Urbanized Watersheds More Sensitive To Climate Change

December 20, 1996

SAN FRANCISCO, Calif. -- A regional investigation of stream flow in urbanized areas could help planners, hydrologists and climate modelers decipher future effects of increased precipitation and temperatures, according to a Penn State researcher.

"There have been a great many studies of stream flow, temperature and precipitation, but they have all been done on undeveloped watersheds that are minimally impacted by humans," says Dr. David De Walle, professor of forest hydrology. "No one, to our knowledge, has formally looked at urban watersheds in this way."

De Walle is trying to see how urbanization affects sensitivity of stream flow to climate change. Using population data from the U.S. Census Bureau, precipitation information from the National Weather Service and stream flow data from the U.S. Geological Survey, he is looking at four regions of the U.S. -- the Northeast, Southeast, Northwest and Southwest over the past 50 years.

In each area, De Walle is comparing five urban watersheds with five rural watersheds. Analysis of data for the northeastern sector is complete.

"It is obvious that stream flow increases with an increase in precipitation, but in the Northeast, we find that with a higher population and the same precipitation, stream flow increases more," De Walle told attendees at the American Geophysical Union Conference today (Dec. 19) in San Francisco.

The study looked at areas such as Saddle River, N.J., which in 1940 had 518 people per square mile and in 1980 had 2590 people per square mile. This area is 100 percent urbanized and had an increase of 70 percent in stream flow. Rock Creek, Md., which had 648 people per square mile in 1940 and 3238 per square mile in 1980, has a 43 percent increase in stream flow. The undeveloped area of Bushkill Creek in the Poconos had a population of 8 people per square mile in 1940 and in 1980 had 39 people per square mile.

"There were some small effects on stream flow in rural areas, but nowhere near what we see in urban areas," says De Walle. "In general, there is some decrease in stream flow with increase in temperature in urban areas, but in the Northeast, this does not seem terribly important."

Increases in stream flow boost the potential for flooding and in the Northeast at least, the areas of most increase -- the most developed -- are those areas that stand to lose the most in property and human life if flooding occurs.

"If the greenhouse effect causes global warming, everyone is pretty confident that temperatures will increase, but how much precipitation will change is hotly debated," says De Walle. "We are interested in the effects of precipitation because it is the least reliable of the two variables in the climate models."The global climate models can provide regional predictions, so De Walle will dedicate the final year of this two-year project, funded by the U.S. Environmental Protection Agency, to the other three regions and to comparing how climate change will effect stream flow in all four regions.
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Penn State

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