ROLE OF EVAPOTRANSPIRATION MODELS IN IRRIGATION SCHEDULING

M OST evapotranspiration (El) models are based on physical principles controlling evaporation and the conservation of mass and energy, and use daily climatic data. ET models coupled with irrigation models are valuable tools because they enable trained and ex- perienced irrigation specialists to provide irrigation scheduling services at a reasonable cost. Estimated standard deviations of mean daily ET for 1- to 30-day periods (Sa-) at Akron, Colorado; Davis, California; Kimberly, Idaho; and Lompoc, CA varied from 0.9 to 1.3 mm/day. Sa- decreased to 0.4 to 0.7 mm/day for 15- to 30-day periods. Standard errors of. ET estimates (Sy.x) with a com- bination equation based on 243 days of data from Kimberly, ID were normally distributed. The Sya for daily values was 1.0 mm/day. The Sy .x decreased in- versely with the square root of the number of days for up to 30 days and was similar to those reported for other areas using models that operate on daily climatic data. A summary of factors affecting confidence levels in irrigation scheduling is presented along with the ex- pected standard deviations. Generally, the error in esti- mating irrigation applications exceeds estimated El' errors. The error in measuring soil moisture is gener- ally smaller than estimated ET and irrigation applica- tion errors.