Variability Analyses of Alfalfa-Reference to Grass-Reference Evapotranspiration Ratios in Growing and Dormant Seasons

Alfalfa-reference evapotranspiration $\left({\mathrm{ET}}_r\right)$ values sometimes need to be converted to grass-reference ET $\left({\mathrm{ET}}_o\right)$ , 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 ${\mathrm{ET}}_r$ to ${\mathrm{ET}}_o$ ratios ( $K_r$ values) for different climatic regions for the growing season and nongrowing (dormant) seasons; and (2) determine the seasonal behavior of $K_r$ values between the locations and in the same location for different seasons. Monthly average $K_r$ 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). ${\mathrm{ET}}_r$ and ${\mathrm{ET}}_o$ values that were used to determine $K_r$ 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 $K_r$ values determined by the same and different methods exhibited substantial variations among locations. For example, the $K_r$ 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 $K_r$ values among locations justifies the need for developing local $K_r$ values because the values did not appear to be transferable among locations. In general, variations in $K_r$ 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 $K_r$ values had less variability among locations than those obtained with other methods. The FAO56 procedure $K_r$ 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 $K_r$ values compared with the ASCE-PM method at all locations. Some of the methods, including the ASCE-PM, produced potentially unrealistically high $K_r$ values (e.g., 1.78, 1.80) during the nongrowing season, which could be due to instabilities and uncertainties that exist when estimating ${\mathrm{ET}}_r$ and ${\mathrm{ET}}_o$ 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 ${\mathrm{ET}}_r$ and ${\mathrm{ET}}_o$ values rarely exist, it appears that the approach of ${\mathrm{ET}}_r$ to ${\mathrm{ET}}_o$ ratios calculated with the ASCE-PM method is currently the best approach available to derive $K_r$ values for locations where these measurements are not available. The $K_r$ values developed in this study can be useful for making conversions from ${\mathrm{ET}}_r$ to ${\mathrm{ET}}_o$ , 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 $K_r$ values are not available.