Sources of Variability of Evapotranspiration in California

Abstract

The variability (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) of potential evapotranspiration estimates (ETo) and related meteorological variables from a set of stations from the California Irrigation Management System (CIMIS) is studied. Data from the National Climatic Data Center (NCDC) and from the Department of Energy from 1950 to 2001 were used to validate the results. The objective is to determine the characteristics of climatological ETo and to identify factors controlling its variability (including associated atmospheric circulations). Daily ETo anomalies are strongly correlated with net radiation (R n ) anomalies, relative humidity (RH), and cloud cover, and less with average daily temperature (T avg ). 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. A characteristic circulation pattern is associated with anomalies of ETo and its driving meteorological inputs, R n , RH, and T avg , at daily to seasonal time scales. This circulation pattern is dominated by 700-hPa geopotential height (Z 700 ) anomalies over a region off the west coast of North America, approximately between 32Њ and 44Њ latitude, referred to as the California Pressure Anomaly (CPA). High cloudiness and lower than normal ETo are associated with the lowheight (pressure) phase of the CPA pattern. Higher than normal ETo anomalies are associated with clear skies maintained through anomalously high Z 700 anomalies offshore of the North American coast. Spring CPA, cloudiness, maximum temperature (T max ), pan evaporation (E pan ), and ETo conditions have not trended significantly or consistently during the second half of the twentieth century in California. Because it is not known how cloud cover and humidity will respond to climate change, the response of ETo in California to increased greenhousegas concentrations is essentially unknown; however, to retain the levels of ETo in the current climate, a decline of R n by about 6% would be required to compensate for a warming of ϩ3ЊC.

Progress on incorporating climate change into management of California’s water resources

“…One relatively large group of model projections that was recently examined for California provides a range of about 2.5 to 9 degrees Celsius temperature rise for Northern California by 2100. An analysis of the distribution of the projections generally showed a central tendency at about 3 degrees Celsius of rise for 2050, and about 5 degrees Celsius for 2100 (Dettinger, 2005).…”

“…In the next section about energy budget the relationships between climatic factors are discussed along with relative influences they exert. Although Hidalgo et al (2005) concluded that to maintain ET o in the current condition requires a decrease in R n of about 6 percent to compensate for a temperature increase of 3ºC, they did not account for increased stomatal resistance that is likely to result from higher CO 2 concentration, which is discussed in detail later in this chapter. Hidalgo et al were clearly correct in that the effects of climate change on ET o are difficult to forecast because of the uncertainty of cloud cover and relative humidity.…”

“…Among the criteria used to select these climate change scenarios were ability of the models to represent El Niño events and availability of the data for analysis to meet the January 2006 governor's deadline for the report (Cayan, 2005). In addition, both models estimated historical climate trends reasonably well.…”

Generalized regression neural networks for evapotranspiration modelling

“…56 (Allen et al, 1998). The CIMIS Penman equation, described in detail in Hidalgo et al (2005) (see http://wwwcimis.water.ca.gov/cimis/ infoEtoCimisEquation.jsp), is:…”

The potential of different ANN techniques in evapotranspiration modelling

“…56 (Allen et al, 1998). The CIMIS Penman equation is also described in detail in Hidalgo et al (2005) …”