Space-based detection of wetlands' surface water level changes from L-band SAR interferometry
Introduction
Interferometric Synthetic Aperture Radar (InSAR) is a powerful technique for detecting small changes (cm level resolution) of the Earth's surface over a wide area. The method is widely used to study earthquake and volcanic induced crustal deformation (e.g., Burgmann et al., 2000, Massonnet and Feigl, 1998, Massonnet et al., 1993), land subsidence (e.g., Amelung et al., 1999, Baer et al., 2002, Bawden et al., 2001, Buckley et al., 2003), landslides (e.g., Colesanti and Wasowski, 2006, Dai et al., 2002, Kimura and Yamaguchi, 2000), mining and large-scale geotechnical projects (e.g., Raucoules et al., 2003, Tesauro et al., 2000), and glacier motion (e.g., Goldstein et al., 1993, Mohr et al., 1998). The above applications measure the displacement of solid surfaces, whether they are rocks, soil, buildings, or ice (glacier). However, water level changes in wetlands may also be measured by InSAR (Alsdorf et al., 2000, Wdowinski et al., 2004a). This application works only in wetlands and floodplains where woody or herbaceous vegetation emerge above the water surface. The horizontal water surface and the vertical vegetation allow a double-bounce reflection of the transmitted radar signal back to the satellite (Richards et al., 1987). In non-vegetated flooded areas the water surface acts as a mirror scattering away the entire radar signal, because of the satellite's off-nadir transmission angle, typically 20°–45°.
Space-borne SAR data have been acquired since the 1970's by several civilian satellites operating in the C- and L-band ranges (5.3 and 1.275 GHz corresponding to 5.66 and 23.5 cm wavelength, respectively). The longer wavelength L-band SAR data (23.5 cm wavelength) is considered more reliable for the InSAR technique in vegetated areas (Rosen et al., 1996, Zebker et al., 1997). The first applications of the InSAR method to floodplains and wetlands were conducted using L-band data acquired over the Amazon basin (Alsdorf et al., 2000) and southern Florida's Everglades wetlands (Wdowinski et al., 2004a). Recent studies have shown that the shorter wavelength C-band SAR data (5.66 cm) can also be used for wetland InSAR, mainly in woody wetland environment (Kim et al., 2005, Lu et al., 2005, Wdowinski et al., 2004a, Wdowinski et al., 2004b).
In this study, we use L-band SAR data to study water level changes and the derived hydrological conditions in the Everglades wetlands. The data were acquired over southern Florida during the years 1993–1996 by the Japanese Earth Resources Satellite (JERS-1). It is the most complete L-band dataset over a large-scale wetland acquired to date, containing seven repeat passes of two tracks that cover most of the Everglades wetlands. Because the Everglades consists of natural and managed wetland portions, as well as diverse vegetation types (woody, herbaceous, prairie, and saltwater mangroves), this large JERS-1 dataset allows a rigorous evaluation of the wetland InSAR method to various wetland environments.
In our previous study (Wdowinski et al., 2004a), we used three JERS-1 passes of one track acquired over six month period in second half of 1994. Here we expand this work in both spatial and temporal coverage. Our 2004 study focused on a limited region, specifically the Water Conservation Areas (WCA) located south of Lake Okeechobee (Fig. 1), where water level changes are largest and are dominated by the operation of human-made water control structures. In the current study, we use a more complete dataset (two tracks with seven repeat orbits) extending over a three year period. We also study a larger area, the entire south Florida wetlands, including both managed and natural flow wetland environments, as well as more diverse vegetation types. We find that the natural and managed flow environments have very different InSAR signatures, reflecting the different conditions in the two environments.
Section snippets
South Florida wetlands and hydrological background
South Florida is characterized by a large amount of precipitation (1200–1500 mm/yr) and very flat topography, resulting in a large volume of surface water that slowly drains to the ocean, evaporates, or infiltrates into the subsurface. The natural undisturbed system of south Florida, which existed until the beginning of the 20th century, includes the watershed of the Kissimmee River, Lake Okeechobee, and the Everglades (Fig. 1b). The Kissimmee River drains precipitation and runoff north of Lake
SAR data and InSAR processing
Our study is based on a large L-band SAR dataset acquired over south Florida during the years 1993–1996 by the JERS-1 satellite. The dataset includes twelve archived JERS-1 passes of two adjacent descending tracks; five repeat passes of the eastern track (463) and seven repeat passes of the western one (464) (Fig. 1, Fig. 2). Although the data are archived and delivered by 75 × 75 km2 scenes, we restored the original continuity of the data by concatenating the scenes, forming 225 × 75 km2
Results
We produced nine strip interferograms of south Florida showing phase changes between 44 and 396 days during the years 1993–1996 (dashed lines in Fig. 2). Eight of these are shown in Fig. 3, which includes interferograms of both the eastern track (upper panels) and the western track (lower panels). One interferogram (eastern track spanning over the period 1994-06-26/1994-12-19) is omitted, because of limited space of Fig. 3. It is of a similar quality and we refer to this interferogram in our
Hydrological analysis
The L-band interferograms presented here (Fig. 3, Fig. 4) provide high spatial resolution (100–300 pixel resolution) maps of surface water level changes over broad wetland area. In order to utilize these space-based observations for hydrological application, we need additional information, because the InSAR measurements are relative in both time and space. In time, the measurements provide the change in water level (not the actual water level) that occurred between the data acquisitions
Discussion
Our InSAR results show that L-band SAR data is very useful for detecting water level changes in most investigated wetland types. The coherence level and, hence, quality of InSAR observations depends on the scattering environment and the interferogram's time span. Highest coherence levels are obtained in the urban and woody wetland environments (Table 1). Over herbaceous wetland vegetation, the coherence level is higher for the one-repeat orbit cycle (44 days) and degrades as the interferogram
Conclusions
Interferometric analysis of twelve JERS-1 SAR passes acquired over south Florida during the years 1993–1996 show that L-band interferograms are very useful for detecting wetland water level changes. Although our study is limited to south Florida it has implication to many large-scale wetlands, because south Florida wetlands are very diverse, consisting of woody, herbaceous, prairie, and salt-water mangrove, as well as managed and natural flow environments. Furthermore a dense stage monitoring
Acknowledgements
We acknowledge the support and collaboration of the Department of the Interior, U.S. Geological Survey and the Florida Water Resources Research Center, University of Florida, under Grant No. 04HQGR0160, as well as, NASA and ONR. The work of S-W Kim was funded by the Korea Research Foundation Grant funded by Korea Government (MOEHRD, Basic Research Promotion Fund: No. M01-2004-000-20345-0). We thank Matt Pritchard for his help with the ROI_PAC software, Virginia Walsh for access to the
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