Databases > Evapotranspiration Data > Explanation
California Evapotranspiration Data
Explanation
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.