Evapotranspiration (ET) can be measured in the field using a lysimeter, which measures the actual ET through the changes in mass of a soil monolith with an actively growing crop. Reference ET can also be calculated using a combination of equations and climate data from a local weather station. The two combination equations currently available in the Colorado Agricultural Meteorological Network (CoAgMet) website are the ASCE standardized Penman-Monteith and the Penman-Kimberly equations. This study compared alfalfa ET measured by a lysimeter in the center of a 4.06 ha furrow-irrigated field at the Arkansas Valley Research Center in Rocky Ford , CO to the calculated values from the two combination equations in periods of reference conditions from 2008 to 2011. An automatic CoAgMet weather station which is 482.803 meters away from the lysimeter collected climate data which was then used in calculating alfalfa reference ET by the ASCE standardized Penman-Monteith and Kimberly-Penman. The comparison between the calculated ETr using the Penman-Kimberly to ETr measured by the lysimeter had a Root-Mean-Square Error (RMSE, mm) of 0.65 in 2008, 1.14 in 2009, 1.52 in 2010 and 3.23 in 2011. Agreement was very good in 2008 and worst 2011. These results were confirmed by the Index of Agreement (d), which were 0.98 in 2008, 0.94 in 2010, 0.91 in 2010 and 0.63 in 2011. This same comparison was done using the calculated ETr from the (ASCE) standardized Penman Monteith equation. The RMSE (mm) using this equation and compared with the measured ETr from lysimeter was 0.70 in 2008, 1.32 in 2009, 1.48 in 2010 and 3.47 in 2011. The d values for this comparison were 0.98 in 2008, 0.91 in 2009, 0.91 in 2010 and 0.61 in 2011. Agreement was best in 2008 compared to other years, and conditions this year led to better performances of the two equations when compared to the lysimeter. Agreement between the two equations and the ETr varied with cutting cycle and growing season conditions.