Detecting deforestation with multitemporal L-band SAR imagery: a case study in western Brazilian Amazônia

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1 INT. J. REMOTE SENSING INPE eprint: v , 1 8, PrEview article Detecting deforestation with multitemporal L-band SAR imagery: a case study in western Brazilian Amazônia R. ALMEIDA-FILHO*{, A. ROSENQVIST{, Y. E. SHIMABUKURO{ and R. SILVA-GOMEZ{ {Instituto Nacional de Pesquisas Espaciais (INPE), Caixa Postal 515, São José dos Campos, SP, Brazil {Swedish Space Corporation, PO Box 4207, Solna, Sweden (Received 30 November 2005; in final form 12 April 2006 ) Applications of L-band SAR data to map deforestation are generally based on the assumption that undisturbed forests consistently exhibit higher radar backscatter than deforested areas. In this Letter we show that depending on the stage of the deforestation process (slashing, burning and terrain clearing), this assumption is not always valid, and deforested areas may display a stronger radar return backscatter than primary forest. The analysis of multitemporal SAR images, supported by several Landsat Thematic Mapper (TM) images and field knowledge, showed that wood materials left following the deforestation practices function as corner reflectors, causing an initial increase in the radar backscatter, which then subsequently decreases over time as the debris on these fields are removed. 1. Introduction Using Landsat-5 Thematic Mapper (TM) imagery, the National Institute for Space Research (INPE) has been monitoring annual gross deforestation in the Brazilian Amazônia over the past two decades (INPE 2002). At the 16-day revisit frequency of Landsat TM, at least one free-cloud scene has been obtained annually in the Arch of Deforestation, a region along the east, south and southwest borders of the Brazilian Amazonian forest. The deforestation fronts are presently moving gradually towards Central Amazônia, mainly along the corridors of the Cuiabá-Santarém and Transamazônica roads. Additionally, secondary roads built by municipalities are also leading to new hotspots of deforestation throughout Amazônia. While continued space-borne mapping and monitoring of this activity is critical, a study by Asner (2001) indicated that it is highly improbable to obtain an annual coverage of Landsat TM (or equivalent optical systems) with 10% or less of cloud cover for the northern half of the Amazônia, which is severely plagued by cloud cover. This means that monitoring annual deforestation using optical imagery alone will be limited, particularly given the increased activity in Central Amazônia. Several authors (e.g. Luckman et al. 1997, Saatchi et al. 1997, Angelis et al. 2002) have established a link between Synthetic Aperture Radar (SAR) L-band (,25 cm wavelength) backscatter and forest structure/biomass. Thus, since SAR data can be *Corresponding author. rai@dsr.inpe.br International Journal of1 Remote Sensing ISSN print/issn online # 2006 Taylor & Francis DOI: /

2 R. Almeida-Filho et al. acquired consistently on a repetitive basis regardless of weather conditions, these data could potentially be used to complement or replace optical systems in an operational programme to map and monitor deforestation in Amazônia. The use of L-band SAR imagery to map deforestation has commonly been based on the premise that undisturbed tropical forests consistently exhibit higher backscatter than deforested areas. However, Stone and Woodwell (1988) and Almeida-Filho et al. (2005) have demonstrated that deforested areas in the Brazilian Amazônia do not always display an unambiguous backscatter pattern at L-band and that the differentiation between primary forest, vegetation regrowth, and slashed area may be inconsistent. As a contribution to this relevant theme, this Letter discusses the use of multitemporal L-band SAR imagery to detect deforestation in a 1600 km 2 study area located in Rondônia State (figure 1), which has been a focus of extensive deforestation in the Brazilian Amazônia. 2. JERS-1 SAR and Landsat TM images To support the interpretation of the SAR imagery, five Landsat TM scenes, acquired between August 1994 and July 1996, were used (table 1). Before interpretation, they were geometrically rectified to the Universal Transverse Mercator (UTM) coordinate system, based on a first-degree polynomial rectification algorithm. Evaluation of the registration accuracy yielded root-mean error values equivalent to 0.8 pixels. Figure 1. Location of the study area in Rondônia State, Brazilian Amazônia. Table 1. JERS-1 SAR and Landsat TM data used in the study. JERS-1 SAR Landsat TM 4 August August October May July September November July

