The weather largely determines the use of water, or evapotranspiration (ET), by most crops during times when the plants are healthy and fully shade the ground. Even under full-cover conditions, however, the evapotranspiration of various crops can vary significantly with differences in stomatal or surface resistance, reflectance, and aerodynamic roughness. However, during early stages of crop growth, transpiration is very limited, and the controlling factor in water use is basically the moisture status of the soil surface. How frequently the surface receives water from rain or irrigation—along with the weather conditions—largely determines evapotranspiration rates.

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Support was provided by a number of agencies including the University's Water Resources Center, the State Department of Water Resources, and by the U. S. Bureau of Reclamation through the Center for Agricultural and Economic Development, Iowa State University. Others providing major assistance with this long-term study were Dennis Orr, Allen Servis, Donald Bradley, Ray Dally, and Mrs. H. H. Laid-law. Wilson Goddard under the supervision of Dr. F. A. Brooks had major responsibility for development of the large floating lysimeter.