Reference and crop evapotranspiration in South Central Nebraska. I: Comparison and analysis of grass and alfalfa-reference evapotranspiration

In Nebraska, historically, there have been differences among the water regulatory agencies in terms of the methods used to compute reference evapotranspiration (ET_ref) to determine actual crop water requirements and hydrologic balances of watersheds. Because simplified and/or empirical temperature or radiation-based methods lack some of the major weather parameters that can significantly affect grass and alfalfa-reference ET (ET_o and ET_r) the performance of these methods needs to be investigated to help decision makers to determine the potential differences associated with using various ET_ref equations relative to the standardized ASCE Penman-Monteith (ASCE-PM) equations. The performance of 12 ET_o and five ET_r equations were analyzed on a daily basis for south central Nebraska from 1983 to 2004. The standardized ASCE-PM ETo and ET_r values were used as the basis for comparisons. The maximum ASCE-PM ET_o value was estimated as 12.6 mm d^-1, and the highest ET_r value was estimated as 19 mm d^-1 on June 21, 1988. On this day, the atmospheric demand for evaporation was extremely high and the vapor pressure deficit (VPD) reached a remarkably high value of 4.05 kPa. The combination-based equations exhibited significant differences in performance. The 1963 Penman method resulted in the lowest RMSD of 0.30 mm d^-1 (r² = 0.98) and its estimates were within 2% of the ASCE-PM ET_o estimates. The 1948 Penman estimates were similar to the 1963 Penman (r²=0.98, RMSD = 0.39 mm d^-1). Kimberly forms of alfalfa-reference Penman equations performed well with RMSD of 0.48 mm d^-1 for the 1972 Kimberly-Penman and 0.67 mm d^-1 for the 1982 Kimberly-Penman. The locally-calibrated High Plains Regional Climate Center (HPRCC) Penman method, ranked 6th, performed well and underestimated the ASCE-PM ET by 5% (RMSD = 0.56 mm d^-1). Most of the underestimations occurred at the high ET range (>11 mm) and this was attributed to the upper limits applied by the HPRCC on VPD, (2.3 kPa) and wind speed (5.1 m s^-1). The lack of ability of the radiation methods in accounting for the wind speed and relative humidity hindered the performance of these methods in the windy and rapidly changing VPD conditions of south central Nebraska. The 1977 FAO24 Blaney-Criddle method was the highest ranked (seventh) noncombination method (RMSD = 0.64 mm d^-1, r² = 0.94). The FAO24 Penman estimates were within 4% of the ASCE-PM ET_o. Overall, there were large differences between the ASCE-PM ET_o and ET_r versus other ET_ref equations that need to be considered when other forms of the combination or radiation and temperature-based equations are used to compute ET_ref. We recommend that the ASCE-PM ET_o or ET_r equations be used for estimating ET_ref when necessary weather variables are available and have good quality. The results of this study can be used as a reference tool to provide practical information, for Nebraska and similar climates, on the potential differences between the ASCE-PM ET_o and ET_r and other ET_ref equations. Results can aid in selection of the alternative method(s) for reasonable ET_ref estimations when all the necessary weather inputs are not available to solve the ASCE-PM equation.