Developing a Hydrogeomorphic Wetland Inventory: Reclassifying National Wetlands Inventory Polygons in Geographic Information Systems

The National Wetlands Inventory (NWI) is the most extensive inventory of wetland resources in the U.S., but it has limited ability to contribute to characterizations of wetland functions. We provide a methodology for reclassifying NWI polygons into Hydrogeomorphic (HGM) classes to facilitate monitoring wetland functions. We conducted this reclassification using spatial and attribute queries within Geographic Information Systems (GIS) for wetlands in central Oklahoma. Once classified, 149 randomly selected wetlands in four HGM classes (depressional, lacustrine fringe, riverine, and impounded depressional) were field verified. The overall accuracy of the GIS classification was 60%. Inherent issues with NWI due to attribute accuracy, spatial accuracy, and map age accounted for >50% of the misclassified sites in this analysis. The results from this analysis were also used to provide an inventory of wetlands in each HGM class and subclass based on user’s accuracy metrics. Reclassifying NWI polygons into HGM classes can assist with determining the spatial distribution and relative abundance of specific wetland classes which allows for more focused wetland restoration and monitoring efforts. However, the error rate associated with reclassification should be calculated to ensure that incorrect conclusions are not drawn regarding the abundance of HGM wetland classes and their associated functions.