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 the 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 (until it reaches the sea), and in a broad sense is our fresh water supply.
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 that becomes 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 more and newer land cover data.
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 mountains areas mean annual yields tend to be less than or equal to (≤)15 centimeters per year (cm/y). Yields are uniformly ≤15 cm/y in the Great Plains and Southwest. Yields east of the Great Plains tend to exceed 30 cm/y 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 land occupies 24 percent of the contiguous U.S. and yields 23 percent of its mean annual water supply. Federal agencies differ greatly in terms of the water supply, because of differences in amount of land they manage and in the elevation and rainfall that occur on those lands. For example, Forest Service lands yield 18 percent of the water supply from 11 percent of the land area whereas BLM lands yield 2 percent of the water supply from 9 percent of the land area.
Different land cover database layers yield different water supply results:
Based on the National Land Cover Database (NLCD), forests occupy 26 percent of the land area of the contiguous U.S. and yield 46 percent of the mean annual water supply, whereas rangelands occupy 37 percent of the land and yield only 14 percent of the water supply. Notably, results by land cover depend on which land cover data are used, which in turn reflects the different definitions used to distinguish among cover types.
Landfire PHYS land cover data show that forests occupy 29 percent of the land and provide 50 percent of the water supply, whereas rangelands occupy 30 percent of the land and provide 7 percent of the water supply.
U.S. Forest Service Forest Inventory and Analysis cover data show that forests occupy 34 percent of the land and provide 59 percent of the water supply.