Reference evapotranspiration (ETref, specifically ETo or ETr) calculated from local mesonets can be used in combination with crop coefficients to estimate crop ET (ETc) demand. However, estimating crop coefficients under deficit or limited irrigation is more complex. These coefficients, the basal (Kcb, the ratio of transpiration to ETref) and water stress (Ks, transpiration reduction) coefficients, become dynamic and highly variable under water stress, and are best estimated from field observations including soil water content, canopy cover, and other indicators of water stress such as canopy temperature. Here we demonstrate a field experiment at the Limited Irrigation Research Farm in Greeley, CO, where we used canopy cover data obtained from imaging and canopy temperature obtained by infrared thermometers to estimate daily Kcb and Ks, respectively. Current experiments also integrate unmanned aerial vehicle (UAV) obtained spatial data, as well as numerical crop modeling information, as well as comparing to FAO-56 based scheduling. Results, opportunities, and challenges of these techniques are shown in this talk. These techniques have potential to optimize irrigation management in precision irrigation applications, including variable rate irrigation on center pivots.
