Rice mapping using multi-temporal imagery in Monsoon Asia

Abstract

Rice is a major food staple in Monsoon Asia, a region covering south, southeast, and east Asia. However, with rapid increases in population and urbanization, the demand for rice production in ever decreasing areas of arable land is becoming critical. Up-to-date information about rice-planting areas is therefore important for sustainable development in this region. Because of frequent cloud cover and heavy precipitation in the rice-growing season, rice mapping on the basis of optical remote sensing is often a challenge. Synthetic-aperture radar (SAR) is a potential alternative because of its all-weather, day-and-night imaging capabilities. SAR sensors frequently acquire images at multiple frequencies and polarizations that are ideal for mapping rice-crop areas1, 2 based on the dynamics of rice backscatter at different stages of growth.3 In January 2006, the Advanced Land Observing Satellite (ALOS) was launched successfully by the Japan Aerospace Exploration Agency (JAXA). The onboard Phased Array-type L-band Synthetic Aperture Radar (PALSAR) acquires SAR images at a wavelength of 23.5cm, in a revisit cycle of 46 days. PALSAR operates in several modes—including fine-beam single and dual polarization, polarimetric, and scanning SAR— enabling data acquisition at spatial resolutions ranging from 6.25 to 50m, with swath widths from 70 to 360km.4 Because of their longer wavelength, PALSAR signals can penetrate rice canopies and provide structural information for rice and other land cover types. These features, coupled with the regional observation strategy, make PALSAR very attractive for rice mapping. We tested the feasibility of this approach using PALSAR imagery of southeast China. Rice is often planted in paddy fields and grows in distinct stages, including transplanting, tillering, ear differentiation, heading, and maturing.2 Rice backscatter in SAR Figure 1. Example composite image of PALSAR frames acquired on 18 June, 3 August, and 18 September 2006, centered on Fuyang City in Zhejiang province (southeast China).