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Agriculture/Soil
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SAR Backscater Response of Various Growth Stages of Wetland Rice Paddy in Malaysia
Result and Discussion
Figure 1 illustrates graphically the preliminary results of the study for site 1: kampung Tanah Seratus. figure 1(a) gives the average radar backscatter coefficient s° is obtained using the following equation:
s° =20 log10 (DN)-K (1)
where DN is the SAR image digital number, K= 63.43dB ( before 16.8.96) and K= 59.68 dB ( after 16.8.96) for ERS-1 data; while K=60.92 dB ( before 16.8.96) and K= 59.75 dB ( after 16.8.96) for ERS-2 data.
Fig 1: (a) Radar backscatter from ERS 1/2 Data (b) Average height of paddy crop above water level
(c) Plant moisture(%O.F.B.) (d) Phenological growth stages of paddy plant.
All the images have been co-registered using SPOT Panchromatic data as reference. Note that s° form ERS ½ are VV-polarization.
Figures 1(b) and (c) shows the average height and moisture contents of the rice paddy crop, respectively ; whereas fig1(d) gives the growth stages of paddy during the study period. the following observations can be made:
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on 27th April 1996, the paddy was at its germination stage. The field was flooded with water s° was low since there was very little backscatter contribution from the water surface, and the plants were very small.
- On 1th June 1996, after 45 days, the crop was in the vegetative phase. The moisture content of the plant was high and the average height of plant above the water level was about 59cm. In this case, s° was higher since there was more returns due to volume scattering with the plant constituents.
- During the field trip on 11th October 1999, the corp. was in the mature /ripening stage. The plant moisture content was lower than on ht previous trip, but the average height was taller,
s° was still high, indicating that plant moisture and plant height were two important factors that control the radar backscatter returns.
- On 15th September 1996, the paddy crops have been harvested, leaving, bare soil that was ploughed and prepared for the second cropping in the year. The field was also covered with water. Again we note that
s° was relatively low compared to those at vegetative stage, or the ripening stage.
Conclusion
From this study, the backscatter coefficient s° of the rice paddy form ERS ½ does vary at different growth stage. the variation reflects the conditions /states of the plants and soil surfaces due to moisture content and plant height. Thus there is agreat potential for application of SAR images in delineation and monitoring of rice paddy. However, much detailed works need to be done to compare and contrast, qualitatively and quantitatively, for s° and the physical conditions of the fields, through goundtruth measurements and simulations. This will enhance one's confidence in the use of space-borne data.
Reference:
- Le Toan, T.,.F. Wang, Floury, N., K.H. Ding, J.A.Komg, M. Fujita and T. Kurosu, (1997), " Rice Crop Monitoring using ERS0-1 Data bsed on Experiment and Modeling Results,", IEEE Trans, Geoscience & Remote Sensing, Vol.35, No. 1,pp.41-56.
- Ewe, H.T. Chuah, A.Ismail, K.F.Loh, and N.Nasruddin, (1995), " Paddy Crop Monitoring using Microwave Remote Sensing Technique,", Geocarto International, Vol. 10, No.3, pp, 33-41
- Rashid, Abdullah, Padi ( paddy ), penerbit Prisma, Petaling Jaya, Malaysia, 1991.
Acknowledgment
The authors would be like to thank the Director General of Agricultural Department Peninsular Malaysia for providing the lab analysis and fieldwork, the Malaysian Center for Remote sensing for its financial support.
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