Use of Synthetic Aperture Radar in Wetland Studies

Abstract

Wetland, a term not even in common use until mid-20th century, today has become a research hub, claiming 8-10 million km2 land surface of the earth, with greatest proportions in northern boreal regions (60°N) and tropical regions (exactly at equator). Wetlands play a very important role in ecological balance. They decrease the flow velocity of water thus help in flood control. Concentrations of various incoming pollutants (viz. nitrogen and phosphorous) are diluted by transforming and releasing them to atmosphere, which results in increase of water quality. They are capable to retain carbon 20 – 30 % and 20 – 25 % global methane and serve as coast line protectors from storms & degradation. Pioneer investigations in wetlands can be tracked back to 1960s. Studies then centered to botanical surveys and investigations of peat structure, now encompass almost every phenomena influenced due to wetland. SAR, promising its continuous availability throughout the year, regardless of the weather conditions, coupled with its other add-ons (InSAR, DInSAR, PolInSAR) has allured a wide research community worldwide for its exploitation. Rainy season being major source of water, influences wetland dynamics to a great extent, thus information during this season is vital for wetland managers. SAR signal, having unique ability to penetrate clouds, promise this requirement to be fulfilled. SAR signals can also penetrate through tree canopy thus, are capable to inform about the underneath culture. This paper goes through various phenomena of wetlands, which has been addressed successfully by various researchers, using SAR data. Delineation of various themes of wetland (i.e. aquatic vegetation, open water habitat, forest areas and localities of high soil moisture) has become possible with SAR data. Different combinations of multi-frequency, multi-incidence angle, fully-polarimetric data, their interactions in wetland and outputs, through various studies are detailed. An effort has been put to provide the insight of radar-target interactions. Study areas fluctuate from Amazon, through Keoladeo National Park (India), to Kakadu National Park (Australia). An extensive work done at world famous Keoladeo National Park, Bharatpur, India, has been discussed, which deals with various aspects of wetland using SAR data. Reviewed studies explicitly announce the immense potential SAR data inherits, in wetland applications. Availability of multi-channel space borne SARs (multi-incidence angle & multi-polarized) have revolutionized familiarity with wetlands. However, unavailability of a multi-frequency space borne SAR has restricted some interesting outcomes from different studies to be tested on operational basis.