Cultivating Olive Trees. Irrigation – Part 4

Olive trees have specific water requirements and the amount and timing of water application can significantly impact their growth and productivity. Without proper irrigation, olive trees may become stressed and produce lower yields, compromising the economic viability of the orchard. Additionally, in regions where water resources are limited, inefficient irrigation practices can have negative environmental impacts, such as soil degradation and depletion of groundwater resources.

While cultivating olive trees irrigation practices can help to conserve water, reduce soil erosion and nutrient loss, and improve overall soil health, contributing to the long-term sustainability of olive orchards. Therefore, olive growers and producers must prioritize sustainable irrigation practices to ensure the economic and environmental sustainability of their agricultural activities.

Understanding the principles and practices of olive tree irrigation is essential for olive growers and producers. This involves knowledge of factors such as soil type, climate, tree age, and water availability and the use of appropriate irrigation systems and strategies to maximize olive production while minimizing water use.

4.1. Draught Effect on Olive Trees

The olive tree can tolerate low water supplies in the soil because it has developed morphological and physiological characteristics in dealing with drought stress (source).

Therefore, excessively dry seasons with poor rainfall result in reduced vegetative growth and a significant decline in productivity (low yield and alternate bearing) in favor of developing the root system (source).

Olive trees continue to photosynthesize at a high rate, even during prolonged drought. It has excellent efficiency because of its capacity to continue to absorb carbon dioxide and produce carbohydrates in water shortage conditions. A higher photosynthetic rate mainly determines better drought resistance in olive cultivars during drought conditions (source). Usually, when water is not restricting growth, olive trees expand photosynthetic tissues, maximizing light absorption and growth.

For example, the Mediterranean regions tend to have a high atmospheric evaporative demand, water deficiencies, and escalating adverse effects of climate change. The annual rainfall ranges from less than 40 cm (16 in) to over 80 cm (31 in) in southern Greece (source). Therefore the lack of rainfall requires irrigation during the dry season to maintain the constant production of olive orchards.

4.2. Olive Trees Irrigation Benefits

Many studies show that olive trees irrigation has significant benefits in the following (sources 1, 2, 3): 

• Increased yield
• Larger olive fruit size
• Higher fruit number
• Better oil content
• Positive impact on pulp-to-pit ratio, phenology, time of olive maturation

Proper soil water availability promotes vegetative growth, including shoot length, allowing olive trees to generate more buds to serve as the ideal foundation for the following year’s yield (source).

The optimal irrigation management must consider the effects of biennial bearing (or alternate bearing) because stress levels and water requirements are significantly influenced by fruit load. Although the biennial bearing is largely genetically controlled, environmental factors, particularly the weather and growing practices, have a significant impact.

With irrigation, olive production can grow up to five times compared to an olive orchard in dry arid climates. On average, a double production must be anticipated in the Mediterranean climate (e.g., Greece or Italy). The olive yield will depend on the following factors (source): 

• Soil
• Average rainfall
• Temperatures
• Evapotranspiration
• Olive cultivars
• Planting distances
• Other cultural techniques 

For instance, table olives cannot be cultivated without irrigation because proper irrigation management keeps leaves engaged in processes encouraging fruit growth and the accumulation of water reserves in the various olive tree’s organs, especially during the summer drought. 

4.3. Irrigated vs. Non-irrigated Olive Trees

Irrigated olive trees generally have higher yields and better fruit quality than non-irrigated trees. However, non-irrigated olive trees may have a deeper root system and be better adapted to drought conditions, resulting in higher oil quality and better resistance to certain diseases. Additionally, non-irrigated olive orchards may require less management and input costs than irrigated orchards.

Some researchers (sources 1, 2) could not identify any difference in oil content between irrigated and non-irrigated olive trees.

In contrast, other research (source) observed a decreased oil content in the fruits of trees cultivated under severe soil water deficit circumstances. The study indicates that irrigation has a highly cultivar-specific effect on olive fruit and oil yield (source). Despite the growing use of irrigation in olive orchards, the impact of irrigation deficit on the qualitative parameters of olive oil is still little understood.

Increasing irrigation results in olive fruits with a higher water content (i.e., lower oil %). Also, irrigation reduces polyphenol content (source), adjusting olive oil’s bitterness and intense flavor.

