Almond Irrigation Scheduling: Deciding on a Crop Coefficient
Crop coefficients for almond have been studied for nearly 50 years. These values indicate the ratio of water use of almonds in comparison to fully irrigated grass (ETo). Values less than one indicates that almonds are using less water than grass at that period while values greater than one indicate a higher water use. This ratio needs to be multiplied by the daily ETo, whether actual or estimated, to determine the water usage for almonds. The crop coefficients provided below are research derived. Differing techniques have been used to develop the coefficients. Some rely on a “water-balance” method in which soil moisture levels are monitored and maintained to match water use. This method has been utilized by many researchers across the world and has formed the foundation of most irrigation research. A more exact method of this technique involves the use of a weighing lysimeter, which is a giant scale that measures the amount of water used by the tree. Due to a minimal number of lysimeters in the world, studies have not been performed until recently and the results for mature almond water use has yet to be published. Outside of the “water-balance” method, there are newer technologies that have been used to develop crop coefficients. These techniques utilize fluctuations of canopy temperature that occur from transpiration loss. These measurements, when taking into account solar radiation, wind, and ambient air temperature and humidity, can be used to back-calculate the water loss. This method is known as “eddy covariance.” When reviewing the differing crop coefficients, it will be clear that the general trend is greater water use. Some of the upward trend is due to a change in irrigation practices and orchard canopy densities – the switch from surface/flood irrigation methods to pressurized drip and microsprinkler orchards as well as
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