Assesment of Era Sar Data for Tropical Acricultural Crop Monitoring
3. Rice Crop Mapping and Monitoring
Study area and Data Basis
A study area of approximately 10x10 Km size was selected in West Thailand (99.5 deg. E, 1.0 deg N). The rice growing area is irrigated, flat, homogeneously, and has large individual fields of at least one hectare size, Multi-temporal ERS-1 SAR data were available at eight acquisition dates, namely 22-Nov-91, 7-Oct-92, 24-Feb-93, 11-Jun-93, 20Aug-93, 29-Oct-93 and 3-Dec-93. Within the whole area ten sample areas of approximately ten hectares each were selected fir detailed backscattering studies of rice field. Extensive ground measurements were taken in parallel to ERS-1 data acquisitions during the main growth period Aug-Dec 1993. Plant height, plant moisture content, plant density, number and size of leaves, stalk diameters, height of standing water were measured, together with more general observations regarding the state of the water/soil surface, state of plants, and a Spot panchromatic image.
Rice plant growth and Radar signature of rice-fields
Rice has three major growth phases, the vegetative, reproductive and ripening phase. In the selected study area the growth cycle lasts 120 days, and two harvests per year are common. The main growth period lasts from August to December, a second crop grows from April to July.
In ERS SAR images rice fields appear dark during the vegetative phase when the field are flooded. During the reproductive phase radar backscattering increases and reaches a maximum in the early ripening phase. This maximum may plausibly be attributed to multiple radar reflections between vertical plant structures and the horizontal water surface at a growth stage when penetration to the surface is still possible. Later, during the ripening phase, the scattering from the volume of the canopy increases but penetration to the water or soil surface decreases leading to a slight darkening in the radar image.
Result-Rice area mapping
Rice acreage can be retrieved from multi-temporal radar imagery making use of the unique backscattering signature of rice fields which is significantly different from that of any other land-cover. Fig. 1 shows a multi-temporal image taken 6-Jun-93, 29-Oct-93, where the rice growing area appears in dark grey and can easily be discriminated from other land covers.
Figure 1 Multi-temporal ERS-1 SAR image of Kanchanaburi, W-Thailand. Rice fields (dark, centre) are clearly
discriminated from other land-use categories.
A simple, pixel-based maximum likelihood classification was carried out, based on four Gamma MAP speckle filtered radar images (6-Jun-93, 20-Aug-93, 3-Dec-93). The result of the classification is shown in Fig. 2. The classification accuracy for rice versus other land-use classes is 89% (Aschbacher et al., 1995; Aschbacher, 1995).
Figure 2: Rice area based on four ERS - 1 SAR data acquisitions (6-Jun-93, 20-Aug-93, 29-Oct-93, 3-Dec-93). Grey scale: rice (dark), water (bright), other land-use classes (black).
