Evapotranspiration Measurements Using Weighing Lysimeters: The Rocky Ford Experience.

Two precision weighing lysimeters were installed at Rocky Ford in the Arkansas Valley of Colorado.  The lysimeters have been used to track the soil water balance and estimate the evapotranspiration (ET) of furrow irrigated crops grown in the Arkansas Valley and to develop local crop coefficients (Kc) for use with the ASCE standardized reference ET equation. Five years (2008 – 2012) of alfalfa hay data have been collected.  This presentation will discuss the factors that affected the accuracy of crop ET measurements from the Rocky Ford lysimeters.  Design factors (e.g., soil reconsolidation around lysimeters; load cell calibration; mechanical failure; fetch) as well as crop (uniformity of growth; water status) and management (irrigation system, irrigation timing and uniformity, field operations) factors will be discussed.  Sources of ET measurement uncertainties will also be highlighted.