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.