3 These images were interpreted following the methodological procedure adopted by INPE in the PRODES Project (INPE 2002), presented in Shimabukuro et al. (1998) and Almeida-Filho and Shimabukuro (2002). The procedure involves segmentation, region classification, and editing techniques, applied over fraction images derived from a spectral mixing modelling approach (Adams et al. 1995). Based on the field data and experience of the authors in image analysis, the adopted procedure assures the accuracy of a visually interpreted land cover/land use map. Three SAR scenes acquired by the Japanese Earth Resources Satellite (JERS-1) were used in the study (table 1). The JERS-1 SAR, which operated at L-band frequency, HH polarization and with a west-looking off-nadir angle of 35u, was in operation from February 1992 to October The ground resolution was 18 m in both range and azimuth directions and the swath width was 75 km. The SAR images were processed to standard level 2.1 (3 looks, 16 bits ground range), according to the procedures described in Almeida-Filho et al. (2005). In order to quantify the radar return from field classes, the backscatter coefficients were computed following the procedure presented by Shimada (2001). The images were subsequently co-registered relative to the Landsat TM images and resampled to the same spatial resolution (30 m630 m). 3. Results and discussions INPE eprint: sid.inpe.br/eprint@80/2006/ v Remote Sensing Letters Landsat TM colour composites (figure 2) acquired in July 1995 (a) and September 1995 (b) show the land use/land cover in the study area. Based on the analysis of the set of Landsat TM scenes (table 1), regions outlined in white correspond to areas deforested up to August 1994, and regions outlined in yellow correspond to areas deforested between August 1994 and July These areas are of particular interest for our discussion relative to ambiguous radar backscatter from newly deforested areas. Deforested areas enhanced in the Landsat TM scene acquired in July 1995 are characterized mainly by hues of magenta, indicating significant occurrence of soil exposure. Hues of light green within the domain of the deforested areas indicate secondary vegetation regrowth, and dark green shades in both scenes of figure 2 correspond to undisturbed primary forests. While areas of undisturbed primary forest remain stable, the Landsat TM image acquired in September 1995 displays significant changes in the domain of the deforested areas, compared with the scene acquired two months before. Most of the deforested areas, especially those slashed between August 1994 and July 1995 (outlined in yellow) appear in dark shades, indicating they had been burned some time during the period between July and September. The JERS-1 SAR scene acquired in October 1995 shows radar backscatter coefficients (su) ranging from db to 24.6 db. These values cover four classes of radar return (figure 3), which correspond to the land use/land cover types identified in Landsat TM images (figure 2). Class 1, with a radar backscatter coefficient (su) of db represents deforestation up to August 1994; class 2 (su527.8) corresponds to areas of undisturbed primary forest; class 3 (su526.6 db) to areas of vegetation regrowth; and class 4, with the highest radar backscatter (su525.6), to forest areas slashed/burned between August 1994 and July Table 2 shows the radar backscatter coefficients (su) and corresponding standard deviations for the above-mentioned classes. 3

4 R. Almeida-Filho et al. (a) (b) Figure 2. Landsat TM colour composites [TM5(R) + TM4(G) + TM3(B)] acquired (a) July 1995 and (b) September 1995 showing the land use/land cover in the study area. Areas deforested prior to August 1994 are outlined in white, and areas deforested between August 1994 and July 1995 in yellow. Dark green corresponds to areas of primary forest. Dark shades in the September 1995 scene indicate that most of the deforested areas were burned (grid is 10 km610 km; north is to the top). The relationships between terrain features and radar returns were also investigated using multitemporal SAR false colour composites. Combining scenes acquired in three subsequent years, two multi-annual composites could be obtained. Figure 4(a) combines SAR scenes acquired in August 1994 (red channel) and October 1995 (green and blue channels), while the second composite (figure 4(b)) combines scenes acquired in October 1995 and November 1996, with similar channel combinations. Although the 1994 image was acquired during the dry season and the 1995 and 1996 scenes during rainy seasons, there is not evidence that eventual differences in soil moisture should influence the interpretation, other than reducing the contrast somewhat between dark and bright areas. In the multi-annual composites, areas subject to no changes in the time period between the image acquisitions appear in different shades of grey undisturbed primary forest in intermediate grey shades and areas deforested before August 1994 in dark grey shades while areas exposed to changes over the considered period appear in hues of either cyan or red, indicating, respectively, an increase or a decrease in the radar backscatter. 4

