Seasonal Change in Wetland Coherence as an Aid to Wetland Monitoring

Abstract

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.

Wetland hydroperiod classification in the western prairies using multitemporal synthetic aperture radar

“…In 2015, however, this area is inundated by temporary surface water. This inundation would reduce the vigour and density of these cattails, and this emergent plant is subject to seasonal die‐off as reported by Brisco et al () in similar vegetated wetland environments.…”

“…This inundation would reduce the vigour and density of these cattails, and this emergent plant is subject to seasonal die-off as reported by Brisco et al (2017) (Brisco et al, 2017;White et al, 2014). Wetland surface water extent was found to change dynamically, corresponding to rainfall occurring seasonally or annually.…”

“…Open water can be covered by varying extents of emergent and floating aquatic vegetation during certain growth stages throughout the year (Brisco et al, ). Significant rain events on May 23–24, 2013 (74 mm), and June 2–3 in 2013 (42 mm) likely promoted riparian and emergent vegetation growth earlier in the growing season, resulting in less open water being identified by the threshold routine, which does not recognize flooded vegetation as a clear unobstructed open water surface (Brisco et al, ; White et al, ). Wetland surface water extent was found to change dynamically, corresponding to rainfall occurring seasonally or annually.…”

“…1.1 | Remote sensing for water mask generation Remote sensing application domains use spatial and temporal data covering large areas that are not geographically limited by in situ access and include flood extent delineation, habitat mapping, and wetland assessments that have been found to greatly enhance water resources monitoring, ecological studies, and infrastructure management (Brisco et al, 2017;Irwin, Beaulne, Braun, & Fotopoulos, 2017;Ozesmi & Bauer, 2002).…”

A Collection of Novel Algorithms for Wetland Classification with SAR and Optical Data

“…In addition to hydrological monitoring of wetlands using InSAR, the interferometric coherence can be used for other wetland applications, such as change detection and classification [51,91]. This is because coherence has a diagnostic function and can be used along with SAR backscatter and polarimetric decomposition techniques for classification of different wetlands.…”