Figure 5.1 Remote sensing products of the Everglades wetland, South Florida. Lowest level (base): L-band SAR image showing the backscatter amplitude of the study area. Second level: SAR interferogram illustrating lateral phase changes between two SAR acquisitions (1994/08/09 and 1994/12/19). Third level: map of water level changes occurring between the two acquisition times (the map is computed by multiplying each phase cycle by 15.1 cm). Highest level (top): high spatial-resolution 3-D map of “absolute” water levels calculated by integrating the space- and ground-based observations. The 3-D map shows dynamic water topography (up to 1 m elevation difference across a 20 km distance) caused predominantly by flood gate operation.

5. Wetland hydrology from space

Wetlands are transition zones, where water flow, nutrient cycling, and the Sun’s energy meet to produce unique and productive ecosystems. They provide critical habitat for a wide variety of plant and animal species, including the larval stages of many ocean fish. Wetlands also filter nutrients and pollutants from fresh water used by humans, and provide aquatic habitats for outdoor recreation, tourism, and fishing. Globally, many such regions are under severe environmental stress, mainly from urban development, pollution, and rising sea level. However, there is increasing recognition of the importance of these habitats, and mitigation and restoration activities have begun in a few regions. A key element in wetlands conservation, management, and restoration involves monitoring its hydrologic system: the entire ecosystem depends on its water supply. In the past, hydrologic monitoring of wetlands was conducted almost exclusively by stage (water level) stations, which provide good temporal resolution, but suffer from poor spatial resolution, as stage station are typically distributed several, or even tens of kilometers, from each other.

InSAR provides the needed high spatial resolution hydrological observations, complementing the high temporal resolution of terrestrial observations. Although conventional wisdom suggests that interferometry should not work in vegetated areas, several studies have shown that both L- and C-band interferograms with short acquisition intervals (1-105 days) can maintain excellent coherence over wetlands. Our recent results [Wdowinski et al., 2004, 2006] indicates that wetlands InSAR provides high resolution water level change maps with 5 cm accuracy and 1-2 cm precision, providing direct observations of flow patterns and flow discontinuities, as well as excellent constraints for high resolution hydrologic flow models.