UCCE Sonoma County
University of California
UCCE Sonoma County

Using Evapotranspiration Data to Determine Irrigation Time

Using Evapotranspiration Data to Determine Irrigation Time

ET = The sum of Evaporation of water from the soil surface plus Transpiration (water loss) from (primarily) leaves of plants. 

ETo = "Reference ET" = The amount of full water use by a well irrigated, mowed grass.  ETo varies daily with changes in temperature, relative humidity, solar radiation and wind. 

To obtain historical ("normal") daily ETo go to the UC IPM Weather Data and Products website.  For any given situation, the UC site will obtain missing values from a nearby station.  The site provides access to daily climatic averages of precipitation and min-max temperatures from weather stations located throughout the state.  It also provides historical daily climatic averages.  Daily and historical climatic averages are also available from the CIMIS (California Irrigation Management Information System) operated by the California Department of Water Resources. In CIMIS, missing values are not obtained from nearby stations.


Kc = "Crop Coefficient" = A factor that is used to convert ETo to potential vineyard ET (ETc).  ETc accounts for the amount of sun interception in your vineyard and increases with canopy growth.

Example - Estimating percent of shaded land surface area to determine a crop coefficientab

A. Row Spacing = 8 ft
B. Vine Spacing = 6 ft
C. Area per vine = 48 ft2
D. Width of measured shaded area between two vines at solar noon = 2.4 ft
E. Shaded area per vine = B x D = 6 ft x 2.4 ft = 14.4 ft2
F. Percent shaded area = E / C = 14.4 ft2 / 48 ft2= .3 or 30%
G. Crop coefficient (Kc) = F x 0.017 (slope of relationship between measured Kc and percent shaded land surface area in Thompson Seedless vines grown in a lysimeter) = 30 x 0.017 = 0.51

aModified from L. Williams, "Irrigation of Winegrapes in California", Practical Winery and Vineyard, Nov-Dec 2001
bThis method will determine a crop coefficient adjusted for shade.  A further adjustment may be required for trellis type.

ETc = "Crop ET" =  The amount of full potential water use by a crop.  ETc varies throughout the season due to canopy size and climate.

ETc = ETo x Kc

= Regulated Deficit irrigation coefficient.  Krdi is known by many as the "management factor," as this coefficient can be adjusted to control vegetative growth, conserve water and meet fruit quality targets.  You may choose to vary this coefficient through the season or keep it constant throughout the season.  The rdi factor you choose is related to your overall irrigation strategy at that vineyard.

Common Krdi values for winegrape irrigation range from 0.35 to 0.65 (35% to 65% regulated deficit).  A Krdi value of 1.00 (no deficit) should be used for young vines.

Rules of thumb for selecting Krdi:
a) For shallow soils, use a higher Krdi
b) Red wine grapes tolerate a lower Krdi than white grapes
c) Deficit irrigation strategies may cause mortality to young vines and should only be used on producing vines. Use a Krdi value of 1.00 (no deficit) for young vines.
d) Low Krdi selections may lower crop yield by fruit dehydration.

Soil Water Contribution 
= The amount of water the vine has access to from the soil.  Early in the season, the rate of extraction of available water is higher than late in the season.  After you begin to irrigate, the remaining available soil water is usually parceled out very slowly, thus many times you see spreadsheets that subtract out equal increments of soil water over a set number of weeks.

Effective In-season Rainfall
= The amount of precipitation that becomes available soil water.  Usually determined for each rain event that occurs during the growing season.

Due to evaporative losses and runoff, the total water volume from a rainfall event does not enter the soil.  Many methods exist for determining the relationship between actual rainfall and effective rainfall.  The following formula provides a practical method for estimating effective rainfall from each event.

Effective Rainfall = [Total Rainfall - 0.25 inches] x 0.8

G; H:
Emission Uniformity
A drip system is rarely 100% efficient, thus the number of gallons you need to give each vine is determined by the Emission Uniformity of the block and the average Application Rate of all of the emitters in that block.

