Wetland Hydrologic Connectivity to Downstream Waters: A Classification Approach and National Assessment
Citation:
Leibowitz, Scott G, Ryan A Hill, Marc H Weber, C. Jones, M. Rains, I. Creed, AND J. Christensen. Wetland Hydrologic Connectivity to Downstream Waters: A Classification Approach and National Assessment. 2017 American Geophysical Union Fall Meeting, New Orleans, LA, December 11 - 15, 2017.
Impact/Purpose:
A 2015 EPA report that reviewed more than 1,200 peer-reviewed papers concluded that non-floodplain wetlands occur along a continuum of connectivity, with a great deal of spatial heterogeneity and temporal variability. A lack of knowledge makes generalizations about connectivity of these wetlands difficult. The report concluded that additional research focused on the frequency, magnitude, timing, duration, and rate of change of fluxes from non-floodplain wetlands to downgradient waters is needed. Given this, we have developed a method to classify the type (riparian, non-riparian surface, and non-riparian subsurface), magnitude, and frequency of hydrologic connectivity between wetlands and downstream waters. The classification also includes an index that assesses relative level of anthropogenic impacts to connectivity. The classification system was designed so that it can be applied at various scales using either geospatial or field data. Here we illustrate the classification system at a national scale by applying it to the conterminous United State, using the National Land Cover Dataset and other nationally available geospatial data. We use these maps to assess patterns in wetland connectivity across the conterminous US. The maps and methods developed herein will ultimately be of use to the Wetlands Division of the Office of Water, and will be published under SSWR 3.01G (Validation of Connectivity Indicators).
Description:
Connectivity has become a major focus of hydrological and ecological studies. Connectivity influences fluxes between landscape elements, while isolation reduces flows between elements. Thus connectivity can be an important characteristic controlling ecosystem services. Hydrologic connectivity is particularly significant, since movement of chemical constituents and biotic flows are often associated with water movement. While wetlands have many important on-site functions, the degree to which they are connected to other ecosystems is a controlling influence on the effect these waters have on the watershed. Wetlands with high connectivity can serve as sources (e.g., net exporters of dissolved carbon), while those with low connectivity can function as sinks (e.g., net importers of suspended sediments). We developed a system to classify wetlands based on type, magnitude, and frequency of hydrologic connectivity with downstream waters. We determined type (riparian, non-riparian surface, and non-riparian subsurface) by considering soil and bedrock permeability. For magnitude, we developed indices to represent travel time based on Manning’s kinematic and Darcy’s equations. Soil drainage class was used as an indicator of frequency. We also included an index that assesses relative level of anthropogenic impacts to connectivity, e.g., presence of canals and ditches and impervious surfaces. The classification system was designed to be applied at various scales using geospatial or field data. We illustrated the classification system by applying it to 4.7 million wetlands in the conterminous United States, using the National Land Cover Dataset and other nationally available geospatial data. We used the resulting maps to assess patterns in wetland connectivity across the conterminous US. While wetland connectivity was dominated by fast, frequent riparian connections nationally, it varied by region, exhibiting characteristic distributions of connectivity. Describing these distributions should promote better management of watershed functions such as flood control and water quality improvement.