Historical reference crop ET reliable for irrigation scheduling during summer

Average daily historical reference crop evapotranspiration was calculated using 13 to 15 years of daily evapotranspiration from 10 sites in California. Maximum average values, generally occurring in June, ranged between 0.25 and 0.32 inches per day at most of the sites. Absolute percent relative errors were minimum during July and August, ranging between 5% and 13% for all but one site. About 25% to 35% of the variation in the historical data reflected year-to-year variation; the remaining variation was due to seasonal trend and variability about the seasonal trend. To prevent overirrigation or underirrigation during deviation from the historical evapotranspiration value, growers should monitor soil moisture or check real-time evapotranspiration.