Thursday, August 26, 2010

Ecological rules of thumb and the 10% impervious rule for watersheds

Conservation planners love general rules of thumb. Rules of thumb are important because they allow us to make decisions without repeating exhaustive ecological studies in every location. By learning from studies conducted in similar ecological systems, we can apply good science while saving time and money. Increasingly, these rules of thumb come in the form of “thresholds”, or points along an environmental gradient (usually anthropogenic or human influenced) where biotic communities shift significantly or experience marked declines (as in diversity or biotic integrity).

One rule of thumb that is well established within the freshwater conservation community is the rule of <10% impervious surface. Several studies in the 1990s and early 2000s showed that biological communities generally began to significantly degrade as a watershed reached approximately 10% impervious. [Note: Impervious means land cover types that do not allow rainfall to infiltrate into the ground, like parking lots, buildings or roads. Such surfaces lead to increased runoff and lower groundwater contributions, leading to increases in both flood and drought conditions.] River conservation groups quickly latched on to these studies to demonstrate the importance of maintaining watershed land cover at <10% impervious or for limiting additional impervious land cover within watersheds that already exceeded 10%.

I believe the 10% rule has sometimes been misapplied in watersheds with low development (much less than 10% impervious). In these watersheds, we should try to provide a buffer (as much as is practical) to avoid significant biotic degradation. This is particularly important when a watershed represents the highest quality example of a particular stream type. But in general, the 10% rule has proven to be a powerful, scientifically-based rule of thumb.

But several recent studies have begun to punch significant holes in the 10% rule. Most recently, a paper by Robert Hilderbrand and colleagues looked at losses of benthic macroinvertebrate taxa with impervious surface percentage in Coastal Plain and Piedmont streams in the Chesapeake Bay watershed in Maryland. They found that about 10% of taxa were already lost from Coastal Plain streams by the time 10% of the watershed was impervious (~4% lost with 5% impervious). However, in Piedmont streams, nearly 40% of taxa were already lost with 10% impervious (nearly 20% lost with 5% impervious). This would indicate that the 10% rule would be entirely inadequate for conserving Piedmont streams. Even for Coastal Plain streams, it doesn’t appear to be ideal.

It is likely that the 10% rule is sufficient in many watershed types, but this and other recent studies provide reason for caution. In particular, we should find studies in watersheds with similar physical and chemical conditions, before applying the 10% rule. Additional studies should be conducted comparing across watershed types. Perhaps for some watersheds, 15% is adequate. But whenever possible, we should look for opportunities for maintaining impervious conditions well below 10%. This will provide a buffer for quality biotic communities to persist in the face of future perturbations—say, if the climate was to become significantly warmer and rainfall patterns were to change significantly over an unusually short period of time.
High density of impervious surface along the Potomac River in Washington D.C. across from beautiful Arlington Cemetary. 

Booth, D. B. and R. Jackson. 1997. Urbanization of aquatic systems: Degradation thresholds, stormwater detection, and the limits of mitigation. Journal of the American Water Resources Association 33: 1077–1090.

Hilderbrand, R.H., R.M. Utz, S.A. Stranko, and R.L. Raesly. 2010. Applying thresholds to forecast potential biodiversity loss from human development. Journal of the North American Benthological Society 29:1009-1016.

Schiff, R., AND G. Benoit. 2007. Effects of impervious cover at multiple spatial scales on coastal watershed streams. Journal of the American Water Resources Association 43:712–730.

Schueler, T.R., L. Fraley-McNeal, AND K. Cappiella. 2009. Is impervious cover still important? Review of recent research. Journal of Hydrologic Engineering 14:309–315.

Wang, L. Z., J. Lyons, and P. Kanehl. 2003. Impacts of urban land cover on trout streams in Wisconsin and Minnesota. Transactions of the American Fisheries Society 132: 825–839


  1. Excellent discussion. My understanding of the 10% threshold was that it was originally almost picked out of the air and the author of the original paper had no intention of setting it as a real value. it only makes sense that each area would be different. Problem is that regulators and land managment agencies like a standard value they can apply to everything. We're using it as a threshold for a cumulative impacts analysis in watersheds in Montana. It works, but is it scientifically sound? Maybe, maybe not.

  2. Thanks for the comment Demian! I hadn’t heard the story about 10% being pulled out of thin air, but don’t doubt it. There were, however, several studies that came up with numbers in the 10% range to support it. This is from David Allan’s “Landscapes and Riverscapes" paper (Allan 2004):

    “A marked decline in species diversity and IBIs with increasing urbanization has been reported from streams in Wisconsin (around 8%–12% IA, Stepenuck et al. 2002, Wang et al. 2000), Delaware (8%–15% IA, Paul & Meyer 2001), Maryland (greater than 12% IA, Klein 1979), and Georgia (15% urban land, Roy et al. 2003). Additional studies (reviewed in Paul & Meyer 2001, Stepenuck et al. 2002) provide evidence of marked changes in discharge, bank and channel erosion, and biotic condition at greater than 10% imperviousness.”

    But I think you are right that every watershed is different, the question is “how different?” I think the 10% rule assumed that the differences weren’t that drastic. But of course it looks like there may be some drastic differences. I think the science—and therefore the application—on this will be iterative, starting out with a general 10% rule, then to estimates for different watershed types, and finally to models to provide us with watershed-specific estimates of an impervious threshold (probably multiple thresholds in fact). But we’re a long way from watershed-specific models and without models, the only way to come up with a watershed-specific number is to measure it as the impervious surfaces accumulate. Of course, that would defeat the purpose.

    But I think you are right on that that is the direction that we need to be thinking.

    Allan, J.D. 2004. Landscapes and riverscapes: the influence of land use on stream ecosystems. Annual Review of Ecology and Systematics 35:257-284.