Alfalfa Reference ET from a Weighing Lysimeter and Estimates from the ASCE Standardized Reference ET Equation in the Arkansas Valley of Colorado

The American Society of Civil Engineers (ASCE) standardized reference ET equation has been prescribed for use in the Arkansas Valley of Colorado for estimation of reference evapotranspiration (ET) and subsequent estimation of crop consumptive use. In 2006, a precision weighing lysimeter (mechanical lever scale — load cell combination) was installed at Rocky Ford, Colorado to verify the performance and predictive accuracy of the ASCE standardized ET equation for calculating alfalfa reference (tall reference) ET values. The undisturbed soil monolith is contained in a tank that is 3 m x 3 m in area and 2.4 m deep. The standard deviation of the weight measurements was less than 0.02%, based on calibration of the load cell with known weights. Alfalfa was established on the lysimeter and surrounding field (4 ha) in 2007. Instrumentation for micrometeorological measurements as well as soil temperature, soil heat flux, and soil moisture measurements were installed. Calculated ET from the ASCE standardized equation (ET_rs) and ET measured from the lysimeter (ET_r) were compared during periods in the 2008 season when the alfalfa crop on the lysimeter and surrounding field were considered to be in reference conditions. Calculated ET_rs agreed well with ET_r from the lysimeter when wind conditions were relatively calm and vapor pressure was relatively stable. However, ET_rs tended to be greater than ET_r under advective conditions. Also, calculated ET_rs was notably less than ET_r from the lysimeter on a number of days late in the growing season. The underlying causes of these differences need further investigation.