Comparison of Measured Alfalfa Reference Evapotranspiration Using a Furrow Irrigated Lysimeter to Calculate Values Using the ASCE Standardized Penman-Monteith Equation in the Arkansas Valley of Colorado.

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 be calculated using a combination of equations and climate data from a local weather station. The equation currently available on the Colorado Agricultural Meteorological Network (CoAgMet) website is the ASCE standardized Penman-Monteith equation. 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 ASCE standardized Penman-Monteith equation in periods of reference conditions, four day periods where at reference stage the alfalfa length was 50cm except in 2010 when it was at 55cm. An automatic CoAgMet weather station, located 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 equation. Four days comparisons between the calculated ETr using the ASCE standardized Penman-Monteith equation to ETr measured by the lysimeter were performed. The first comparison was without any adjustments in the ETr where the Root-Mean-Square Error (RMSE, mm/h) was 0.08 on 9/16/2010, 0.11 on 9/23/2010 and 0.10 on 10/1/2010. Agreement was low in before the adjustment for 2010. These results were confirmed by the Index of Agreement (d), which were 0.98 on 9/16/2010 and 9/23/2010, 0.97 on 10/1/2010, 0.96 on 10/12/2010. This same comparison was done using the calculated ETr from the (ASCE) standardized Penman-Monteith equation after making adjustments where the lysimeter area was adjusted for the four days in 2010. The RMSE (mm) using this equation and compared with the measured ETr from the lysimeter was 0.05 in 9/16/2010, 0.04 on 9/23/2010 and 0.03 on 10/12/2010. The d values for this comparison were 0.99 on 9/162010, 1.00 on 9/23/2010 and 0.99 on both 10/1/2010 and 10/12/2010. The conditions on 10/1/2010 and 10/12/2010 led to better performances of the comparison ETr when compared to the days 9/16/2010 and 9/23/2010.