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California Evapotranspiration Data



Evapotranspiration values have been developed for this section using the method outlined in Food and Agriculture Organization Irrigation and Drainage Publication No. 56

Irrigated agricultural crops in California were modeled on a daily basis for 13 ETo Zones that have been established by the California Dept. of Water Resources (DWR).  These include the major irrigated agricultural areas in California.  California Department of Water Resources ETo Zone Map separates California into 18 zones.  For this project only 13 zones were looked at in determining ETc.  The other zones had very little irrigated agriculture, which was grouped into the closest ETo zone.

Although the ETc values were computed for four major soil types, the tables presented here represent average values of all four soil types. 

The evapotranspiration values are dependent upon assumptions of crop growth stage length, planting and harvest dates, irrigation scheduling, pre-irrigation dates, leaching irrigations, and other parameters that are crop and management specific.  Information was obtained from University of California Crop Calendars, Irrigation and Water District crop calendars, ITRC staff experience, literature searches, and farmer interviews that were conducted by ITRC in spring 2000.

It is important to note that the total annual ETc values in ITRC tables include evaporation during non-growing periods which can be considerable in wet years.  As most published estimates of crop ET are based only on the growing season, you will find that ITRC values can be higher than many other published values.

The small vegetables crop category is based primarily on lettuce in most regions.  It assumes lettuce has been double cropped.  Drip irrigation on small vegetables in the coastal regions assumed sprinkler irrigation for pre- and early season irrigations (3 irrigations total) and surface drip irrigation for the remainder of the irrigation season (about six irrigations of about an 8 hour duration each).

Water stress was taken into account for the scheduling of appropriate crops, such as processing tomatoes and wine grapes.

Note: The difference between the ET values in the water balance section and the ET values in the scheduling and design section is that the ETc values for a water balance must recognize that fields are not blanketed by pristine conditions. Studies in California by the ITRC have concluded that about 10% of acreage in California suffers from bare spots and/or decreased vigor.  Bare spots and decreased vigor causes include, but are not limited to, salinity, non-uniformity of irrigation, under-irrigation on parts of the field, disease, poor initial crop stands, farm implement damage, and pest damage. Obviously, transpiration is lower on bare/weak spots than in the rest of the field.  However, because bare spots are irrigated with the same frequency as the rest of the field, evaporation from these regions is higher than in the rest of the field.  The net result is an overall reduction in actual field crop ETof 7-8%. This will vary by crop because of the difference in growing season and cover.  The details are in the CALFED/ARI Evaporation Study Report.

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