ET Irrigation Scheduling

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ET Irrigation Scheduling

Why is ET 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 root zone 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.


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.




  • 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


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


  • ETc= 2.1 in x 1.15
  • ETc=  2.42 in

Determining 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


In this example we determined that we would need to irrigate this orchard for 122 hours to match last week’s crop demand.

In Conclusion

Creating an irrigation schedule is a great way to ensure you are applying the right amount of water to your crop.  Knowing the environmental conditions as well as your system capabilities is critical when creating your schedule.  Again, for environmental data check out your local CIMIS stations.  For your system capabilities refer to your irrigation map provided by your irrigation dealer.

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