11-medium indica, Shanyou No.63-hybrid medium indica), 3 fertilization levels and 2 replications. The 0.2 hectare experimental field was divided into 24 plots. Rice seedlings were transplanted on June 10th every year. Rice plant height, leaf area index(LAI), total dry biomass
(TDM), tiller developments and phonological phases were measured every 15 days from the 7th day after transplantation.
The Exotech 100 Radiometer used in the experiment has 4 spectral bands: a 0.45-0.52um, b:052-0.60um, C: 0.63-0.69um, D:0.76-0.90um ( the same as the first 4 bands of TM) . All spectral data were collected by a polycordes on selected clear, windless days at 10:00-14:00 Binjing time. The Radiometer is set perpendicularly downward, 2m above the rice canopy. Five testing points were set in every plots. The average value of 5 tests was regarded as the spectral reflectance of the paddy plots. The average value of 5 tests was made once for every 1 or 2 weeks during the whole rice growing period.
Results and discussion
- Fig. 1 shows that the paddy spectral features of blue, green and red bands are: low reflective, low transmissive and highly absorbed. The main reason is that the pigment especially the chlorophy11) strongly absorbs the photon. In red band the reflectance is very low because more than 90% of incident sun light energy is absorbed for rich photosynthesis. During the period from seedling transplantation to heading, long with the increase of rich leaf photosynthesis, the chlorophy11content in rice plant increases rapidly. With the arrival of rice heading time, rice reflectance in red bands increases. After milk stage, decreases but the rice panicle needs more nutrient, therefore the rice reflectance in red band increases more rapildly.
Fig. 1 Paddy reflectance character
There is a little reflectance peak in the green band and it increases with the growth of rice. Because the inner tissue of the rice leaf has high reflectance and high transmission in the near infrared (NIR) band, so there is a high reflectance and low absorption in the NIR band Fig. 2 shows that he NIR reflectance increases with the development during the growing period of the rice.
Fig. 2 Near infrared reflectance of rice
From seedling transplantation to heading stage, the reflectance of NIR increases with the increase of LAI, then a tendency of steadiness follows. During milk stage, along with large amount of nutrient transferred into the panicle and the change of inner tissue of the leaf, the reflectance of NIR decreases (Fig. 3)
Fig. 3 Relationship between LAI and NIR reflectance.
Agronomist had indicated that the value of rice biomass depends and LAI, and the former is closely related to yield. It is, therefore, reasonable to rely on R and NIR bands in studying the relationship between paddy spectrum and rice growth.