5 Remote Sensing Letters Figure 3. JERS-1 SAR scene acquired October The numbers indicate land use/land cover classes with different radar backscatter: (1) deforestation prior to October 1995, (2) undisturbed primary forest, (3) vegetation regrowth, and (4) newly slashed/burned forest (grid is 10 km610 km; north is to the top). Table 2. Radar backscatter coefficient (su) for different land use/land cover classes in the study area. Land use/land cover classes su (db) Standard deviation Class 1 (deforestation prior to August 1994) Class 2 (undisturbed primary forest) Class 3 (vegetation regrowth) Class 4 (slashed/burned forest, July 1995) The composite (figure 4(a)) indicates several areas with increased radar backscatter between the two years (hues of cyan) and one area of decreased radar backscatter (hues of red). Comparison with the Landsat TM scene (figure 2(a)) confirms that the areas of increased radar backscatter correspond to areas deforested between August 1994 and July 1995, as well as to vegetation regrowth in areas deforested prior to In the composite (figure 4(b)) all the areas subject to increased radar backscatter in the composite (cyan) now instead appear in hues of red, thus indicating a backscatter decrease. Analysis of the Landsat TM scenes acquired in 1995 and 1996 confirm that these areas now had been further cleared, and converted into pastures or plantations. One small area appearing in cyan in the composite shows a new field slashed/burned sometime during the time period. Notable in this context is that the observed increases in radar backscatter from areas slashed/burned in the and periods do not conform with the well-established premise that mature forests consistently exhibit higher L-band backscatter compared to deforested areas. The results from this study indicate that the increase in radar backscatter observed in newly deforested areas results from the slashing and burning practices adopted by farmers in the Brazilian Amazônia, who 5

6 R. Almeida-Filho et al. (a) (b) Figure 4. JERS-1 SAR multi-temporal false colour composites [1994(R) (G,B)] (a) and [1995(R) (G,B)] (b) with hues of cyan and red indicating, respectively, increased and decreased radar backscatter over the respective time periods (grid is 10 km610 km; north is to the top). leave fallen trees on the ground to dry for several months, before carrying on with clearing and burning. This practice is confirmed in the Landsat TM images, which show that areas burned in September 1995 had been slashed several months earlier, prior to May This confirms previous results by Stone and Woodwell (1988), who hypothesized that in newly cleared forest, trunks and branches still remaining on the ground function as corner reflectors for the radar return, diminishing over time as they are removed. 4. Conclusions We have shown that deforested areas in the Brazilian Amazônia under certain circumstances may be characterized by higher L-band radar backscatter values than undisturbed primary forest. This occurs because the conversion of primary forest into pasture or agricultural fields involves three different steps before seeding the terrain: slashing, burning, and clearing (removal of stems and branches that were not entirely consumed by the fire). Thus, depending on the time of the SAR image acquisition relative to these steps, radar backscatter from deforested areas can be stronger than from primary forest, contrary to the general premise that primary forests consistently exhibit high L-band SAR backscatter compared to deforested areas. 6

