# Multitemporal Synthetic Aperture Radar Study of Rice Cropping Systems in the Mekong Delta

The aim of this study is to use multitemporal ERS-2 synthetic aperture radar (SAR) images in delineating and mapping areas under different rice cropping systems in the Mekong river delta, Vietnam. Rice cultivation in the Mekong river delta is largely governed by hydrology, rainfall pattern and the availability of irrigation. Given the diversity of the cropping systems, it would be very difficult, if not impossible, to discriminate the areas under different planting systems using one single-date image. Multi-temporal images are required to monitor the spatial and temporal growth patterns of the rice crops and subsequently to identify the cropping systems practiced at a particular region. Multispectral visible/near infrared images such as those from the SPOT or LANDSAT satellites could be used for this purpose. Unfortunately, a large part of the rice growing season coincides with the rainy period resulting in limited availability of cloud-free images throughout the growing season. The use of cloud penetrating SAR would overcome this problem.

The study was confined within a 100 km by 100 km area covered by one ERS-2 SAR scene, (track 75, frame 3411 shifted 30% along track) which includes parts of Soc Trang and Bac Lieu provinces in the Mekong river delta, Vietnam. Seven descending mode ERS-2 synthetic aperture radar images at 35-day repeat intervals acquired during May 5, Jun 9, Jul 14, Aug 18, Sep 22, Oct 27 and Dec 1, 1996 were used in the study.

The image shown here is a multitemporal colour composite image derived from three ERS synthetic aperture radar images of the Mekong River Delta during the early part of the rice growing seasons (red: May 5; green: June 9; blue: July 14) in 1996. The many colours displayed show the different areas under different rice cropping systems. The areas which appear grey are where the backscattering coefficient remained relatively constant throughout the season. These areas correspond to relatively invariant surface features such as the inland melaleuca forests, the coastal mangrove forests, the linear settlements, homestead gardens and orchards along canals and roads; and the built-up areas.

The three main rice seasons in the Mekong river delta are shown in the following table.

Rice Seasons in the Mekong River Delta
| Season | Planting | Harvesting
--- | --- | --- | ---
Dong Xuan (DX) | Winter-Spring | Nov/Dec | Feb/Mar
He Thu (HT) | Summer-Autumn | May/Jun | Aug/Sep
Mua (M) | Rainy Season | Jul/Aug | Dec/Jan

These three rice seasons, in various combinations governed by hydrology, rainfall pattern and availability of irrigation, constitute the variety of rice-based cropping systems practiced in the Mekong river delta.

Major Rice Cultivating Systems in the Mekong River Delta
| Cropping Type | Rice Seasons
--- | --- | ---
1. | Single Rainfed | Mua(M)
2. | Double Irrigated | Dong Xuan (DX) - He Thu (HT)
3. | Double Rainfed | He Thu (HT) - Mua (M)
4. | Triple Irrigated-Rainfed | Dong Xuan (DX) - He Thu (HT) - Mua (M)

A thematic map of rice cropping system was derived using a classification method based on the analysis of the temporal change patterns of the radar backscattering coefficients in the seven SAR images.