Watershed or County Boundaries?

Life and geophysical scientists like me know that natural systems rarely follow geopolitical boundaries, such as states and counties. Knowing how many square miles of wetlands or forest are present and have been lost in a particular county, for example, does not help us readily manage for the sustainability of the entire, connected wetland or forest system. And how we manage a connected system can have significant impacts on the species that utilize it, as well as the provision of ecosystem services that humans benefit from. So, how we choose to aggregate land cover information is an important consideration when making management decisions related to natural systems. And since coastal and riverine habitats are driven (primarily) by hydrology, using a nationally-standardized watershed network to map natural systems just makes sense (at least to me!).

Extent of wetland losses (red) and gains (green) for the Eastern Louisiana Coastal Watershed (yellow boundary) from NOAA’s Land Cover Atlas.

You can see an example of the difference between a county aggregate and a watershed aggregate of wetlands and wetland changes in the images to the right. The top one shows the Eastern Louisiana Coastal Watershed, which has suffered significant wetland loses (shown in red). Examining by county, as in the bottom image of St. Bernard Parish, you’ll notice that not all the wetland losses for the watershed are within the same county, and you might come to different conclusions when considering wetland processes and dependencies. Here I used the NOAA Land Cover Atlas to get both change summaries and a spatial view of the changes, since it’s also helpful to see the distribution of wetlands and changes in a watershed (or county) when attempting to analyze the information. Knowing and visualizing the change-trends can inform the exploration of different management scenarios, or suggest research hypothesis to fully understand cause and effect of habitat changes.

Extent of wetland losses (red) and gains (green) for St. Bernard Parish in Louisiana (yellow boundary) from NOAA’s Land Cover Atlas. Notice how the wetland system is disected, and not fully accounted for, when selecting to view wetland changes by county, instead of watershed.

Watershed planners, conservation practitioners, and scientists are always in need of tools that help us with proposal writing and site-specific project design by giving us an initial understanding of the landscape; and we’re better able to do that when they follow natural systems. I am reminded of all the conservation and restoration proposals I have reviewed for NOAA funding over the years, usually for conservation of a specific habitat type in a watershed, or for an entire watershed if the goal is ecosystem-based watershed planning. The background section of these proposals always sets the context for the watershed, drivers, and conservation need – how big the watershed is; acres of habitats; losses in those habitats over time; and distributions of land uses in the landscape that help tell the story of those habitat trends, such as amounts of agricultural lands and impervious surfaces. Tools that help us look at the change-trends in land cover types over time (increases and decreases), as well as the proximity of habitats to other land cover types, helps us glean the potential causes of the landscape changes and thus possible management actions to explore.

All of this information can be extracted from the Land Cover Atlas, even if you are not a GIS wiz. The watersheds boundaries in the Land Cover Atlas are a recent addition and come from USGS’ national Watershed Boundary Dataset. I hope the next proposal I review will have taken advantage of the available tools and provide good background information on habitat changes by watershed to support their conservation solutions.