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The basic equation for Evapotranspiration (ET) based scheduling is:
ET Crop = Kc x ETo
ET Crop is the crop evapotranspiration, usually measured in inches/day
ETo (or sometimes ETr) is the evapotranspiration of a reference crop (usually either alfalfa or grass)
Kc is a crop coefficient, and is a dimensionless number. Kc tells how much water your crop uses in comparison to the reference crop. For example, Kc = 1.1 means that at the particular instance under consideration, your crop uses 10% more (1.1 x) water as does the reference crop.
Note on Reference Evapotranspiration (ETo):
Much research has allowed us to estimate ETo based on these 4 weather variables:
The weather may vary from day to day and can vary with location. But for a given set of weather conditions, ETo can be used for all crops grown on that day in that location as measured by a CIMIS station.
Note on Crop Coefficients (Kc):
Kc will vary with
These things depend on the crop only, so we can use Kc at all locations where that crop is grown.
Detail on Crop Coefficient
At the detail level, Kc also depends on irrigation management, which may not be the same at all locations. But we often assume the "typical" or "normal" irrigation practice is being used, so Kc is regarded as being transferable from one location to another.
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Kc is subject to 2 adjustments that irrigation management can affect. One has to do with a wet soil surface after an irrigation, and the other has to do with possible soil moisture stress in the last days before an irrigation.
The basic equation for adjusting Kc is this:
Kc = Kcb x K Stress + K evap
Kc is the crop coefficient after the adjustment.
Kcb is called the "basal crop coefficient." Kcb is defined at the crop coefficient that will be observed when the soil surface is dry (no soil surface evaporation), and there is adequate moisture in the root zone to prevent any decrease in relative ET.
K Stress is an adjustment factor that decreases Kc when the soil is so dry that relative ET is decreased. Remember this graph from out earlier studies?
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After the soil dries out to a certain point, the relative ET and relative crop yield will decrease.
Note:
K evap accounts for the extra water use due to evaporation from a wet soil surface. As we have seen, evaporation from a wet soil surface is at its maximum rate for about 1 day after the soil surface is wetted, and then the evaporation amount decreases in stage 2 evaporation, as shown below.
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As stage 2 evaporation decreases to essentially zero, K evap = 0.
Note that when the soil surface is dry and there is enough moisture in the root zone so that relative ET is at 100%, K Stress = 1.0 Under these conditions,
Kc = Kcb x K Stress + K evap
Kc = Kcb x 1.0 + 0
Kc = Kcb
Practical Adjustments
The remaining adjustments to irrigation scheduling have to do with the practicalities of irrigation systems. In the following sections, you will find examples for these irrigation systems:
