Smithsonian researchers report that by accounting for the arrangement of land use in a watershed—not just the percentage of developed or agricultural land—they can better predict ecological conditions within the water. Their new study could have broad implications for land- use managers faced with deciding where to restrict land-use activities or where to apply better land management practices, such as protecting or restoring forested or wetland buffer zones near streams.
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The report, authored by scientists at the Smithsonian Environmental Research Center (SERC) in Edgewater, Md., is published in the November issue of Landscape Ecology.
Using publicly available geographic data from U.S. Environmental Protection Agency and U.S. Geological Survey, the scientists studied 503 watersheds within the Chesapeake Bay drainage basin, analyzing elevation, stream channels and land cover. By incorporating an understanding of landscape function in their assessment, the researchers' work offers improved methods for assessing watershed impacts on aquatic systems and provides potentially powerful tools for restoration and conservation management.
According to the new study, traditional land-use management strategies may be based on oversimplified descriptions of land use arrangement. It's long been known that forested areas can act like sponges, soaking up much of the nutrients from runoff passing through them on the way into a stream or lake, while unforested areas along a waterway are like big leaks that enable runoff from the land into the water.
Current models designed to assess the impact of land use and the effectiveness of these forest sponges (known as riparian buffer zones) have been based on comparisons between the amount of nutrient discharge into a watershed system and the percentage of riparian buffer within a certain distance from streams in the system. This type of analysis doesn't consider whether any of the riparian area is actually functioning as a buffer. The Smithsonian scientists offer an alternative that provides a much more useful picture of how the features of the landscape and land-use patterns interact.
"To function as a buffer for material discharge, a riparian system must be in between the stream and some nutrient source area, such as agricultural lands. There is no guarantee that all forests within a fixed distance of streams function as riparian buffers or that all riparian filters fall within any specified distance of streams," said Matthew Baker, the study's lead author and SERC alumnus (now at Utah State University).
The new methods developed by Baker and SERC researchers Donald Weller and Thomas Jordan account for only buffer zones with the potential to filter materials flowing from uphill sources. Additionally, they factor in such things as the distance of various types of land use to the water, the effect of topography on the direction of runoff from disturbed areas, and the amount of forest along flow paths between nutrient source areas and streams.
"The simple proportions of disturbed land cover types, such as cropland or developed land in a watershed, can be good predictors of aquatic conditions," Weller said. "But focusing on source areas and how things are connected to the stream tells us more of what we want to know."
The study provides methods for managers and policy makers to better understand how land use such as development or agriculture might impact a watershed. And, according to Jordan, "The new analysis could help pinpoint the locations of nutrient leaks in riparian zones, showing where restoration of riparian buffers would be most effective in intercepting nutrients before they reach streams."