Emission uniformity is calculated from sampled discharge of emitters throughout an irrigation block. Once true discharge of emitters has been sampled, emission uniformity can be calculated as:

 [Average discharge of 25% of emitters with least discharge] / [Average discharge of all sampled emitters]

See the following sections of this website regarding how to determine emission uniformity of your system: 

How to Sample Emitters
Calculating Emission Uniformity to Adjust Irrigation Time
Emission Uniformity Calculator Excel Worksheet

After calculating the net irrigation amount, you must adjust that figure as drip systems are rarely 100% efficient.  Most systems should be run slightly longer than one hour for each gallon you want out of a one-gallon per hour emitter.  When deficit irrigation is practiced in a well designed and maintained drip system, then irrigation efficiency is assumed to be the same as Emission Uniformity and can approach 92% At that efficiency, a 1 inch net requirement requires 1.09 inch gross application (1 / 0.92 = 1.09).

Average Application Rate
= The true average discharge of the emitters on each vine.  If the vine has multiple emitters, determine the combined average discharge.  Do not assume emitters to discharge more or less water than specified. 

Example - Calculate hours to irrigate using ETc and Krdi

Sample Date Range: July 1 - 7
Once you decide to begin to irrigate, the Net Irrigation Requirement (in inches or millimeters) for a specific period in the growing season is calculated. (Note: this does not account for the inefficiencies of your irrigation system.)

Net Irrigation Requirement (inches) = ETc x Krdi - soil water contribution - effective rainfall

Etc = 0.84"
Krdi = 0.40
Soil Water Contribution = 0.19"
Effective Rainfall = 0"

 Net Irrigation Requirement = [0.84" x 0.4] - 0.19" -0" = 0.15"

Calculate the gross irrigation amount and then find gallons per vine per week and hours to run the system.

Gallons per vine per week = [Net irrigation requirement (units are inches/wk) x vine spacing (units are square feet) x 0.623] / [emission uniformity (as a decimal fraction)]

EXAMPLE   [0.15 inches per week] x [6ft x 8ft] x 0.623 / [0.92] = 4.87 gallons / vine / wk

How long to run the system per week = [Gallons/Vine/Week] / [Average Application Rate]

EXAMPLE    [4.87 gallons per vine per week] / 0.96 = 5.07 hours per week

Accessing CMIS ETo from the UC Integrated Pest Management website:

  • Go to http://ucipm.ucdavis.edu
  • Under the banner "How to manage pests", click on "Weather data and products."
  • Select a county name and click on "Map" button or just click on "Map" to see the entire state.
  • Click on the county you are interested in.  This takes you to a page that lists each type and location of weather station located in that county and in nearby counties. 
  • Station names ending in '.A' (for 'Automatic') are CIMIS stations and thus contain ETo values.
  • Find the Automatic weather station that you want, then click on "Daily Data"
  • Enter the time period range you are interested in.  You can retrieve daily ETo data from a time period spanning 1 or more years.
  • To download that file to a spreadsheet to calculate average daily ETo values, in the box "Output File Format", click on radio button "comma delimited data file."  Click on "Retrieve data."
  • When the data appear, save it as a text file (.txt) on your hard drive.  Then open MS Office Excel on your computer and select that .txt file to open (make sure Excel can "see" all file types).
  • When the "Text Import Wizard" opens in Excel, select "Delimited" as the original data type.  Click Next
  • Select "comma" as a delimiter and click Finish
  • Save the file to your hard drive as a MS Excel Workbook file (.xls)

University of California Cooperative Extension, Sonoma County
133 Aviation Blvd Suite 109, Santa Rosa, CA 95403  Phone: 707.565.2621  Fax: 707.565.2623
Office Hours:  M-F, 8am-Noon & 1pm-4pm

Like us on Facebook: UCCE Sonoma                        Follow us on Twitter @UCCESonoma 

Webmaster Email: klgiov@ucanr.edu