Irrigation Scheduling Part 1 – Calculating Orchard Water Use

Scheduling of irrigation must take into account the variables of weather, water availability, and plant stress. All of these variables change throughout the irrigation cycle, providing difficulty in determining irrigation timing and amounts of water applied.

Today, lets start with a brief discussion on how to calculate irrigation needs based upon the weather. This weather driven demand for water by the almond tree is called evapotranspiration (ET). ET accounts for the loss of water through surface evaporation as well as the loss of water through the opening and closing of the stomates. We can calculate ET by knowing two things: the demand for water on a reference crop (ETo), and the crop coeficient (Kc) The Kc transforms the reference crop water use (ETo) into almond tree water use (ETc). The reference crop of use throughout California is grass pasture. Therefore, the multiplication of the reference crop water use and the almond crop coefficient will give us water use by an almond tree – or ETc = ETo x Kc.

When we access CIMIS to view weather conditions and water use, the values are always presented for reference crop. Remember to transform these to represent almond water use.

Since almonds change in canopy coverage throughout the year, the corresponding Kc will also change. Below is a list of crop coefficients as provided by the UN FAO* for almond orchards experiencing light to moderate winds with and without cover crops:

Almond Orchard Kc by Month

Dec/
Jan

Feb

Mar

Apr

May

June/
July/
Aug

Sept

Oct

Nov

With cover Crop 0.85 0.85 0.85 0.95 1.05 1.15 1.10 0.90 0.85
Without Cover Crop 0 0 0.50 0.70 0.85 0.90 0.80 0.75 0.65

Remember that these coefficients are based upon research and further research may refine the values listed above. There is currently ongoign research within California to review these numbers.

So, now that we have our water use by our reference crop and the crop coefficient almond, lets look at a sample calculation of water use by a mature almond orchard. We will use the month of May and a Merced County orchard with a cover crop as an example:

Week ETo for the week
(Grass water use)
provided by CIMIS
Kc ETc for the week
(Almond Water Use)
Cumulative Total of water
use by the Almond Orchard
w/ cover crop
May 1st- 7th 1.65 1.05 1.74
8th – 14th 1.20 1.05 1.26
15th- 21st 1.39 1.05 1.46
22nd-28th 1.19 1.05 1.25
29th- 31st 0.72 1.05 0.76 6.47

As we can see above, a mature almond ochard grown under the described conditions used nearly 6.5 inches of water in May of 2010.

Young orchards, less than 7th leaf, will use a slightly lower crop coefficient depending on their size. Canopy coverage of the orchard should be estimated at mid-day by guessing the area of the orchard floor that is shaded by the tree. This percentage should then be doubled and then multiplied by the corresponding crop coefficient. For example, a 3rd leaf orchard covering 40% of the orchard floor will use 80% of the water that a mature orchard would use. Once the canopy shades 50% of the orchard floor, we assume that it will need full ETc.

Next week, we will discuss irrigation timing and soil water holding capacity differences.

*Dorenbos, J. and W.O. Pruitt, 1984:
Crop Water Requirements – Guidelines for Predicting Crop Water Requirements. – FAO Irrigation and Drainage Paper 24, FAO, Rome. ISBN 92-5-100279-7
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