LCMAP Offers Insight on Dynamic Wetlands

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Wetlands are dynamic in nature, growing and shrinking within and between years in ways far less predictable than croplands, forests, or established urban areas.

Map comparing current wetlands extent to extent defined by National Wetlands Inventory

Graphic highlighting expansion of wetlands in east-central North Dakota. The yellow, magenta, and cyan borders show wetlands area defined by the U.S. Fish and Wildlife Service's National Wetlands Inventory (NWI).

Animation showing wetlands change over east-central North Dakota

This Landsat-based animation depicts 33 years of landscape change in east-central North Dakota. The inset map comes from the U.S. Fish and Wildlife Service's National Wetlands Inventory (NWI).

The tools used to study U.S. wetlands on a broad scale often lack the kind of timely detail needed to truly understand what’s changing, when, and by how much. Land and wildlife managers or researchers who work in wetlands tend to start with hydrology datasets akin to snapshots in time, sometimes decades old and wildly out of sync with current conditions.

The Land Change Monitoring, Assessment, and Projection (LCMAP) Initiative at the USGS Earth Resources Observation and Science (EROS) Center promises satellite-derived data with the capacity to fill in critical gaps in those datasets.

LCMAP leans on the deep Landsat archive to characterize Earth surface change across the conterminous United States on an annual basis, from 1985 through the present, for every 30-by-30-meter plot of land in the country.

For wetland ecosystems, these data amount to an unparalleled historical record of landscape conditions, responsive to and representative of the seasonal and long-term fluctuations that pass unrecorded in typical wetland maps.

Animation of land cover change in east-central North Dakota

Animation of Primary Land Cover for LCMAP tile h15v04, over east central North Dakota.

The LCMAP applications team recently explored the possibility of merging Version 1 prototype data with three simplified wetland classes pulled from one of the nation’s most widely-cited hydrology datasets, the U.S. Fish and Wildlife Service’s National Wetlands Inventory (NWI).

The NWI offers detailed information on wetland condition across the United States, but its production is labor-intensive, requiring high-resolution ground-based photography and interpretive expertise. Updates come slowly as a result, trickling into the dataset region by region. Some areas haven’t been updated since the 1970s.

One such area is east-central North Dakota, a region that’s changed considerably over the past four decades thanks to a significant uptick in moisture since the 1980s. Recently-collected aerial photography data clearly shows lakes and ponds expanding beyond NWI borders, but photos alone don’t tell the full story of change.

To learn how LCMAP data might improve wetland mapping for the region, the LCMAP team overlaid its Land Cover and Change Day of Year data products over areas tagged by the NWI as lakes, freshwater ponds and freshwater emergent wetland classes.

The team found that plots classified as “lake” by LCMAP’s land cover product aligned closely with the permanent lakes and ponds in the NWI, but LCMAP noted overall gains in wetland and water classes. Expansions and multiple changes to NWI’s “emergent” wetlands were also common, such as short-term transitions from grass/shrub to wetland or wetland to open water and back again.

LCMAP data therefore offers a new source of information to update NWI in areas with outdated maps.

But LCMAP looks beyond land cover class changes. Many of the impermanent changes were captured by the Change Day of Year product. Most of the NWI “emergent” wetlands saw between one and four changes over the 33-year study period.

The Applications team used the Change Day product not only to make note of those changes, but to place them in a seasonal context by tagging if the changes occurred in spring, summer or fall.

Knowing when a wetland area is most likely to change can be especially useful for wildlife managers who aim to protect specific species and for understanding the types of wetland habitat available.

Shorebirds, for example, need exposed, bare shorelines at certain times of the year. Knowing which areas with cyclical change patterns are most likely to dry out or remain stable at those times might help guide shorebird-focused decisions on wetland management.

USGS LCMAP data overlay on east-central North Dakota wetlands

The USGS Land Change Monitoring, Assessment, and Projection (LCMAP) Version 1 Time of Spectral Change product shows when changes occur in wetlands. In this example shows change in east-central North Dakota.