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TitleSeasonal Change in Wetland Coherence as an Aid to Wetland Monitoring
AuthorBrisco, B; Ahern, F; Murnaghan, K; White, L; Lancaster, P
SourceRemote Sensing of Environment vol. 9, no. 158, 2017., Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20130057
PublisherMDPI AG
Mediapaper; on-line; digital
File formatpdf
AreaOttawa valley
Lat/Long WENS -76.0000 -75.5000 45.5000 45.2500
Subjectsenvironmental geology; hydrogeology; radar imagery; ground probing radar; radar methods; satellite imagery; LANDSAT; satellites; floods; flood potential; wetlands; swamps; soils; vegetation; change detection
IllustrationsLandsat images; location maps; tables; graphs
ProgramRemote Sensing Science
Released2017 02 15
AbstractWater is an essential natural resource, and information about surface water conditions can support a wide variety of applications, including urban planning, agronomy, hydrology, electrical power generation, disaster relief, ecology and preservation of natural areas. Synthetic Aperture Radar (SAR) is recognized as an important source of data for monitoring surface water, especially under inclement weather conditions, and is used operationally for flood mapping applications. The canopy penetration capability of the microwaves also allows for mapping of flooded vegetation as a result of enhanced backscatter from what is generally believed to be a double-bounce scattering mechanism between the water and emergent vegetation. Recent investigations have shown that, under certain conditions, the SAR response signal from flooded vegetation may remain coherent during repeat satellite over-passes, which can be exploited for interferometric SAR (InSAR) measurements to estimate changes in water levels and water topography. InSAR results also suggest that coherence change detection (CCD) might be applied to wetland monitoring applications. This study examines wetland vegetation characteristics that lead to coherence in RADARSAT-2 InSAR data of an area in eastern Canada with many small wetlands, and determines the annual variation in the coherence of these wetlands using multi-temporal radar data. The results for a three-year period demonstrate that most swamps and marshes maintain coherence throughout the ice-/snow-free time period for the 24-day repeat cycle of RADARSAT-2. However, open water areas without emergent aquatic vegetation generally do not have suitable coherence for CCD or InSAR water level estimation. We have found that wetlands with tree cover exhibit the highest coherence and the least variance; wetlands with herbaceous cover exhibit high coherence, but also high variability of coherence; and wetlands with shrub cover exhibit high coherence, but variability intermediate between treed and herbaceous wetlands. From this knowledge, we have developed a novel image product that combines information about the magnitude of coherence and its variability with radar brightness (backscatter intensity). This product clearly displays the multitude of small wetlands over a wide area. With an interpretation key we have also developed, it is possible to distinguish different wetland types and assess year-to-year changes. In the next few years, satellite SAR systems, such as the European Sentinel and the Canadian RADARSAT Constellation Mission (RCM), will provide rapid revisit capabilities and standard data collection modes, enhancing the operational application of SAR data for assessing wetland conditions and monitoring water levels using InSAR techniques.
Summary(Plain Language Summary, not published)
Information about surface water can help a number of applications including hydrology, meteorology, ecology, and agronomy. Ample quantities of water are needed for healthy wetlands. Synthetic Aperture Radar (SAR) satellite imagery is as an important data source for monitoring surface water, especially under inclement weather conditions. This paper evaluates methods to determine if RADARSAT-2, Canada's SAR satellite, can be used to monitor wetland water levels. The results identify that coherence (the degree which surfaces are identical measured on a scale of 0 (low) to 1 (high)) offers promise as a monitoring tool for flagging wetland changes. The cause of this change can then be determined and remedied. This paper identifies additional research needed for operational implementation of this technology for future SAR satellite missions, e.g. the RADARSAT Constellation Mission.

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