Evaluating the ASCE standardized Penman-Monteith equation and developing crop coefficients of alfalfa using a weighing lysimeter in southeast Colorado

Quantification and efficient irrigation application of crop water requirements are potential ways for water resources conservation and sustainability. Reference evapotranspiration (ETref) is a very important variable in the quantification of crop water requirement. The ASCE standardized ETref equation has been recommended recently for calculating ETref. However it has not been tested under Colorado conditions. In addition, crop coefficients (Kc) for use with the ASCE standardized reference ET equation have not been developed in Colorado. Crop coefficients that have been used by the Colorado Division of Water Resources were estimated from Kimberly, Idaho and Bushland, Texas even though the growing conditions such as soil, elevation, climate and environmental factors in Colorado are different than in Kimberly and Bushland. They were developed using reference ET equations other than the ASCE standardized PM ETref equation and later they were adapted for use with the ASCE standardized PM ETref equation (Allen et al., 2007).;The objectives of this study were to test the performance of the ASCE standardized ETref equation for calculating alfalfa reference ET under southeast Colorado conditions and to develop compatible crop coefficients of alfalfa that apply to the region. A corollary objective was to determine if the full version of the Penman-Monteith equation could better match measured alfalfa ET from the lysimeter when reference conditions were satisfied.;A precise weighing lysimeter was used to measure alfalfa ET and to develop crop coefficients of alfalfa at Rocky Ford in Southeast Colorado. The lysimeter was filled with a 3 m x 3 m x 2.4 m undisturbed soil monolith and alfalfa (Genoa variety) was planted in the lysimeter and in 4 ha of surrounding field in August 2007. Alfalfa was harvested four times in each of the 2008, 2009 and 2010 growing seasons. ETref was calculated using the hourly ASCE standardized PM ETrs (standardized reference evapotranspiration for tall reference crop) and full version Penman-Monteith equations using climate data from an automatic weather station installed at the lysimeter site. Crop coefficients of alfalfa were calculated by dividing daily measured ET from the lysimeter by the corresponding daily ASCE standardized reference ET.;Season total alfalfa ET from the lysimeter ranged from 1179 mm to 1455 mm. Maximum daily water use of alfalfa was around 14.4 mm/day in 2010 season due to relatively high maximum temperature, high solar radiation and high wind speed. Average daily ET for 2008, 2009 and 2010 was 5.7 mm/day, 6.0 mm/day and 6.9 mm/day, respectively.;Hourly calculated ETrs values agreed well with measured ET from the lysimeter whenever alfalfa was under reference conditions (height of at least 50 cm and no soil water stress). Residuals between calculated ETrs and measured lysimeter ET increased as air temperature increased and as relative humidity decreased. Greater residuals were obtained when 80 % of the footprint length was not in the field. During some periods, there was lack of adequate fetch and this contributed to greater differences between the ASCE standardized ETrs and lysimeter ET. Good agreement between ASCE standardized ETrs and lysimeter ET was obtained when at least 80% of the ET flux footprint was inside the alfalfa field.;The alfalfa growth stage, climate, precipitation and soil water content were major factors that shaped the crop coefficient curves. The first cutting cycle, which had slower growth due to cooler weather, had smaller crop coefficients, whereas later cutting cycles with rapid growth had larger crop coefficients. The maximum crop coefficients were below 1.2 in 2008 (water stressed) and at or slightly above 1.2 in 2009 and 2010. The Kcr values greater than 1.0 were due to ETc from the lysimeter being greater than ETrs from ASCE standardized PM. Periods when alfalfa in the lysimeter was taller than alfalfa in the immediate surroundings and when the canopy extended outside the lysimeter boundary (3 m x 3 m) contributed to Kcr values greater than 1.0. Precipitation interception by the alfalfa canopy increased evaporation and caused outliers in the crop coefficient values. Crop coefficients were greatly affected by soil water content. A reduction in the alfalfa crop coefficients was observed at the end of some cutting cycles that coincided with reductions in soil water content.;The average leaf area index (LAI) at a height of 50 cm was 4.34 m 2/m2 and there was a high correlation between LAI and alfalfa height with R2 of 0.94, but the relationships were not the same as suggested by Allen et al. (1994). ETref values calculated by the full version of the Penman-Monteith equation deviated more from the lysimeter ET compared to the ASCE standardized ETref equation.;Full version of Penman Monteith showed very good agreement with ASCE standardized ET. Using the full version of the Penman-Monteith equation did not improve agreement with lysimeter ET at reference conditions.