Quantifying the role of National Forest System and other forested lands in providing surface drinking water supply for the conterminous United States
The U.S. Department of Agriculture’s Forest Service manages more than 779,000 km2 (193 million acres) of national forests and grasslands (collectively, National Forest System [NFS] lands) that play a significant role in providing clean, fresh water for local ecosystems and economies. This water is sometimes transferred hundreds of kilometers away to also serve big cities through inter-basin transfers (IBTs). The contribution of NFS lands to surface drinking water supplies for public water systems has not been assessed at the national scale while accounting for IBTs. The Forest Service Water Supply Stress Index (WaSSI) model was modified to provide estimates of 2001–2015 mean annual surface water supply and the proportion of mean surface water supply originating on 172 NFS land units and other forested lands at the 12-digit hydrologic unit code scale across the conterminous United States (CONUS) while accounting for water transfer through IBTs. Predictions of the proportion of surface water supply originating on NFS and other forested lands were linked to specific downstream communities and populations, using surface drinking water intake information from the U.S. Environmental Protection Agency Safe Drinking Water Information System database of public water systems. A new database of 594 IBTs was compiled for this study, ranging from 0.01 million m3 yr-1 to 8,900 million m3 yr-1, for a total transferred volume of 116,894 million m3 yr-1. Overall, NFS lands comprised 9.2 percent of the total CONUS land area but contributed 12.8 percent of the surface water supply. In the West, NFS lands comprised 19.2 percent of the total land area but contributed 46.3 percent of the 478.7 billion m3 yr-1 surface water supply; in the East, NFS lands comprised about 2.8 percent of the total land area and 3.8 percent (66.6 billion m3 yr-1) of the surface water supply. In total across the CONUS, NFS and other forested lands comprised 28.7 percent of the total land area but contributed 46.0 percent of the surface water supply. Approximately 45.8 million people derived >10 percent of their surface drinking water supply from NFS lands, and 22.6 million people received >50 percent of their surface drinking water supply from NFS lands. Approximately 125.5 million people, about 39 percent of the total population in the CONUS in 2017, derived >10 percent of their surface drinking water supply from NFS and other forested lands, with 83.1 million people receiving >50 percent of their surface drinking water supply from NFS and other forested lands. In addition to those populations receiving surface drinking water supply from their local public surface drinking water intakes, 12.6 million people were served by public water systems that purchased surface drinking water supply from other public water systems deriving >10 percent of their surface drinking water supply from NFS lands. This study provides a systematic accounting of NFS and other forested lands for surface drinking water supply. Our results can aid water resource and forest managers in developing integrated watershed management plans at a time when climate change, population growth, and land development threaten water supplies.