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Chapter 4: Evaporation and Evapotranspiration

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pages 55-79 (doi:10.13031/swce.2013.4) in Soil and Water Conservation Engineering, 7th Edition . Copyright 2013 American Society of Agricultural and Biological Engineers, St. Joseph, Mich.
Authors:   Rodney L. Huffman, Delmar D. Fangmeier, William J. Elliot, Stephen R. Workman
Keywords:   Soil, water, conservation, environment, Evaporation, 4.1 Evaporation from Water Surfaces, 4.2 Evaporation from Land Surfaces, Evapotranspiration, 4.3 Transpiration Ratio, 4.4 Evapotranspiration Definitions, Evapotranspiration Estimation Me

Introductory paragraphs: Two phases of the hydrologic cycle of particular interest in agriculture are evaporation and transpiration. About three-fourths of the total precipitation received on land areas of the world returns directly to the atmosphere by evaporation or transpiration. Most of the balance returns to the ocean as surface or subsurface flow.

Evaporation is the transfer of liquid surface water into vapor in the atmosphere. The water molecules, both in the air and in the water, are in rapid motion. Evaporation occurs when the number of moving molecules that break from the water surface and escape into the air as vapor is larger than the number that re-enter the water surface from the air and become entrapped in the liquid. Evaporation, which may occur from water surfaces, wet leaf surfaces, or from water on soil particles, is important in water management and conservation.

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