Impervious Surfaces

How much of the Piscataqua Region watershed is currently covered by impervious surfaces and how has it changed over time?

In 2015, 5.6% of the land area of the Piscataqua Region watershed was covered by impervious surfaces. This is an increase of 1,257 acres of impervious cover or 0.2% of the land area since 2010.


Impervious surfaces are man-made features, such as parking lots, roads, and buildings, that do not allow precipitation to infiltrate into the ground. When precipitation falls on impervious surfaces, it runs off those surfaces carrying pollutants and sediments into nearby waterways. Watersheds reach a tipping point around 10% impervious cover19, beyond which water quality impacts become increasingly severe.
No increase in the number of watersheds and towns with greater than 10% impervious cover and no decrease in the number of watersheds and towns with less than 5% impervious cover.

The 2015 update to this dataset represents a new, improved baseline for impervious surface across the region due to the use of higher resolution imagery and different processing methodology. Impervious surface values reported in the 2013 State of Our Estuaries report using 30-meter satellite imagery (63,214 acres) were greater than those reported using the improved and more accurate 1-foot orthoimagery (45,377 acres) in this report. In 2015, 46,634 acres (5.6% of the land area) of impervious surface were mapped representing an increase of 1,257 acres (0.2% of the land area) since 2010 (45,377 acres).

Watersheds with greater than 10% impervious surface coverage of land area are around the Hampton-Seabrook estuary, the Piscataqua River and the Route 16 corridor along the Cocheco River. Impervious surfaces in 2015 in each of the Piscataqua Region subwatersheds are shown as a percentage of land area in Figure 1.1.

Communities with the highest reported impervious surface percentages were found in Portsmouth (26.7%), New Castle (20%), and Seabrook (20%), while the largest increase of impervious surfaces between 2010 and 2015 occurred in Rochester (122 acres), Wells (64 acres), Seabrook (64 acres), Dover (56 acres), York (42 acres), and Sanford (39 acres). Communities with the smallest increases in impervious surfaces occurred in Madbury (4 acres), New Castle (2 acres), and Brookfield (2 acres). Small increases in impervious surfaces may be a result of limited availability of buildable lots. Town-by-town information on impervious surfaces in 2015 is shown in Figure 1.2.

Between 2010 and 2015 population in the Piscataqua Region watershed increased 6% (21,760 people), and impervious surfaces increased 2.7% (1,257 acres). For every one person increase in population, impervious surface increased .06 acres. However, as shown in Figures 1.1 and 1.2, the amount of impervious cover is not evenly spread across the watershed. For more discussion on population and housing trends in the watershed refer to Housing Permit Approvals.

Figure 1.1 Percent impervious cover by subwatershed (HUC-12) as of 2015.

Figure 1.2 Percent impervious cover by town as of 2015.