Use of Atmometers in Estimating Evapotranspiration

Studies conducted by the California Department of Water Resources during the past 10 yr show that Livingston black and white spherical atmometers provide a practical means of estimating monthly and seasonal evapotranspiration. The difference of evaporation between the black and white atmometers is affected primarily by shortwave energy, whereas evaporation from either the black or white atmometer provides an index of the total amount of energy available for evapotranspiration. Various studies have been made, the results of which indicate that the type of background such as bare ground, dry range, or irrigated pasture has little effect upon the difference in evaporation between the black and white atmometers. Furthermore, the height above the surface of the ground or crop likewise has little or no effect upon the difference of evaporation. Monthly crop coefficients have been developed for some of the principal crops in the Central Valley of California, which provide evapotranspiration estimates consistently as good or better as can be obtained using coefficients developed from United States Weather Bureau Class A evaporation pan data.