University Park, Pa. -- Penn State researchers recently concluded a two-year mapping project detailing the extent of impervious surface area (ISA) for the state of Pennsylvania. Pennsylvania is the first state to have mapped ISA available at the state scale.
Project collaborators, Dr. Toby Carlson, professor of meteorology, Eric Warner, research associate with the Penn State Institutes of the Environment, and Deborah Slawson, a planning consultant for Adams and York counties, mapped the ISA, using a highly unique method developed by Carlson that determines ISA based on satellite measurements and derived fractional vegetative cover maps. The team completed two sets of maps: the first based on 1985 data and the second based on 1995 data. Maps based on 2000 data currently are being developed. Digital map information for ISA and fractional vegetation is available on the Pennsylvania Spatial Data Access (PASDA) web site (www.pasda.psu.edu/access/newdata.shtml).
"ISA is a term that refers to artificial surface cover, such as roads, buildings, sidewalks, and parking lots, through which water can not evaporate to the atmosphere or infiltrate into the soil," explains Carlson. Rainfall cannot infiltrate into the soil; therefore, it is converted quickly into storm water and channeled into rivers, streams, or other waterways, serving as a major conduit for pollutants.
"ISA has widespread environmental implications, some of which are just beginning to be understood says," Carlson. "ISA has the potential to affect water quality, surface hydrology, and surface microclimate. The amount of ISA lends itself to a quantitative assessment of urban sprawl. Knowing the location and extent of ISA in a watershed could aid in predicting urban flooding and indicate where non-point sources of pollution might be affecting the water supply."
Diffuse sources of pollution associated with peak storm runoff over impervious surfaces are the second leading cause of water pollution in the United States and threaten the quality of wetlands as well as the efficacy of existing clean water regulations.
Increases in ISA can affect water resources in many ways. Carlson reports, "Both the quantity and timing of runoff, as well as the chemistry of non-point source pollution constituents in stream discharge, are affected by changes in ISA, which facilitates rapid flushing of surfaces such as parking lots and roads. An increase in ISA increases direct runoff but decreases the amount of ground water recharge.
"There is a reasonably direct correlation between the amount of impervious surface area and amount of surface runoff - there is also an inverse relationship between the percentage of impervious surface area in a watershed and the viability of the streams in the watershed and the overall health of the watershed - so for planning purposes it is important to know the extent of ISA in a watershed," says Carlson.
"It is quite unique to measure ISA from a satellite," states Carson. "Our first step was to estimate fractional vegetation cover based on solar reflectance in both the visible and near infared part of the spectrum from Thematic Mapper (TM) data that was resampled to 25 meter pixels (approximately 82 square feet). The next step was image classification. Each pixel was classed as urban, woodland, short vegetation/bare, or water. The final step was to use algorithms to calculate ISA from the satellite-derived fractional vegetation cover and classification images."
The theory behind the calculation of ISA from satellite measurements is based on observations that in urban areas impervious surface area is inversely related to vegetation cover. Previous research has shown that non-vegetated surface areas in urban or residential areas are almost entirely impervious.
Since regional land cover changes brought about by human activity tend to occur incrementally, communities often do not realize the extent of their development and therefore, the possible changes in their environment. In this regard, satellite data hold great potential for practical application to regional planning and urban ecology because mapped ISA patterns can provide snapshots of urbanization and the human use of the landscape, and allow one to view the changing panorama over period of decades.
The Pennsylvania Department of Transportation sponsored the project as part of their ongoing effort to examine the regional impacts of highway construction.
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Contacts:
Patricia Craig (814) 863-0037 plc103@psu.edu
Vicki Fong (814) 865-9481 vfong@psu.edu