Temporal Backscatter Behavior of Rice
This study aimed at understanding the backscatter behavior of rice over its whole life circle, and
the relationship between the rice structure parameters and its backscatter coefficients. Based on a
good understanding of backscatter characteristics of rice, we can then suggest the best date for
radar data acquisition to monitor the rice growth and estimate the yield. Figures 1 and 2 show the
backscatter coefficients of rice extracted from calibrated RADARSAT images acquired from
summer to autumn in 1996 and from spring to summer of 1997 respectively. These are produced
by applying the calibration procedure to the digital number of Radarsat image to produce
s 0 for a
training area (20 pixel by 20 pixel) of the multi-temporal RADARSAT dataset.
Figure 1. Backscatter coefficients of rice as a function of time (1996)
Figure 2. Backscatter coefficients of rice as a function of time (1997)
Figure 3. Theoretical backscatter model of rice (based on the
backscatter behavior of medium-late mature rice of 1997)
In figure 4, there are five types of rice: 1) medium mature rice. 2) late mature rice. The backscatter
coefficient of late mature rice is slightly higher than medium mature rice on August 4. 3) Late
transplanted rice, which has been transplanted about 25 days later than the normal spring rice, due
to a cold spring and low temperatures in 1996. The RADARSAT image acquired on April 25
shows the backscatter coefficients of these fields as very low, responding similar to a still water
body. The backscatter coefficients from the RADARSAT images taken after April 25 increase
gradually, until September when the rice is harvested. Its life span is relatively short, and the
backscatter coefficients are lower. 4) single rice and 5) autumn rice. These two rice types
demonstrate how farming activities can change within a year, switch between fish ponds and rice
fields. Spring rice was a rice field in spring then switched a fish pond in autumn. Conversely,
Autumn rice was a fish pond in spring then switched to rice field in autumn. This practice is very
common in Zhaoqing area, as well as many other parts of China. In general, this swap of fish
pond and rice implies that the quality of the field is less than excellent, and that the rice yield is
normally lower. Based on the classification results of the 1996 multi-temporal fine mode
RADARSAT dataset, A land cover map was produced clearly showing the five types of rice, as
well as other vegetation covers and targets.
Figure 4. Backscatter intensity of the targets in relation to
polarization and frequency (based on the CV-580 data)
In figure 2, there are four major types of rice with different growth cycles. With the knowledge
gained from the rice backscatter behavior of 1996, we can distinguish between rice types quite
easily. They are early mature rice, medium mature rice, medium-late mature rice, and late mature
rice. Their life span is about 80 days, 100 -115 days, 110-120 days and 120-125 days
respectively. A false-color fusion image is produced based on the multi-temporal Standard mode
RADARSAT images acquired in the spring of 1997. A rice distribution map is produced using the
7 RADARSAT scenes. If a type of rice has higher backscatter value in April, at the early stage or
seedling development period, as found with late mature rice, then it was transplanted earlier than
the other types of rice. If a type of rice has higher backscatter coefficients in July, the late stage
or the harvest period, the rice is harvested later than other type of rice, as found with medium-late
mature rice. From figure 2 we can conclude that late mature rice has the longest lifetime. The
medium-late mature rice was transplanted a few days later than medium mature rice and late
mature rice, and harvested later. The medium mature rice was harvested before July 22. From
figure 2, we can see that early mature rice was harvested before June 28. It has very lower
backscatter coefficients since early harvest period. It is concluded that from the early stage, 25
days after transplanting, and late stage, 80 days after transplanting, we can distinguish the rice life
span easily, which is an important information for rice yield estimation. The life span is directly
related to the yield and quality of rice. This information is provided by local agronomists. It has
been proved in real life. The farmer used to grow three rice crops a year in past, nowadays, they
only grow two rice crops a year and they make more money from rice farming.
