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Crop
Curve
A crop will experience different water needs at different growth stages of development
during the year. The best way to represent
this is with a crop curve.
The curve
is generated by plotting the
crop coefficient (Kc)
versus the time of season.
The example shows that in the crop development portion of the graph the Kc increasing. This is due to the development and maturation of the leaf surface.
As the number and size of the leaf increases then
the number of stomata increases so the transpiration rate increases. The transpiration rate increase is directly
related to the ETc. This section of the curve in the EID service district usually starts at bud
break and ends at full canopy development.
During the mid-season section the crop is setting and maturing the fruit and stretches from
full canopy to harvest. The crop curve
normally ends with leaf fall.
Typically
full canopy is reached by mid-May and the harvest occurs in September to October. Therefore the water needs experienced
during the crop development and late season sections of the curve are met with seasonal
rainfall, so the mid-season water needs are met through irrigation. This example also demonstrates that
this particular crop uses more water than the reference crop during the mid-season
because the Kc is greater than 1.
Knowing the growth stage for a crop is critical to determine the water use and needs
for the crop. Water stress experienced
by the crop at certain stages of growth will affect the crop yields and/or quality. An example of this in the District is
wine grapes grown on 5C root stock.
If this root stock experiences any water stress before harvest then fruit maturation
is halted to protect the vegetative growth.
This can severely reduce the yield and quality of the grapes that can be eventually
harvested.
The crop curve shown above is adequate for the majority of the crops grown in the
District since these are perennial crops with well established root zones. Annual or row crops on the other hand can have crop curves with many growth
stages and many different water requirements. A hybrid corn crop curve from
Texas has 17 different growth stages with as many crop coefficients. In this situation it is extremely important to know which stage of growth the crop is in to predict the water requirements for that growth stage.
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