Jul 21, 2009

Sources of Variability of Evapotranspiration in California

Hugo G. Hidalgo, Daniel R. Cayan, Michael D. Dettinger
Journal of Hydrometeorology
  • Studies the variability (1990–2002) of potential evapotranspiration estimates (ETo) and related meteorological variables from a set of stations from the California Irrigation Management System (CIMIS)
  • Uses data from the National Climatic Data Center (NCDC) and from the Department of Energy from 1950 to 2001 to validate the results
  • Aims to determine the characteristics of climatological ETo and to identify factors controlling its variability (including associated atmospheric circulations)
  • States daily ETo anomalies are strongly correlated with net radiation (Rn) anomalies, relative humidity (RH), and cloud cover, and less with average daily temperature (Tavg)
  • States the highest intraseasonal variability of ETo daily anomalies occurs during the spring, mainly caused by anomalies below the high ETo seasonal values during cloudy days
  • States a characteristic circulation pattern is associated with anomalies of ETo and its driving meteorological inputs, Rn, RH, and Tavg, at daily to seasonal time scales
  • States this circulation pattern is dominated by 700-hPa geopotential height (Z700) anomalies over a region off the west coast of North America, approximately between 32° and 44° latitude, referred to as the California Pressure Anomaly (CPA)
  • States high cloudiness and lower than normal ETo are associated with the low-height (pressure) phase of the CPA pattern
  • States higher than normal ETo anomalies are associated with clear skies maintained through anomalously high Z700 anomalies offshore of the North American coast
  • States spring CPA, cloudiness, maximum temperature (Tmax), pan evaporation (Epan), and ETo conditions have not trended significantly or consistently during the second half of the twentieth century in California
  • States because it is not known how cloud cover and humidity will respond to climate change, the response of ETo in California to increased greenhouse-gas concentrations is essentially unknown; however, to retain the levels of ETo in the current climate, a decline of Rn by about 6% would be required to compensate for a warming of +3°C