www.elsevier.com/locate/rse Multisensor Hydrologic Assessment of a Freshwater Wetland Jessika To ¨yra ¨ ,* Alain Pietroniro, † and Lawrence W. Martz* T his article evaluates the use of synthetic aperture radar when the SPOT scene was classified in combination with the Radarsat S7 scene. Elsevier Science Inc., 2001. All (SAR) and visible/infrared (VIR) satellite imagery for map- Rights Reserved. ping the extent of standing water in the Peace-Athabasca Delta during spring and summer of 1998. SAR images con- tain data about the geometric and electrical characteristics INTRODUCTION of the objects, while VIR images contain information about the reflectivity of objects. Radar pulses can also penetrate Although wetlands only cover about 6% of the land surface vegetation to some degree depending on the wavelength and of the world, they provide an important role in the global ecosystem (Mitsch and Gosselink, 1986). Wetlands support vegetation thickness. It is hypothesized that since Radarsat a unique habitat for a great variety of hydrophytic plants, and SPOT images contain complementary information, flood fish, wildlife, and insects. From a hydrologic perspective, mapping will be more efficient when the two image types wetlands improve water quality, recharge ground water, are used in combination. Radarsat SAR and SPOT multi- control floods, and protect shorelines. Since the availability spectral imagery from May 1998 and July 1998 were used of water is the dominant mechanism driving the ecosystem for the flood mapping. A Radarsat S2 image (27.5° incidence response, accurate and frequently updated maps of flood- angle) was obtained for May 1998. To evaluate the inci- water extent are necessary for wetland management. This dence angle effect on flood mapping, a Radarsat S1 image information is difficult to obtain using ground surveys due (27.5° incidence angle) and a Radarsat S7 image (47.0° to logistical problems and high costs. Satellite remote sensing incidence angle) were obtained for July 1998. The Radarsat has become an important and cost-effective tool for provid- scenes were calibrated and filtered, and all imagery were ing this information in a temporal context (Carter, 1982). orthorectified to minimize geometric distortion. A Mahala- Many studies have utilized the low reflectance of water nobis distance algorithm was used to classify the SPOT in the infrared bands of SPOT, Landsat TM, and Landsat scenes, the Radarsat scenes, and a combination of the two MSS satellites to map the extent of surface water (Moore into open water, flooded vegetation, and nonflooded land. and North, 1974; Rose and Rosendahl 1983; Ackleson et al., The results indicate that flood mapping in both spring and 1985; Bennett 1987; Jaques 1989; Pietroniro and Prowse summer conditions has significantly higher accuracy when 1996). In the case of forested wetlands, Carter (1982) has Radarsat and SPOT imagery are used in combination, rather suggested that the extent of flooding will be most efficiently than separately. However, it is important to use Radarsat detected on visible and infrared (VIR) imagery when vege- imagery acquired at low incidence angles. Classification tation is absent or during the dormant, leafless season. As the deciduous vegetation becomes foliated, tree canopies of the SPOT scene combined with the Radarsat S1 scene obscure the floodwaters and most flood boundaries are achieved significantly better results than those obtained difficult to distinguish. On the other hand, the vegetation cover and composition that can be observed on visible and infrared imagery may provide indirect information about * Department of Geography, University of Saskatchewan, Saska- the floodwater boundaries. toon, Saskatchewan, Canada † National Water Research Institute, Environment Canada, Saska- Synthetic Aperture Radar (SAR) is useful for flood toon, Saskatchewan, Canada mapping because of its sensitivity to moisture differences Address correspondence to J. To ¨ yra ¨, Dept. of Geography, Univ. of and its potential to penetrate clouds and vegetation. Open Saskatchewan, 9 Campus Dr., Saskatoon, SK, Canada, S7N 5A5. E-mail: water acts as a specular reflector in the microwave spec- jessika.toyra@ec.gc.ca Received 3 March 2000; revised 10 July 2000. trum and yields a very low radar backscatter signal, while REMOTE SENS. ENVIRON. 75:162–173 (2001) Elsevier Science Inc., 2001. All Rights Reserved. 0034-4257/00/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(00)00164-4