Alfalfa ET and Crop Coefficients From a Weighing Lysimeter in the Arkansas Valley of Colorado.

A precision weighing lysimeter at Rocky Ford, Colorado was used to track the soil water balance and estimate the evapotranspiration (ET) of furrow irrigated 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 from water balance.  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 from 2008 to 2011. There were four hay cutting cycles in all years. Total ET values of alfalfa hay in the lysimeter for the four growing seasons were 1333 mm (1 Apr 08 – 5 Nov 08), 1179 mm (24 Mar 09 – 5 Oct 09), 1474 mm (30 Mar 10 – 28 Oct 10), and 1603 mm (7 Apr 11 – 30 Nov 11). The Kc (daily lysimeter ET / ASCE standardized reference ET) curves for four cutting cycles are presented. Possible causes of Kc values persisting above 1.0 at peak alfalfa growth are also given.