Abstract
Water 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.
Highlights
There is an increasing need for improved management of freshwater resources as these resources are becoming increasingly strained and more valuable
We have found that long-term measurement of coherence provides information about the vegetation characteristics of wetlands that is additional to that provided by radar backscatter alone, and we report those findings here
We have investigated the effects of the perpendicular baseline on coherence
Summary
There is an increasing need for improved management of freshwater resources as these resources are becoming increasingly strained and more valuable. The causes are many: increasing populations, increasing agricultural irrigation that often results in over-use or salinization, human populations moving to sunny dry climates with limited water supplies, pollution by agriculture and industry, industrial consumption and climate change. Wetlands are the most vulnerable landscape feature subject to climate change [1]. Even Canada, a country with abundant water in most places, is experiencing water issues. Drought years strain water resources for cities (e.g., Regina) and rural areas (southern Alberta and Saskatchewan). Industrial use of water is stressing resources. A notable example is the tar sands oil extraction, which requires large quantities of water from the Athabasca River
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