Ontogenetic Irrigation Scheduling

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William G. Gensler

Agricultural Electronics Corporation

Tucson, Arizona


The two decisions that must be addressed in irrigation scheduling are when to water and how much to water. Measurements to support these decisions can be made with sapwood water content sensors. These sensors are resident in the sapwood of the main stem for at least twelve consecutive months. This period encompasses the dormancy period prior to bud break in the spring, the seasonal production of fruit during the summer and the return to dormancy in the fall. Water content in the tree as a whole can be determined from sapwood water content sensors during spring, summer, fall and winter.

In a standard installation, fifteen sensors are implanted in fifteen consecutive trees or vines in a single row. Measurements are taken every half hour, 24/7. In 2009, data from these sensors and the corresponding observation of fruit development has led to the method of ontogenetic irrigation scheduling. The basic hypothesis of this method will now be discussed.

Basic Hypothesis

The plant undergoes a sequence of four phases in its twelve month cycle.

Phase 1: Blossom Development and Bloom

Phase 2: Fruit Development

Phase 3: Fruit Maturation

Phase 4: Post Harvest and Winter Dormancy

The development cycle of the fruit is contained within the twelve month cycle of the plant. In botany (and biology as a whole), this development cycle is referred to as the ontogeny of the fruit. Ontogenetic irrigation scheduling is water application that conforms to and enhances the plant's intrinsic water requirements during all four phases described above.

Although the reproductive cycle is associated with a particular part of the plant structure, namely the fruit, the reproductive cycle provokes concomitant phases in other parts of the plant. For example, the water content of the sapwood is adjusted by the plant to support the requirements of fruit development in each phase of fruit ontogeny. The hypothesis of ontogenetic irrigation scheduling is that the level of sapwood water content is correlated with the fruit development phases. Setting levels of sapwood water content is tantamount to setting levels of fruit water content. This hypothesis is supported by data taken in wine grapes and almonds in 2009 wherein the sapwood water content levels have followed the ontogenetic pattern of fruit development.

Ontogenetic Irrigation Scheduling

The sapwood water content is intrinsically set by the ontogenetic phase of the fruit from pre-bloom to harvest. Under fully irrigated conditions, the plant sets an upper bound to the sapwood water content. Further irrigation will not result in an increase in water content. The plant will accept water to rehydrate the canopy in the short term but will return to the upper bound in a day, or at most, two or three days. Any water application when the plant is in an upper bound and full canopy hydration condition will elicit no response in the sapwood water content. The sapwood water content target level can be used as a measure of the upper bound for the entire twelve months, pre-bloom to harvest, harvest to pre-bloom. Furthermore, this target level should be valid from year to year. The upper bound is site sensitive.

If the irrigation manager chooses to operate below the upper bound, then water application can be restricted. If the irrigation manager wants to determine the upper bound a sequence of stepped water applications will determine the upper bound. If the irrigation manager chooses to operate at the upper bound during each ontogenetic phase, then water application should be periodic to hold the sapwood water content to the upper bound.

January, 2010

An example of the sapwood sensor output is shown below. This report covers the period April 1 through December 1, 2009.

Sapwood Results

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