7 Remote Sensing Letters In the context of the present study, and with continuous needs for satellite data within INPE s annual deforestation mapping activities, the launch of Japan s Advanced Land Observing Satellite (ALOS) in January 2006, is very timely. Apart from two optical instruments, ALOS carries an L-band Synthetic Aperture Radar (PALSAR), which provides observations in both HH and HV polarizations, thereby improving possibilities for differentiation between primary forest, newly slashed/ burned areas, and vegetation regrowth. Important to note about PALSAR in this context, which makes the sensor particularly relevant for mapping changes over large areas, is that a systematic observation strategy has been implemented for the sensor (Rosenqvist et al. 2004), by which all land areas on the Earth will be acquired in a temporally and spatially consistent manner, at least twice per year on a repetitive basis during the lifetime of the satellite. In the case of the Amazonian Basin, blanket observations at fine resolution are scheduled 3 5 times annually, i.e. several times during the slash/burn cycle, thereby increasing the probability for unambiguous discrimination of deforestation of the type described in this paper. Acknowledgments The National Institute for Space Research (INPE) provided Landsat TM scenes and the JAXA Earth Observation Research and Application Center (EORC) provided SAR data, within the framework of the JERS-1 SAR Global Rain Forest Mapping (GRFM) Project. The authors also thank three anonymous reviewers for their constructive criticisms of the manuscript, and colleague Ramon Freitas for his help in post-processing the SAR data. References ADAMS, J.B., SABOL, D., KAPOS, V., ALMEIDA-FILHO, R., ROBERTS, D.A., SMITH, M.O. and GILLESPIE, A.R., 1995, Classification of multispectral images based on fractions of endmembers: application to land-cover in the Brazilian Amazon. Remote Sensing of Environment, 52, pp ALMEIDA-FILHO, R. and SHIMABUKURO, Y.E., 2002, Digital processing of a Landsat-TM time-series for mapping and monitoring degraded areas caused by independent gold miners, Roraima State, Brazilian Amazon. Remote Sensing of Environment, 79, pp ALMEIDA-FILHO, R., ROSENQVIST, A., SHIMABUKURO, Y.E. and SANTOS, J.R., 2005, Evaluation and perspectives of using multi-temporal L-band SAR data to monitor deforestation in the Amazônia. IEEE Geoscience and Remote Sensing Letters, 2, pp ANGELIS, C.F., FREITAS, C.C., VALERIANO, D.M. and DUTRA, L.V., 2002, Multitemporal analysis of land use/land cover JERS-1 backscatter in Brazilian tropical rainforest. International Journal of Remote Sensing, 23, pp ASNER, G.P., 2001, Cloud cover observation of Brazilian Amazon. International Journal of Remote Sensing, 22, pp INPE (NATIONAL INSTITUTE FOR SPACE RESEARCH) 2002, Monitoring of the Brazilian Amazonian forest by satellite , Realised Brochure (São José dos Campos, Brazil, INPE), 18 pp. LUCKMAN, A., BAKER, J., KUPLICH, T.M., YANASSE, C.C.F. and FRERY, A.C., 1997, Study of the relationship between radar and regenerating tropical forest biomass from spaceborne SAR instruments. Remote Sensing of Environment, 60, pp ROSENQVIST, A., SHIMADA, M., WATANABE, M., TADONO, T. and YAMAUCHI, K., 2004, Implementation of systematic data observation strategies for ALOS PALSAR, PRISM and AVNIR-2. Proceedings of the International Geoscience and Remote 7

8 Remote Sensing Letters Sensing Symposium (IGARSS 04), Anchorage, USA, September 2004, 7, pp SAATCHI, S.S., SOARES, J.V. and ALVES, D.S., 1997, Mapping deforestation and land use in Amazon rainforest using SIR-C imagery. Remote Sensing of Environment, 59, pp SHIMABUKURO, Y.E., BATISTA, G.T., MELLO, E.M.K., MOREIRA, J.C. and DUARTE, V., 1998, Using shade fraction image segmentation to evaluate deforestation in Landsat Thematic Mapper images of the Amazon region. International Journal of Remote Sensing, 19, pp SHIMADA, M., 2001, User s Guide to NASDA s SAR products, Version 3 (Tokyo: National Space Development Agency of Japan). STONE, T.A. and WOODWELL, G.M., 1988, Shuttle imaging radar A analysis of land use in Amazônia. International Journal of Remote Sensing, 9, pp