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Puget Sound Basin NAWQA Study
U.S. Geological Survey
934 Broadway,
Suite 300
Tacoma, WA 98402

(253) 552-1600
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Puget Sound Basin NAWQA

Geographic Information Systems (GIS)

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To study the relationships between land use and water quality using geographic information systems (GIS). Our two main areas of activity are developing and analyzing spatial data.

Developing spatial data

Creating a land-use map from satellite and county data

We are creating a land-use map by combining tax assessor maps that show land parcels with Landsat satellite imagery. The parcel maps are good sources of information for differentiating between different urban land use types (such as residential, commercial, and industrial areas). Satellite imagery is a good source of information for differentiating between agricultural and forested areas. By combining the two sources of information, we are able to obtain a greater level of detail than we could from just using a single source. When a composite map is completed, it could be used to estimate areas that are contributing to nonpoint source pollution.

Using satellite imagery to create maps showing changes in land-use patterns

The Puget Sound NAWQA is using satellite data from 1972 and 1992 to map changes in urban, agricultural, and forest areas. Once the 1972 and 1992 data are classified, the two satellite images are compared to determine which land uses are being transformed into other land-use types. We will then use this information to link land-use and water quality trends in the basin. For example, we can use this satellite imagery to determine whether an increase in impervious surface causes an change in the intensity of surface water flows.

Analyzing spatial data

Estimating nonpoint source nutrient loads in study basins

The Puget Sound NAWQA is estimating the total nutrient loads to our study basins. Through improved land-use maps, we will refine estimates of nutrient loads presented in a previous NAWQA report by our surface water specialist, "Nutrient Transport in Rivers of the Puget Sound Basin, Washington, 1980-93" (in press). We will also use sample data collected from three basins to calibrate a nonpoint source model.

Estimating landscape fragmentation

We are testing different methods to measure landscape fragmentation. Statistical methods will be used to test whether fragmentation impacts water quality. Fractals and other landscape metrics are being tested to measure fragmentation at different landscape scales.

Estimating the probability of finding nitrate in wells

A groundwater specialist on the Puget Sound NAQWA study team developed a regression equation for estimating the likelihood of finding nitrates in groundwater at a given locations within the Puget Sound Basin. Erwin, M.L., and Tesoriero, A.J., 1997, Predicting ground-water vulnerability to nitrate in the Puget Sound Basin: U.S. Geological Survey Fact Sheet 061-97, 4 p. This equation was used as input to a geographic information system that calculated the probability of finding nitrates at a given location based on the land-use types around that location.

Information on Puget Sound Basin NAWQA Sampling Sites

For information about GIS in the Puget Sound Basin, please contact the Puget Sound Basin Study Unit Chief at (253) 552-1600, or email

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