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ET Irrigation Scheduling Under Microirrigation

Irrigating with microirrigation systems depends less on stored soil moisture than when irrigating with sprinkler or surface irrigation systems. Instead, the irrigation frequency is chosen by the operator, the crop ET is determined during the irrigation interval, and the irrigation amount replaces the soil moisture depletion caused by crop ET since the last irrigation.

Step 1: Determine the irrigation frequency.

Step 2: Track crop ET to determine when cumulative crop ET since the last irrigation.

Step 3: Determine the irrigation amount.

Step 1: Irrigation Frequency

During peak ET periods, drip systems will likely be operated daily while microsprinklers may be operated every 3 to 4 days. During periods of lower ET, the irrigation interval will likely be greater.

Example:Almonds irrigated using drip irrigation in early June near Modesto, CA.
The almonds will be irrigated daily.

Step 2: Determine crop ET

ET information is available as Historical ET - average year ET, and as Real-time ET - ET information based on current weather information. Historical ET information is ready to use with the estimated ET being tabulated for the entire year. Using Real-time ET will require you to access reference crop ET information and then adapt it for use with your crop.

Example:Almonds irrigated using drip irrigation in early June near Modesto, CA.

The almonds will be irrigated daily. Almond ET using historical ET information for almonds in Modesto during June 1-15. would be:

3.28 in.
15 days
0.22 in/day

Since irrigations are daily, the net irrigation amount during this period would be 0.22 inches.

Step 3: Determine the irrigation amount

Microirrigation systems can be very efficient, but some water is still lost during irrigation. Assuming a microirrigation irrigation efficiency of 85 to 95% for a well designed, well maintained microirrigation system is usually satisfactory. The Gross Irrigation Amount includes the Net Irrigation Amount plus water lost to inefficiencies.

Gross Irrigation
Amount (in)
= Net Irrigation Amount (in) x 100
Irrigation Efficiency (%)

Example: The Net Irrigation Amount is 0.22 inches. Assume a drip irrigation efficiency of 85%.

Gross Irrigation
Amount (in)
0.22 in
x 100=0.26 inches
85 (%)

To convert the inches of water to gallons of water, the plant spacing needs to be considered using the following formula:

Water use (gallons)  = Crop Spacing (ft2)  x ET (inches)  x 0.623

Example: For the example above and a tree spacing of 16' x 22' (352 ft2), the gallons of applied water would be:

Water use (gal)  = 352 ft2  x 0.26 in  x 0.623  = 57 gallons

Summary of example:

During the beginning of June, drip irrigation should occur daily with 0.26 inches being applied each day.  If the tree spacing were 16' x 22', the daily applied water would be 57 gallons.