CIMIS Data Calculating Instructions

• The Distribution Uniformity (DU) of your irrigation system which is a number, given as a percentage, assigned to your irrigation system by a soil conservation analysis. Use .85 (85%) if you are not sure of your efficiency or don't have pressure regulators or pressure compensating sprinklers.

• The crop coefficient (Kc) provided in the table below.

• The number of trees per acre. There are 43,560 square feet per acre. You can take the tree spacing, 20'X20' for example which will give you 400 square feet per tree or approximately 109 trees per acre. Use the spacing table given below, as a guide.

• The output of your sprinkler in gallons per hour. If you are not sure, take a liquid measuring device or a gallon container and collect the water from one sprinkler for 5 minutes. Multiply the volume of water measured times 12 to get gallons per hour. You can take a sample sprinkler to the irrigation supply house to find its output.

• The electrical conductivity of your water (ECw). If you use district water you can get the information from them or get yourself an inexpensive EC meter. Use the formula to calculate the leaching fraction (LR). If you are not sure use 10%.

• The estimated electrical conductivity of your soil (ECe) corresponding to acceptable yield potential. If this number is not available to you then you could multiply times 2 your ECw. For example, where ECw = 0.9 dS/m your corresponding ECe would be 1.8 dS/m.

Don't bother with this number if you intend to use a fixed LR, such as 10%, as mentioned above.

The following formula is for full grown trees with a canopy ground cover exceeding 65%.

A more detailed explanation with some advanced calculations follows:

((ETo X Kc / DU) X 27,154 gal per acre inch / number trees per acre / sprinkler output in gallons per hour)) +10% LR = hours of water needed per tree per day.

Explanation:

Evapotranspiration of crop = Evapotranspiration number from CIMIS data (ETo) X crop coefficient from table (Kc) (closest day or the average ETo for the previous week)

ET crop = ETo X Kc

This will give you the evapotranspiration in inches for your avocado tree.

Distribution Uniformity (DU) of the irrigation system, from soil conservation analysis or .85 = 85% if you are not sure. (If your grove is located on hilly terrain with no pressure regulators, DU will surely be lower than 85%).

ETc = ETo X Kc / DU

For example: if CIMIS reports ETo to be .22 and the coefficient for the month is .65 (65%) and the Distribution Uniformity DU is .85 (85%) then your ETc (Evapotranspiration coefficient) is as follows:

ETc = (0.22 X .65) / 0.85 = 0.168 inch

There are 27,154 gallons per acre inch. When multiplied out, the water requirement when ETo = .22 inch (ETc = .168 inches) would be as follows:

.168 inch X 27,154 gal/acre inch = 4,568 gal/acre

If your tree spacing is 20' X 20' then you have approximately 109 trees per acre (this does not account for roads and pads). Thus :

4,568 gal/acre / 109 trees/acre = 42 gal/tree

In most areas there is a need for leaching as a result of salt buildup mostly from your irrigation water. The leaching fraction could be calculated using the formula:

LR = ECw / (5(Ece) - ECw)

If you prefer not to do this calculation, a 10% LR is reasonable to assume. You can also look up the data in the table for leaching fraction at the end of these instructions.

Thus if your irrigation requirement per tree per day is 42 inches, then a 10% LR will add another 4.2 gal/tree to a total of about 46 gal/tree (you can multiply your resulting water requirement by 110% or 1.1 to get the same result). If your sprinkler output is 17 gallons per hour then your irrigation schedule should be 46 gal/tree / 17 gal/hr = 2.7 hours per tree per day.

* It is important, during rainfall months that rain events greater than 1/4 inch are subtracted from ETo.

The Crop Coefficient for Avocados

Recently published research by University of California scientists indicates a year-round crop coefficient of 0.86 more accurately represents actual irrigation water demands than previously published numbers by month.

Back to irrigation calculator.


CIMIS Calculator by: Reuben Hofshi, Shanti Hofshi, and Ben Faber