The 1990-2002 history of potential evapotranspiration estimates (ETo) and related meteorological observations from California Irrigation Management System (CIMIS) stations provide insights into the long-term and large-scale determinants of evapotranspiration variability around the State. ETo is associated with irrigation water demands, and thus is particularly important to the State’s agricultural economy. Data from the National Climatic Data Center (NCDC) and from the Department of Energy from 1950 to 2001 validate the findings. Daily ETo anomalies are strongly correlated with net radiation (Rn) anomalies, relative humidity (RH) and cloud cover, and are less correlated with average daily temperature (Tavg). The highest intraseasonal variations of ETo daily anomalies occur during the spring, mainly as excursions toward low ETo rates during cloudy days. The circulation pattern associated with anomalies of ETo, at daily to seasonal timescales, is shared with its driving meteorological inputs, Rn, RH and Tavg at the same time-scales. This shared circulation pattern is dominated by positive 700 hPa geopotential (Z700) anomalies over a region off the west coast of North America, approximately between 32 and 44 degrees latitude, and has been referred to as the California Pressure Anomaly (CPA). High cloudiness and lower than normal ETo are associated with the low-height (pressure) phase of the CPA pattern. Higher than normal ETo anomalies are associated with clear skies maintained through anomalously high Z700 anomalies off the west coast. The likely response of ETo in California to increased greenhouse-gas concentrations is essentially unknown; however, to retain current levels of ETo, a decline of Rn by about 6% would be required to compensate for a warming of +3ºC.