ET Irrigation Scheduling

Determining Orchard Water Requirements

ET Irrigation Scheduling

Why is Irrigation Scheduling important? 

Scheduling and planning your irrigation to fit your crop needs is important for several reasons.  Proper irrigation scheduling enables your irrigator to apply the exact amount of water to fit your goals. Over irrigation not only wastes water, energy, and labor, but also leaches valuable nutrients past the effective rootzone and out of the crop's reach.  On the other hand, under irrigation causes crop stress and results in lower yields.  

Orchard water requirements are a combination of all water needs for your orchard including crop use, excess water for leaching, compensation for non uniformity in the irrigation system, etc.  To determine actual crop use we evaluate the evapotranspiration (ET) for the orchard.  The term ET is a combination of losses for both the soils (evaporation) and the crops leaves (transpiration).  

Accurate rainfall totals and ET information for your location is critical in creating an efficient and effective irrigation schedule.

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Local ET data can be found from various sources, CIMIS offers free ET and Rain data from local weather stations throughout California.  For more accurate data, a weather station on your own ranch will offer ET and Rain data from your exact location.

To the left you can see the data gathered from a personal Davis weather station.  The ET for the last 7 days is shown, as well as the estimated rainfall for the next 7 days.  With this data, you can accurately refill your soil profile week to week.

ET Scheduling

The ET you get from local or personal weather stations is ETo which represents water use by a reference crop, which is grass pasture.  When using ET to irrigate your crop you need to apply your crop coefficient to your ETo data.  The crop coefficient (Kc) is a multiplier representing the water use by your specific crop, giving you ETc which is the number you will use to schedule your irrigation.  Below is a chart with several Kc values for Almonds during the irrigation season.

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EXAMPLE:

GIVEN:

Past 7 days ETo in July- 2.1 inches
Goldhamer Model (Kc) for July- 1.15
Expected 7 Day Rain- 0 inches
Microirrigation System- 75% efficiency
Acreage- 100 acres
Typical Flow Rate- 1200 GPM

FORMULAS:
ETc = ETo x Kc
Gross Irrigation Inches = ETc / DU
In/Hr = (GPM x 96.3) / area in sq.ft.
Hours = Inches Needed / (in/hr)

ANSWER:
ETc= 2.1 in x 1.15
ETc=  2.42 in

Determine Irrigation Amount:
Microirrigation systems are highly efficient however, some water does get lost in each irrigation.  We will assume a 75% efficiency on this system.

Gross Irrigation Inches= 2.42 in / 75%
Gross Irrigation Inches= 3.23 acre inches (crop use and DU compensation

In/hr= (1200gpm x 96.3) / (100ac x 43,560sq.ft.)
In/hr= 115,560 / 4,356,000
In/hr= .0265 inches per hour

Hours= 3.23 in / .0265 inches per hour
Hours= 121.89 hour

SUMMARY:
In this example we determined that we would need to irrigate this orchard for 122 hours to match last weeks crop demand.


Once your required irrigation amount is established, the next step is determining your irrigation frequency.  This will vary from ranch to ranch based on your irrigation system application rate, soil type, and water holding capacity.  For more information on determining your soil type and understanding its characteristics and holding capacity check out The Almond Boards Irrigation Improvement Continuum .   We will be covering this topic in a later newsletter.

Cathleen Olson