3. Rice Calendar
In the Zhaoqing test site, there are two crops per year; early season rice and late season rice. There are five major growth periods in the life cycle of rice. 1) Transplanting period: rice plant seedlings are transplanted from the seedbed to the paddy field. The transplanting date depends on the weather, especially temperature; 2) Seedling developing period: the seedling splits up and begins to develop a root system; 3) Ear differentiation period; 4) Heading period: headings begin to form; 5) Mature period: the rice plants mature and are ready to be harvested. Temporally, these five periods for early season rice are March 25 ~ April 5, April 15 ~ 25,
May 10 ~ 30, June 10 ~ 25, July 5 ~ 31. For late season rice, the growth stages occur as follows: July 20 ~ August 5, August 10 ~ 20, September 1 ~ 30, October 1 ~ 20, November 1 ~ 25 respectively. The exact dates of each period vary with the weather condition during the whole life of rice and are totally rice life span dependent. Plant maturity rates and life span are species dependent.
This rice calendar only illustrates a conventional law of rice growing behavior.
With progress in agricultural technology, it is known that a long growing season is favorable to high photosynthetic rates and high rice yield. For this reason, the transplanting date of rice is getting earlier and the life span is getting longer. For instance, the local farmers grow three rice crops a year in past, but currently, they only grow two crops a year. In USA, there is only one rice crops a year. It starts in early may then harvests in end of October. The rice life circle is around 170 ~ 180 days. The yield reaches 6.2 ton/ha, which is higher than rice yield of 5.9 ton/ha in China. Improved varieties and cultural practices have contributed significantly to yield in last decades too.
4. 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 for a training area (20 pixel by 20 pixel) of the multi-temporal RADARSAT dataset.
In figure 1, 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 fishponds and rice fields. Spring rice was a rice field in spring then switched a fishpond in autumn. Conversely, Autumn rice was a fishpond 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 fishpond 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 1. Backscatter coefficients of rice as a function of time (1996)
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.
Figure 2. Backscatter coefficients of rice as a function of time (1997)
Based on the knowledge we accumulated in our researches in Zhaoqing test site started in 1993, we propose a theoretic backscatter model for rice. It is mainly suit for rice monitoring using RADARSAT data. The backscatter coefficients from flooded rice field before transplanting, in another words, the water surface recorded by RADARSAT is around -25 dB. After transplanting, in the first 20 days, the backscatter coefficients from paddy rice field are -15 dB. Then entering the seedling-developing period, from 20 days to 45 days, the backscatter coefficients from paddy rice field increase to -12 dB. From 45 days to 75 days, within the ear differentiation period, the backscatter coefficients from paddy rice field increase to -9 dB. In the middle of ear differentiation period, about 65 days, the backscatter coefficients reach the highest point -6 dB. Then gradually drops to -8 dB and keep this value during the mature period. The theoretic backscatter model is expressed in figure 3. It is produced by cubic polynomial that simulated the backscatter coefficients of rice as function of time. It mainly represents the backscatter behavior of medium-late mature rice. Medium-late mature rice is transplanted in April 1 and harvested in July 30. The late mature rice and medium mature rice are transplanted earlier than medium-late mature rice. Medium mature rice is harvested earlier than medium-late mature rice. This model can be applied to other type of rice and modified in accordance with rice life span, transplant date and harvest date. Then we can use this model to calculate the backscatter coefficients of rice at certain growth stage. During the mature period of spring rice, it is flooding season (Guo et al, 1999), so the backscatter coefficients of rice can be much higher
than normal value since the high moisture or dew effect.
The results from the studies carried out in 1996 and 1997 imply that the most important radar data acquisition time for rice monitoring is at the end of the seedling period and beginning of ear differentiation period, which is late April for spring rice and late August for autumn rice. For calculating the acreage of planted rice, two radar data acquisitions are required: one near the end of April, and another in the middle of May during the ear differentiation period. The first data acquisition tells the biggest difference between rice and other targets. The two data sets acquired in two dates is used for automatic classification. For rice yield estimation, three radar data acquisitions are required: one at the end of April, one in the middle of May, and one at the end of June. For autumn rice or late season rice, the corresponding date can be found by checking the rice growth calendar.
