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Title: Topographic indices for improved mapping of forested wetlands

item LANG, M - Us Forest Service (FS)
item McCarty, Gregory
item OSTERLING, R - University Of Maryland

Submitted to: Wetlands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2012
Publication Date: 12/12/2012
Citation: Lang, M.W., McCarty, G.W., Osterling, R.A. 2012. Topographic indices for improved mapping of forested wetlands. Wetlands. 33:141-155.

Interpretive Summary: Forested wetlands are one of the most difficult types of wetland to map. This is especially true in low topographic relief areas such as on the Delmarva Peninsula. We report the development of new topographic metrics for mapping wetlands which are based on high resolution elevation data from LiDAR remote sensing. LiDAR (Light Detection And Ranging) is a rapidly evolving technology for generation of highly accurate and detailed elevation data for landscapes. Two topographic metrics were found to be very useful for mapping wetlands. One metric predicts how water will move and redistribute on land surfaces and the other estimates influences of local relief and possible influence of near surface groundwater on wetland hydrology. Used together, these metrics provided great synergy of information for mapping location and predicting hydrology of wetlands.

Technical Abstract: The best management of wetlands and associated ecosystem services requires accurate and up to date knowledge of wetland location and character. Unfortunately even the most accurate US wetland maps contain relatively high levels of error in areas that are difficult to map, such as forests. Furthermore, the mapping process used to create finer scale wetland maps relies on manual interpretation which is time and resource intensive, as well as highly dependent on the skill of the photoanalyst. Recently developed remote sensing technologies and techniques have the potential to improve the detail and reliability of wetland maps and the ability to characterize wetland condition and function in an automated environment. We investigated the accuracy of forested wetland maps produced using digital elevation models (DEMs) derived from Light Detection and Ranging (LiDAR) data and multiple topographic metrics, including multiple topographic wetness indices (TWIs), a FD8 TWI enhanced to better incorporate information on the presence or absence of water outlets, normalized relief, and hybrid TWI/relief in the Coastal Plain of Maryland. LiDAR based mapping products were compared to highly accurate maps of inundation and the most up to date and accurate wetland map available. The TWI based on FD8 flow routing was better correlated with inundation than the TWIs based on D8 and D8. The enhanced FD8 TWI mapped inundation over approximately 65% of the study area with an accuracy of 95% and the hybrid product mapped inundation throughout the entire study area with an accuracy of 70%. Mapping methods based on topographic metrics contained fewer errors of omission than the currently available aerial photograph based wetland map, included similar areas of flooded forest, and were significantly correlated with hydroperiod. These results indicate that LiDAR based topographic indices have the potential to improve the accuracy and automation of fine scale wetland mapping.