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South Central Region Office
Department of Water Resources

(559) 230-3300

Street and Mailing Address:

3374 East Shields Avenue
Fresno, CA 93726-6913

Home Land and Water Use Land & Water Use Data Collections Agricultural Climate and Evaporation Data

DWR has been researching the relationship between crop water use, or evapotranspiration (ET), and evaporation from United States Weather Bureau (U.S.W.B.) Class 'A' evaporation pans located in irrigated pastures for over 50 years. The program operated many research plots to document pan evaporation in different regions of the state and verify the ET of a grass reference crop as well as the ET of important individual crops.  The combined years of research allowed the determination of crop specific coefficients used to estimate crop ET from pan evaporation.  These research sites, called Agroclimatic stations, consist of an evaporation pan and instruments measuring other meteorological parameters (air temperature, precipitation, wind movement, and solar radiation) that influence pan evaporation rates.  A statewide Agroclimatic Station network continues to be operated today. DWR Bulletins 113 through 113-4 document the activities and data collected from this program, presents data and methodologies for estimating crop or vegetative water use and also includes data from other researchers.  Results from this program ultimately led to the ability to reasonably estimate crop ET from pan evaporation for hydrologic studies and irrigation scheduling.

As part of this research, the South Central Regional Office has been measuring evaporation from U.S.W.B. Class 'A' pans. Evaporation measurements are taken from evaporation pans located at standardized sites (irrigated pastures) with static water levels maintained in the pans by float valves and water supply tanks. The environment surrounding the evaporation pans is critical for obtaining quality evaporation measurements.   The sites are visited at least weekly to measure pan evaporation.  Weekly measured pan evaporation is used to calculate ET of eight major crops.  These data are disseminated locally for use in scheduling irrigations, and compile the annual Crop Water Use Reports.

Evaporation Data

A summary of monthly evaporation from pans near Bakersfield from 1958-2010 and at California State University at Fresno from 1968-2010 are available through the link below. These data are arranged in tables of monthly averages, standard deviation and standard error, including data collection descriptions and an evaporation pan photo.  Evaporation pan data for specific months is available in the annual tables located in the Agroclimatic Data. 
Typical Evaporation Site
Typical standardized site (irrigated pasture) with a U.S.W.B. Weather Bureau Class 'A' pan, tank and DWR agroclimatic station.

AVERAGE MONTHLY EVAPORATION FROM CLASS 'A' PAN IN IRRIGATED PASTURE ENVIRONMENTS NEAR BAKERSFIELD, CALIFORNIA FROM 1958-2010
Evaporation in inches
Month Average STD DEV STD Error
January 1.44 0.34 0.05
February 2.25 0.45 0.06
March 4.13 0.71 0.10
April 5.95 0.86 0.12
May 8.35 0.82 0.12
June 9.58 0.79 0.11
July 9.94 0.82 0.11
August 8.85 0.71 0.10
September 6.62 0.64 0.09
October 4.47 0.43 0.06
November 2.24 0.36 0.05
December 1.35 0.36 0.05
Mar-Oct Total 57.83 0.72 0.10
Jan-Dec Total 65.11 0.61 0.08

AVERAGE MONTHLY EVAPORATION FROM CLASS 'A' PAN IN IRRIGATED PASTURE ENVIRONMENTS AT CALIFORNIA STATE UNIVERSITY AT FRESNO FROM 1968-2010
Evaporation in inches
Month Average STD DEV STD Error
January 1.26 0.28 0.04
February 2.08 0.41 0.06
March 3.94 0.77 0.12
April 6.03 0.86 0.13
May 8.75 1.03 0.16
June 10.43 0.92 0.14
July 11.02 0.73 0.11
August 9.67 0.68 0.11
September 6.99 0.57 0.09
October 4.42 0.49 0.07
November 2.25 0.40 0.06
December 1.21 0.30 0.05
Mar-Oct Total 61.26 0.76 0.12
Jan-Dec Total 68.07 0.62 0.10

Click here to download printer friendly table.

Agroclimatic Station Data

The Agroclimatic station data is collected weekly along with pan evaporation.  Annual tables are generated for each station summarizing monthly instrument measurements from each instrument.  Below are links to annual tables for Agroclimatic stations near Bakersfield and at California State University at Fresno from 1992-2010 viewable in Adobe Reader PDF format.

Fresno, CSU

1992 | 1993 | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010

Bakersfield 12S

1992 | 1993 | 1994 | 1995

Bakersfield 14W

1994 | 1995 | 1996

Wasco 7E

1997 | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010

Lamont 2NW

1992 | 1993

Note: Agroclimatic station records depict actual measured U.S.W.B. Class pan evaporation. Pan evaporation reported in the Crop Water Use Reports is the pan evaporation used to calculate estimated crop evapotranspiration. The crop water use is an estimate of evapotranspiration, a very important component in hydrologic investigations. The surrounding environment around evaporation pans is critical for obtaining quality evaporation measurements. At locations, where environments do not meet standardized criteria or severe weather events occur, adjustments to pan measurements (based on correction tables) may be required before estimating crop evapotranspiration.