Variability of Pan-based and Penman-based Evapotranspiration Estimates in California

Daily data from 29 meteorological stations from the California Irrigation Management Information System (CIMIS) covering the period from 1990 to 2002, and from 7 pan-evaporation stations from the National Climatic Data Center (NCDC), were used to characterize variability and trends in reference evapotranspiration (ETo) in California. ETo daily anomalies in California exhibited higher variance during the spring compared to other seasons. The higher variance is associated with larger ETo seasonal values -due to the higher spring irradiance- compared to winter and autumn, combined with large ETo reductions associated with relatively common cloudy days. Although the irradiance is also high during the summer, ETo estimates during this season are very close to the seasonal -clear sky- medians and therefore have low variance, due to the much lower frequency of occurrence of cloudy days. The combination of high seasonal values and relatively frequent cloudy days is only observed during the spring, the most variable season in terms of ETo. Atmospheric circulations at 700 mbar pressure levels (Z700) over a region off the West Coast of North America, approximately between 30 and 45 latitude, are most closely associated with the historical ETo variations. ETo is significantly positively correlated (r=+0.7) with Z700 in this region at daily, monthly and seasonal time scales, especially during the spring, through atmospheric circulation influences on the variability of radiation, relative humidity and cloudiness. This correlation pattern was identified using the CIMIS data (1990 to 2002) and its robustness was verified using the pan data (1965 to 2000). This pattern is consistent with other studies that have showed connections between atmospheric circulation in the eastern Pacific and precipitation and streamflow variations in California. Estimates of spring actual evapotranspiration from 1984 to 2002 computed based on the CIMIS data showed moderate, but significant increasing trends in most of the Central Valley, generally associated with increasing trends in minimum temperature. Future work will verify these trends using the NCDC data.