Green Infrastructure

REALITY CHECK FOR MAINTAINING THE NATURAL WATER BALANCE IN URBAN AREAS: About 5.5% of developed land in the continental United States is covered by impervious parking lots! – a research finding by US Geological Survey

Note to Reader:

Parking lots may be a significant source of pollution, but up until now there has been no quantitative estimate of the areal extent of parking lots in the U.S.

Estimating the presence of paved surface parking lots in the conterminous U.S. from land-use coefficients for 1974, 1982, 1992, 2002, and 2012

The United States Geological Survey (USGS) has estimated the impervious parking lot coverage for all 3,109 counties in the continental USA. The research study led by James Falcone indicates that about 5.5% of developed land in the 48 connected states is covered by impervious parking lots!

The study reports that there were more than 275 million registered motor vehicles in the U.S. in 2018. Accommodating that number of vehicles requires an enormous network of parking lots, the vast majority of which are made of impervious pavement that rainwater cannot infiltrate.

Until now, researchers have been unable to gauge the full extent of impervious parking lot coverage in a scientifically sound way.

In May, the USGS released a new model that uses land-use data to estimate the amount of land in the lower 48 U.S. states covered by impervious parking lots. Findings from the model could be valuable for urban planners and watershed managers as they plan new developments or retrofit existing areas where runoff pollution is a major issue, according to James Falcone of the USGS.

Proving the Model

Having a robust body of data from the six focus cities gave researchers the opportunity to test how well their model’s estimates aligned with real-world parking lot coverage. In all but one case, the model’s estimates were within 1% of actual coverage figures.

In the case of Hartford, Conn., however, data was available only for a 14-km²area, at most one-sixth the size of the next largest study area. The model’s estimates were inaccurate by about 5% in this case.

Researchers say the inconsistency underscores an important point about using the tool: While its estimates of impervious cover are useful for modeling large areas such as cities and watersheds, the data becomes far less useful for study areas smaller than 80 km².