5. A Proposed Scenario for Operational Rice Monitoring and Yield Estimation
The value of a given data set and research project is measured in its practicality, and applicability to the real world. Researchers at the Institute of Remote Sensing Applications (IRSA), through a program entitled "SAR Technology for Rice Growth Status Monitoring and Land Cover Mapping" under the support of National High Technology Program, have strive to provide information on rice growth status and yield to both national and local government agencies, as well as to commercial users. The final goal of this project is to establish an operational rice monitoring and yield estimation system. This paper presents the research achievements of this program. Figure 4 outlines the operating procedures for rice monitoring and yield estimation as followed for this study. The entire working procedure can be divided into four parts:
5.1. Data Input:
5.1.1. Multi-Temporal and Multi-Parameter Radar Data
The radar data provides the latest information on rice growing status and the accurate acreage of planted rice. For single frequency and polarization radar data like RADARSAT, two or three data acquisitions are required: one about 20 days after the transplanting period, and one at the end of seedling development period. For the multi-parameter radar data (i.e. multi-frequency or multi-polarization), one data acquisition is required during the early stage of rice growth. The another critical radar data acquisition is at the end of the heading period.
5.1.2.Static Supporting Data
A digital elevation model (DEM) is required to understand the rice distribution limitations and to aid in image processing. In addition, soil maps help to know the soil quality and its effect on rice production. Knowledge of the crop calendar helps to decide the optimum radar data acquisition date, and explain the rice growth stage at the acquisition time. The crop calendar may shift a few days at the transplanting period, in accordance with the change of temperature.
5.1.3.Dynamic Supporting Data
Meteorological data, including temperature, total sun illumination, rainfall etc. are important in deciding the conventional yield of rice on a regional scale. Knowledge of tillage activities provides information on the seeding of the rice and transplanting methods, which is directly related to the yield per unit area.
In the early stages of the study, data sources B and C determine the rice yield model. Data source C will be a dominant factor in determining the annual yield.
5.2. Supporting Database
There are a series of database and numerical models to support the image processing and the yield estimation work.
- The rice yield model is established based on the input source of static and dynamic data, which directly go to join the rice yield estimation procedure.
- Database ground control points (GCPS) are collected and used to register the acquired imagery to the stored DEM, as well as to other images from different dates or sources.
- Representative training areas for classification are selected for both rice and other targets. In general, the sites should be regular and stable.
- A rice theoretic backscatter model is a numerical model developed based on radiation transfer theory, which is used to explain rice backscatter behavior. A rice experiment backscatter model is established by studying the multi-temporal RADARSAT data of 1996 and 1997. These two models are used to classify the radar data.
5.3. Image Processing
- Registration of the imagery to existing data and maps using a DEM and GCPs.
- Filtering to remove the speckle noise and small pitches before and after classification.
- Extraction of linear features such as ditches between fields. The width of these linear features is enlarged by the corner reflection effect, allowing the linear features to be easily seen on an image. The area of these linear features should be calculated and considered in the estimation of crop acreage. Linear feature extraction method and the amount of area that should be considered in acreage estimations is a topic in need of further research.
- Classification procedure to separate different types of rice and assess the separability of rice from other targets. In this study, a neural network classifier (Liu and Shao, 1996) from the PCI software package was used to classify the multi-temporal RADARSAT data.
- Based on the classification and linear feature extraction, a precise calculation of the acreage of planted rice can be made and combined with the rice yield model and rice experiment backscatter model to make the first yield estimate and to forecast the acreage of planted rice. Meteorological data can help in the estimation and forecasting of the final yield after the harvest. During the harvest season, there may be some hazardous weather such as summer storms and flooding.
- Creation of new maps, and updating of existing databases with new results and knowledge.
