Biological Systems Engineering


Date of this Version



Published in Journal of Irrigation and Drainage Engineering, March/April 2008.


Alfalfa-reference evapotranspiration (ETr) values sometimes need to be converted to grass-reference ET (ETo), or vice versa, to enable crop coefficients developed for one reference surface to be used with the other. However, guidelines to make these conversions are lacking. The objectives of this study were to: (1) develop ETr to ETo ratios (Kr values) for different climatic regions for the growing season and nongrowing (dormant) seasons; and (2) determine the seasonal behavior of Kr values between the locations and in the same location for different seasons. Monthly average Kr values from daily values were developed for Bushland, (Tex.), Clay Center, (Neb.), Davis, (Calif.), Gainesville, (Fla.), Phoenix (Ariz.), and Rockport, (Mo.) for the calendar year and for the growing season (May– September). ETr and ETo values that were used to determine Kr values were calculated by several methods. Methods included the standardized American Society of Civil Engineers Penman–Monteith (ASCE-PM), Food and Agriculture Organization Paper 56 (FAO56) equation (68), 1972 and 1982 Kimberly-Penman, 1963 Jensen-Haise, and the High Plains Regional Climate Center (HPRCC) Penman. The Kr values determined by the same and different methods exhibited substantial variations among locations. For example, the Kr values developed with the ASCE-PM method in July were 1.38, 1.27, 1.32, 1.11, 1.28, and 1.19, for Bushland, Clay Center, Davis, Gainesville, Phoenix, and Rockport, respectively. The variability in the Kr values among locations justifies the need for developing local Kr values because the values did not appear to be transferable among locations. In general, variations in Kr values were less for the growing season than for the calendar year. Average standard deviation between years was maximum 0.13 for the calendar year and maximum 0.10 for the growing season. The ASCE-PM Kr values had less variability among locations than those obtained with other methods. The FAO56 procedure Kr values had higher variability among locations, especially for areas with low relative humidity and high wind speed. The 1972 Kim-Pen method resulted in the closest Kr values compared with the ASCE-PM method at all locations. Some of the methods, including the ASCE-PM, produced potentially unrealistically high Kr values (e.g., 1.78, 1.80) during the nongrowing season, which could be due to instabilities and uncertainties that exist when estimating ETr and ETo in dormant season since the hypothetical reference conditions are usually not met during this period in most locations. Because simultaneous and direct measurements of the ETr and ETo values rarely exist, it appears that the approach of ETr to ETo ratios calculated with the ASCE-PM method is currently the best approach available to derive Kr values for locations where these measurements are not available. The Kr values developed in this study can be useful for making conversions from ETr to ETo, or vice versa, to enable using crop coefficients developed for one reference surface with the other to determine actual crop water use for locations, with similar climatic characteristics of this study, when locally measured Kr values are not available.