Benefits of forested lands for the Nation's drinking water

Forested lands provide the cleanest and most stable water supply compared to other land types and make a disproportionate contribution to surface water supplies.  This study  quantified the contribution of National Forest System and other forested lands to Public Water Systems across the contiguous U.S., or conterminous U.S. (CONUS), while accounting for inter-basin transfers (IBTs), or the transfer of water from one geographically distinct area to another. These types of transfers can sometimes move water long distances (hundreds of kilometers or more) to serve large cities. Forest land managers and water utilities can use this information to prioritize forests and  sub-watersheds  (i.e., smaller watersheds delineated within larger watersheds) for fuels reduction and restoration treatments, watershed management, and other projects to protect or enhance surface water supplies.

This story map provides an overview of the aggregate contribution of National Forest System and other forested lands across the 48 contiguous states. We provided a list of linked  additional resources  at the end of this map.

The Forest Service currently manages 779,000 square kilometers (193 million acres) of public lands in 42 states and Puerto Rico (National Forest System). This study focused on the 151 national forests, 20 national grasslands, and 1 National Recreation Area within the contiguous U.S. Within the National Forest System, these units are managed by administrative units that contain one or more forests or grasslands. Only the land managed by the National Forest System within these units were considered in this study.

• A new database of these transfer systems was developed for this study that identified 594 IBTs transferring from 0.01 million cubic meters (2.64 million gallons) to 8,900 million cubic meters (2,351 billion gallons) per year, based on average transfer volumes from 2001 through 2015.
• In total these inter-basin transfers moved 116,894 million cubic meters (30,880 billion gallons) per year of water over a distance of 36,339 kilometers (22,580 miles).
• More than half of these transferred water from sub-watersheds where more than 50% of surface water originated on National Forest System and other forested lands, and most of those were in Western states.
• These inter-basin transfers supplied surface water to major urban areas. For example, 69% of the water transported to the greater Los Angeles area through inter-basin transfers originated on forested lands, and 82% of Phoenix’s imported water originated on forested lands.

• National Forest System and other forested lands comprised 28.7% of the total land area studied but contributed 46.0% of the total surface water supply.
• In the Western states, National Forest System lands accounted for 9.2% of the total land area and contributed 46.3% of the western surface water supply.

• 45.8 million people obtained more than 10% of their surface drinking water supply from National Forest System lands through their public water systems, representing 33% of the total population served by public water systems with surface water intakes.
• 82% of the population served by public water systems with surface water intakes in the Western states obtain more than 10% of their surface drinking water supply from National Forest System lands.
• 125.5 million people derived more than 10% of their surface drinking water supply from National Forest System and other forested lands, representing 91% of the total population served by public water systems with surface water intakes.

Additional resources are available that can be used in conjunction with the information in this story map for data analysis and assessments at the National Forest System proclamation unit and public water system level, including:

•  Report  documenting methods and results for the contiguous U.S.
•  Research Data Archive  providing tabular data at the National Forest System proclamation unit, public water systems, and sub-watershed level
•  Summary of public water systems and populations receiving surface drinking water supply from National Forest System lands  showing one-page summaries of the individual contribution of 172 National Forest System units to surface water supply
•  National Forest System Unit Explorer  allowing exploration of maps and data tables at the National Forest System proclamation unit level [ The help page ]
•  National Forest System Unit Query Tool  allowing fine-scale analysis and custom data overlays at the National Forest System proclamation unit, public water system, and sub-watershed level [ The help page ]