Potential water consumption for irrigation scheduling in North Dakota was typically calculated from a reference Evapotranspiration (ET ref) using the Jensen-Haise method and its associated crop coefficient (K c) curves developed in the 1970's and 1980's. The ET ref method proposed by the American Society of Civil Engineers, Environmental and Water Research Institute (ASCE-EWRI) reference evapotranspiration task force has shown to be more accurate and therefore more widely used than any other methods. However, to apply the ASCE-EWRI method for irrigation scheduling requires a corresponding change of the K c curves associated with the Jensen-Haise method. In this paper, a comparison of ET ref estimates for 11 methods, including the ASCE-EWRI and the Jensen-Haise methods, was conducted using 18 years of data collected in southeastern North Dakota. The results show that the annual ET ref by the Jensen-Haise method was nearly the same as the ASCE-EWRI grass ET ref , but with a higher Root Mean Square Deviation (RMSD), 0.903 mm d-1 , and a lower coefficient of determination (R 2) 0.8659. The ET ref comparison for the growing season only shows an RMSD of 1.007 mm d-1 , R 2 of 0.7996 and 8.13% overestimation. The ET ref by the Jensen-Haisemethod has a higher monthly ET ref than the ASCE-EWRI in June, July, and August, and a lower monthly ET ref for all other months in an 18 year period. The ET ref comparisons also show that the modified Penman method used by the High Plains Regional Climate Center (HPRCC Penman) has the best accuracy and correlation with the ASCE-EWRI ET ref method. Indeed, all alfalfa based ET ref methods, including Kimberly Penman and HPRCC Penman, show better performance than the grass based ET ref methods, including FAO24 Penman, FAO24 Radiation, FAO24 Blaney-Criddle, Priestley-Taylor, Hargreaves, and the Jensen-Haise methods. Ir ri g ation & D r a in ag e Sys te m s E ng ineer in g
