Crop coefficients (Kc) are used with reference evapotranspiration (ETo)
rate to estimate specific crop water use and
needs. These coefficients are dimensionless
numbers (usually between 0.1 and 1.2) that are multiplied by the ETo value to arrive
at a crop ET (ETc). The resulting
ETc can be used to help an irrigation manager schedule when irrigations should occur
and how much water should be put back in the soil.
An example of how quickly the ETo can change and affect the ETc is seen below.
This is a graph of nut crop water use during a specific time period during the 2007
growing season. This crop has a Kc of 0.8. The ETc is represented by
the blue line while the ETo is the top line. Therefore, on any given day the
crop uses 80% of the water that the refernence crop uses during the height of the
growing season. This graph shows that the ETo varies from 0.12 inches per
day to over 0.33 inches per day in just a two week period near the end of July.
If a grower is using an irrigation controller that is set to deliver a certain amount
of water on a daily schedule independent of weather parameters then the grower probably
would have over irrigated on July 22 and under watered on August 3 due to changes
in crop water use casued by the weather.
Crop coefficients vary by crop, stage of growth of the crop, by some cultural practice,
and the elevation the crop is being grown.
Citrus trees have smaller coefficients than peach trees when the peach tree is in
full leaf. Coefficients for annual
crops will vary widely through the season with a small coefficient in the seedling
stage to large coefficients when the crop is at full cover. Pear orchards with cover crops between
the tree rows will have larger coefficients than orchards without cover crops since
these cover crops also utilize soil moisture.
Apples grown on a south facing slope at 1,500’ will use about twice as much
water as apples grown on a north facing slope at 3,000’ due to the changes in temperature
and humidity of these two setting, and this is reflect in the Kc values for the
The crop coefficients tend to be constant over a wide range of growing areas with
extremes in weather conditions. This
will not be a problem for the irrigation manager if he knows what the coefficients
are for his crops and he has access to regional and/or specific ETo information. With these two pieces of information
the manager can calculate the water use on any given day during the growing season.
Crop coefficients have their roots in the agricultural setting but the same principles
can be applied in the urban landscape setting.These coefficients have been published and are free to the public. Use the following links to find the appropriate coefficient: