Our fresh water supply begins as precipitation falling on land and fresh waters. From there the water naturally evaporates from the land or vegetation, percolates down to groundwater aquifers, or flows toward sea via rivers and streams. Water that evaporates is unavailable for use until it falls again elsewhere as precipitation. What remains is available for use by humans and other species, and in a broad sense is our fresh water supply (until it reaches the sea).
We estimated water supply across the contiguous 48 states for the period 1981-2010. Political, administrative, and land cover boundaries were mapped over the gridded water supply estimates to indicate the amount of water available in respective land areas. These water supply estimates are an update of those provided by Brown et al. (2008). Compared with Brown et al., these new estimates incorporate more recent precipitation and temperature data, apply a different water yield model, and utilize newer land cover data.
Forests occupy 26% of the land area of the contiguous U.S. but yield 46% of the mean annual water supply. Rangelands occupy 37% of the land but yield only 14% of the water supply, largely due to higher temperatures and lower rainfall that typify much of this land cover type.
In the West, the highest yields are concentrated in the mountainous areas of the north Pacific Coast, the Sierras of California, and the northern and central Rocky Mountains. Away from these mountainous areas, mean annual yields in the West tend to be ≤15 cm/yr. Away from these mountains areas mean annual yields in the West tend to be ≤15 cm/yr.
Yields are uniformly ≤15 cm/yr in the Great Plains and Southwest.
Yields east of the Great Plains tend to exceed 30 cm/yr except for areas along the eastern edge of the Great Plains, some areas near the Great Lakes, and areas along the south Atlantic coast including Florida.
Federal agencies differ greatly in terms of water yielded from the lands they administer. Differences are explained by the amount of land they manage and the elevation and rainfall that occur on those lands. Forest Service lands yield 18% of the water supply from 11% of the land area, and BLM lands yield 2% of the water supply from 9% of the land area.