Citation:

Leibowitz, S., M. Rains, I. Creed, D. Aldred, R. Hill, M. Weber, AND C. Jones. Wetland Hydrological Connectivity: A Classification Approach and Continental Assessment. 10th Intercol International Wetlands Conference, Changshu, CHINA, September 19 - 24, 2016.

Impact/Purpose:

A recent EPA report that reviewed more than 1,200 peer-reviewed papers concluded that non-floodplain wetlands – including the vast majority of geographically isolated wetlands (GIWs), or wetlands surrounded completely by uplands – 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 (overland, shallow groundwater, or deep groundwater connectivity), magnitude, and frequency of hydrologic connectivity between GIWs and downstream waters. The classification also includes an index that assesses relative level of impact 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 contiguous United States. This is done using the 2.6 million stream catchments (i.e., local drainages to stream reaches) in the National Hydrography Dataset Plus (NHDPlus), along with information from EPA’s StreamCat dataset. We also apply the classification to the entire North American continent, using national and global geospatial datasets. We use these maps to assess patterns in GIW connectivity across the US and North America. 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 biota flows are often associated with water flow. 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 larger landscape. Specifically, 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). Here we focus on so-called “geographically isolated wetlands” (GIWs), or wetlands that are completely surrounded by uplands. While these wetlands normally lack surface water connections, they can be hydrologically connected to downstream waters through intermittent surface flow or groundwater. To help quantify connectivity of GIWs with downstream waters, we developed a system to classify GIWs based on type, magnitude, and frequency of hydrologic connectivity. We determine type (overland, shallow groundwater, or deep groundwater connectivity) by considering soil and bedrock permeability. For magnitude, we developed indices to represent travel time based on Manning’s kinematic and Darcy’s equations. Frequency is determined based on recurrence intervals of storm events sufficient to saturate soils. We also include an index that assesses relative level of impact to connectivity, e.g., presence of canals and ditches, impervious surfaces, and others. The classification system is designed so that it can be applied at various scales using either geospatial or field data. Here we illustrate the classification system by applying it to the contiguous United States, using the 2.6 million stream catchments (i.e., local drainages to stream reaches) in the National Hydrography Dataset Plus (NHDPlus), along with nationally available geospatial data. We also apply it to the entire North American continent, using national and global geospatial datasets. We use these maps to assess patterns in GIW connectivity across the US and North America.

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