September 4, 2002

ET Variance in Irvine

ET in Irvine, Revisited     

As you know, your irrigation water allocation from Irvine Ranch Water District is based upon Eto, a calculated value based upon weather data.  On their website, at, click on Conservation, then Landscape/Agriculture, then Weather Station Info.  Here you can see the Eto data for 1997 through 2001, the current year weekly Eto, as well as an overview of how allocations are calculated.  At Centennial Services, we use the IRWD Eto data to help manage your irrigation water use.  We also access Eto data from the statewide system known as CIMIS, which maintains a weather station in central Irvine.  If you are interested, you too can see this info at

IRWD uses weather station technology similar to the CIMIS system, and has divided its service area into three Zones, each of which is served by its own weather station.  These three  zones are the “Coastal” (encompassing Newport Coast west of the ridgeline, not UCI, not Turtlerock), the “Foothill” (Foothill Ranch and Portola Hills, and “Central” (everything else).  The weather station serving the Central zone is located at IRWD headquarters on Sand Canyon.  The following map shows this location, the approximate location of the CIMIS station #75, along with an approximation of the IRWD “Coastal” and “Foothill” areas.

We have been tracking and comparing the cumulative and weekly Eto from both sources, with some interesting results.  Here is a comparison of the Eto for the periods shown from the central IRWD station and the CIMIS station 75:

                                 CIMIS weather              IRWD Central                                                 
Period                    station 75                        weather station             % IRWD/CIMIS
1996                            54.38                       no data                                   -----
1997                            53.11                       48.62                                     91.5%
1998                            49.48                       43.87                                     89%
1999                            51.8                         45.86                                      89%
2000                            50.37                       46.64                                     93%
2001                            45.71                       39.1                                        85.5%
2002 through 6/30   24.47                       20.11                                      82.2%
July 2002                     6.17                         4.90                                       79.4%
August 2002              5.41                          4.42                                        81.7%

As you can see, Eto recorded at the IRWD Central weather station has been lower than CIMIS since records from IRWD start, and it is dramatically lower for 2001 and 2002. Everyone knows weather conditions vary from year to year, month to month and day to day. Conditions also could vary significantly or slightly within a defined geographic area or “zone”.  The Central zone, asdefined by IRWD, includes your site.  The question raised by the variance in Eto shown in this chart is, How much of, and what parts of, the IRWD Central Zone have significantly different Eto than that recorded at the Central zone’s weather station?

Apparently, a study of this situation is either planned or underway, involving IRWD, UCI, and the University of California Cooperative Extension, and IRWD has or will put in place some
additional weather stations within their service area  to gather more data.  We are looking forward to learning more about this situation, and will pass on what we know when we know it (or, what we think we know when we think we know it?).


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