Click on “Download PDF” for the PDF version or on the title for the HTML version.

If you are not an ASABE member or if your employer has not arranged for access to the full-text, Click here for options.

A precision weighing lysimeter at Rocky Ford, Colorado was used to measure the evapotranspiration (ET) of well-watered alfalfa hay grown in the Arkansas Valley and to develop local crop coefficients (Kc) for use with the ASCE standardized reference ET equation. The mass of an undisturbed soil monolith contained in a steel tank (3 m x 3 m area; 2.4 m deep) was continuously monitored with a calibrated load cell to determine alfalfa hay ET. Alfalfa was established on the monolith and surrounding field (4 ha) in 2007. Hourly micrometeorological parameters measured above the lysimeter were used in the ASCE standardized tall reference ET equation. Crop height and soil water content were monitored weekly in 2008 and 2009. There were four hay cutting cycles in both years. Total ET of alfalfa hay in the lysimeter for the two growing seasons was 1333 mm (1 Apr 08 5 Nov 08) and 1179 mm (24 Mar 09 5 Oct 09). Average daily ET was 6.1 mm/d and 6.0 mm/d in 2008 and 2009, respectively. Preliminary Kcs (daily lysimeter ET / ASCE standardized reference ET) were persistently greater than 1.0 (values up to 1.4) during latter portions of the 2nd, 3rd, and 4th cutting cycles, which indicated possible unaccounted energy in the ASCE standardized ET calculations or a clothesline effect on the lysimeter. Further analyses of weather data inputs to the standardized reference ET equation and of lysimeter alfalfa canopy conditions are needed before alfalfa Kc curves are finalized.