Measuring Wetlands

As part of a volunteer team that cleared a walking trail in my neighborhood, I helped to shovel a straight path through a small berm. Seeking to solve another problem at the same time, a member of the party started to use the dirt he removed to fill in a nearby puddle on the trail. After the first shovel-full, someone said, “You’re spoiling a wetland,” to which the shoveler replied, “Not so: the soil’s Munsell chroma is greater than 2.0, so we’re safe from the law.”

This kind of distinction is serious business in the building industry, which is increasingly finding a shortage of land and opposition from people who want to protect the vanishing wetlands and their wildlife species. In fact, the United States (Section 404 of the Clean Water Act) forbids the filling in of wetlands, especially new construction on them. Various states have implemented wetland regulations of their own.

To be classified as a wetland, a piece of property need not be wet at the moment, but only on average over time. As evidence for time-averaged immersion in water, one criterion for a wetland is for the soil to have a Munsell chroma less than 2.0, which indicates the soil has been underwater for so long that it fails to oxidize—and acquire colorfulness—from exposure to the air. Soil experts use the Munsell soil color chart to determine the Munsell chroma for this assessment. (See the references below.)

The Munsell soil color chart is related to the Munsell Book of Color, an atlas of color designed by Albert Munsell (1858–1918), a painter and color scientist. The atlas, a set of colored reflecting samples pasted into the book, is organized around three basic attributes of color: hue, associated with dominant wavelength; value, associated with lightness; and chroma, associated with colorfulness or departure from neutral gray. On any page of the atlas, the perceived color differences between adjacent samples are designed to be equal to each other.

After the standardization body Commission Internationale de l’Éclairage (International Commission on Illumination) developed its 1931 standard observer and standard illuminants, the Munsell color atlas underwent a renotation, which assigned specific color coordinates to each Munsell notation. Implicit in this renotation was a specific illuminant (standard illuminant C) under which the patch and its real-world counterpart were to be viewed.

When you’re in the field comparing soil samples with the Munsell charts, you must deal with several problems. First, field conditions make it hard to keep the charts clean for accurate matches. Secondly, the matches that are made depend on the illuminant, and the spectrum of daylight can change from moment to moment, as clouds pass or the sun sets. Thirdly, the soils themselves are often heterogeneous enough to make matches with a color chart very difficult. These problems are bound to result in ambiguous and contradictory assessments of whether a particular soil’s chroma is greater than 2.0.

Perhaps future soil experts will use an inexpensive and accurate portable spectrophotometer held near the soil sample. In that case, the light source would be constant, the instrument aperture would average out the soil granularity that is distracting to human vision, and the light source spectrum and geometry—a controlled property of the spectrophotometer—would be held constant for all observations. The measurement would be objective. You could even assess the chroma threshold inside the instrument, avoiding the need for Munsell charts or the need to keep them clean. Accuracy and reproducibility would, of course, be matters for further research.

Delineation of wetlands is a more complicated issue than just assessing soil color: A wetland must also pass the tests of specific vegetation and hydrology. However, soil color is an important component, and there will be increasing demand to be able to assess it more reproducibly.

By the way, for mosquito-phobic readers, I’m told that the Walter Reeds of the future will be able to get exemptions from the wetlands law if yellow fever or the like returns to plague us.

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