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On the Ratio of Evaporation to Precipitation

James E. McDonaldUniversity of Arizona

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The evaporation ratio ɛ is defined as the ratio of the evaporation actually occurring under natural conditions over a given area to the precipitation falling on that area. Data drawn from a variety of sources are used to obtain simple climatological estimates of ɛ. The mean value of ɛ over all the world's land surfaces is probably near 0.65, with individual continents ranging from 0.60 for North America to 0.87 for Australia. For the United States, ɛ averages about 0.73, with local ɛ values ranging from near 1.0 in the southwestern deserts to essentially zero in the Pacific Northwest coastal slopes and in portions of Maine. A chart depicting the generalized pattern of ɛ over the United States east of the Rocky Mountains is presented.

1 Work done with the support of the Rockefeller Foundation under the University of Arizona Arid Lands Program.

The evaporation ratio ɛ is defined as the ratio of the evaporation actually occurring under natural conditions over a given area to the precipitation falling on that area. Data drawn from a variety of sources are used to obtain simple climatological estimates of ɛ. The mean value of ɛ over all the world's land surfaces is probably near 0.65, with individual continents ranging from 0.60 for North America to 0.87 for Australia. For the United States, ɛ averages about 0.73, with local ɛ values ranging from near 1.0 in the southwestern deserts to essentially zero in the Pacific Northwest coastal slopes and in portions of Maine. A chart depicting the generalized pattern of ɛ over the United States east of the Rocky Mountains is presented.

1 Work done with the support of the Rockefeller Foundation under the University of Arizona Arid Lands Program.

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