Look at the section entitled, “What Kinds of Scientists Did
This Research?” What do you notice about the kinds of
scientists who are involved with this study? You probably noticed that three different kinds of scientists worked together to study drinking water watersheds in the United States. You might have also noticed that two of the scientists study relationships. When different kinds of scientists come together to conduct a study, each scientist brings a different perspective, a different kind of knowledge, and perhaps different scientific methods. These differences strengthen the study. One person may see or understand things that another person may not.

A watershed is an area of land where all the water that is underground within the area and all the water that drains off of its surface goes to the same place (figure 1).

Watersheds contain streams or rivers that carry surface water toward the oceans, and they may contain lakes or reservoirs (re zə vwärz) (figures 2a and 2b).

Watersheds hold groundwater, which also flows into streams and rivers. In the United States, the largest watersheds are defined by the Continental Divide (figure 3).

One of these large watersheds drains into the Pacific Ocean. The other large watershed drains into the Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. Smaller watersheds are contained within each larger watershed in a nesting pattern (figure 4).

Scientists estimate that 80 percent of all U.S. freshwater resources begin in forests. Approximately 60 million U.S. citizens rely directly on national forests for their water. About two-thirds of the U.S. population use drinking water from surface sources. Surface sources of drinking water include streams, rivers, lakes, and reservoirs (figure 5). People who manage drinking water sources have recognized the relationship between conservation of natural land and improved drinking water quality. When land is covered with vegetation, surface water is cleaner. Cleaner water means lower water treatment costs and safer drinking water.

Managers take a number of actions to protect drinking water. Starting in 1974, with the passage of the Safe Drinking Water Act, managers focused on detecting sources of water pollution. By 1996, people had begun to think differently about how to provide clean drinking water. People realized that protecting surface source water was more effective and efficient than just detecting pollution. In 1996, amendments to the Safe Drinking Water Act shifted the focus from detection of pollution to protection of source water. A big part of protection is managing drinking water watersheds so that the surface water is clean. This kind of management means that the land is purposely managed to protect surface sources of drinking water.

The 1996 Safe Drinking Water Act amendments also required each State to assess the condition of its source water. These assessments are important, as they alert managers to the condition of drinking water watersheds. If a watershed’s natural condition becomes degraded or the land is developed for other uses, for example, managers can address the problem to protect water quality. Unfortunately, State-by-State assessments do not provide a regional or national understanding of drinking water watersheds. Each State might do its assessment a little differently. Watersheds often cross State boundaries, and watersheds are managed across State boundaries (figure 6). The scientists in this study, therefore, were interested in conducting a national assessment of drinking water watersheds that crossed State boundaries.

First, the scientists located all surface drinking water intakes. A surface drinking water intake is a pipe in lakes, streams, rivers, or reservoirs that takes water from the source and carries it to a water treatment plant (figures 8, 9a, 9b, and 9c). The scientists found 5,265 of these intakes across the United States.

The scientists then used maps to trace backwards from the water intakes. Tracing backwards from the intakes enabled scientists to identify the watershed for each intake. Each watershed included all the land area supplying water to the intake. Then, the scientists identified in which of the 21 hydrologic regions of the United States each watershed belonged (see figure 6).

The scientists needed more information to do their analysis. The scientists used information from an existing national database on land cover. This database is made up of satellite photographs taken of Earth’s surface (figures 10 and 11). Land cover is the physical cover at Earth’s surface. Examples of land cover include forest, grass, asphalt, water, and buildings. Some of these land covers are classified as urban, such as buildings. Some of these land covers are classified as natural, such as forests.

Using this database of photographs, the scientists noted changes between 1992 and 2001 in land cover for areas identified as urban, natural, agricultural, and water. They identified, for example, whether the amount of urban land expanded or got smaller between 1992 and 2001 for each of the drinking water watersheds.

Next, the scientists looked at another database called the Protected Areas Database of the United States. This database identifies land that is protected from development. Development is what happens when land is converted from natural to urban or agricultural uses. The Protected Areas Database identifies public land and conservation land. The scientists figured out how much land in each of the drinking water watersheds was protected. Land that is protected is less likely to be developed into roads and buildings, or used for agriculture.

Finally, the scientists put all of this information together into a computer. For each of the drinking water watersheds, the scientists figured out how much of the land was urban, natural, agricultural, or covered in water. They figured out percentages for 1992 and 2001. By looking at data for 1992 and 2001, the scientists figured out if each watershed was becoming more urban, more agricultural, more natural, or
had not changed. Then, the scientists calculated the percentage of land in each watershed that was protected from further urban and agricultural growth.

Most major U.S. rivers are used to supply drinking water to Americans. The scientists discovered that about two-thirds of surface drinking water intakes are in rivers. One-third of the intakes are in lakes and reservoirs. Nationwide, most of the land in drinking water watersheds is covered with plants and trees. The scientists found that the median watershed percentage of natural land cover nationwide was 77.1 percent (table 1).

The scientists also discovered that about 3 percent of the land in U.S. drinking water watersheds was protected from development. The Western United States, however, has a higher percentage of protected land in drinking water watersheds. This higher percentage is due to the large amount of public land in the Western United States (figure 14). Public land is generally kept in a natural condition with limited development.

Between 1992 and 2001, land cover changed in some of the drinking water watersheds. About 23 percent of the watersheds lost at least 1 percent of their natural vegetation. About 5 percent of the watersheds gained at least 1 percent in natural vegetation. In the Southeast and Ohio hydrologic regions (see figure 6), 42 percent of the watersheds lost at least 1 percent of their natural vegetation.

About 9 percent of the drinking water watersheds had at least a 1-percent increase in urban land between 1992 and 2001. Most of this increase was in the Eastern United States. Three-fourths of all watersheds nationwide, however, increased in urban land by some percentage between 1992 and 2001.

The scientists found that drinking water watersheds are mostly covered with natural vegetation. In some areas of the United States, however, watersheds contain a high percentage of urban land. Twenty percent of the Nation’s
drinking water watersheds have less than 50 percent natural vegetation. In addition, 8 percent of the Nation’s drinking water watersheds contain at least 20 percent urban land. Over time, drinking water watersheds are losing natural vegetation and are becoming more urbanized. These trends indicate that many surface sources of drinking water may become more exposed to pollutants.

This study shows the importance of taking a regional and national look at U.S. drinking water supply. U.S. population increases will put more land use demands on drinking water
watersheds. As more land becomes urbanized, drinking water source protection will likely become more difficult unless the percentage of protected lands increases as well.