According to several studies on the impact of irrigation on the fatty acid composition of olive oil, irrigation increases free fatty acids in the oil (source), can change the composition of fatty acids (source), and improve organoleptic characteristics of the olive oil (source).

Choosing between irrigated and non-irrigated olive trees depends on local conditions, available resources, and management goals.

4.4. Factors Influencing Olive Tree Water Needs

Determining the water requirements of olive trees is important to ensure that they receive the right amount of water to grow and produce fruit. Here are some factors to consider when determining the water requirements of your olive trees:

• Soil type

The type of soil your olive trees are growing in affects how much water they can hold and how quickly it drains. Soils with high clay content have more water and drain more slowly than soils with high sand content. Understanding your soil type can help determine how often and how much water to apply.

• Climate

Temperature, humidity, wind, and solar radiation affect how much water olive trees need. On hot, dry, and windy days, olive trees lose water through transpiration more quickly and may require more frequent watering.

• Planting density

The spacing of your olive trees will affect the type of irrigation system needed. If the trees are planted closely together, a sprinkler system with a low profile and a low trajectory may be required to avoid spraying water over the trees.

• Age of trees

Young olive trees require more frequent watering than mature trees. As olive trees grow and mature, they become more tolerant of drought and require less water. The water requirements of olive trees are typically highest in the first few years after planting.

• Cultural management (e.g., fertilizing, pruning)

Proper pruning can improve water uptake efficiency and reduce water stress, while over-fertilization can increase water demand and reduce water use efficiency. In addition, soil management practices such as mulching and cover cropping can help conserve soil moisture and reduce water loss from evaporation.

• Crop Stage

The water requirements of olive trees vary throughout the growing season. For example, olive trees require more water during fruit development than during the dormant period.

• Method of irrigation

The efficiency of the irrigation system you use can affect the water requirements of your olive trees. Drip irrigation, for example, delivers water directly to the root zone and can be more efficient than sprinkler irrigation.

Nevertheless, there are some crucial periods in the olive tree’s annual growth cycle when the tree requires water the most. 

The first one starts with bud differentiation and goes all the way to flowering and, consequently, fruit set. During these stages, a water shortage might interfere with flower development, resulting in fewer flowers for inflorescence, an increase in ovarian abortion, and a reduced fruit set. During this period, there were no water shortages in the soil in the Mediterranean countries such as Greece or Italy. 

During the second period of olive growth (when the pit hardens), olive trees are most resistant to water shortage (source).

In the third period, when the olive oil is accumulated, an olive tree appears again to be very sensitive to water stress (source)

4.5. Calculation of Water Needs in Olive Orchard

For the calculation of the water requirements in an olive orchard, a few formulas are taken into account these factors:

1. Tree age and size: Young trees require less water than mature trees, and smaller trees need less water than larger trees.
2. Soil type and quality: The soil type and quality can affect water holding capacity and water uptake efficiency.
3. Climate and weather conditions: Temperature, humidity, wind, and rainfall can all affect the water needs of olive trees.
4. Tree spacing and canopy cover: Trees with broader spacing and lower canopy cover may require less water than trees with denser canopies.
5. Irrigation method: The irrigation system’s efficiency can affect water delivery and uptake.
6. Management practices: Cultural management practices such as fertilization, pruning, and weed control can affect the water needs of olive trees.
7. Potential evapotranspiration (ETP): ETP estimates the amount of water lost from the soil through evaporation and transpiration by plants under ideal climatic conditions. It indicates the maximum amount of water required to meet plants’ water needs in a given area.

The water deficit will be determined by the difference between the amount of water needed by the olive, and the amount of water available in the soil will determine the water deficit. The deficit will be compensated through irrigation to maximize the olive’s potential for productivity.

The amount of irrigation water needed for olive trees depends on various factors such as soil type, climate, tree age, and planting density. However, the following steps can be used as a general guideline to calculate irrigation water use:

• Determine the water requirement of olive trees in your region based on the climate data and tree age.
• Measure the soil’s water-holding capacity and estimate the amount of water available to the olive trees.
• Calculate the irrigation water requirement by subtracting the available water from the water requirement of the olive trees.
• Determine the irrigation system’s efficiency and adjust the irrigation water requirement accordingly.
• Monitor the soil moisture regularly and adjust the irrigation schedule as needed.

4.5.1. Irrigation Formula

There are several methods for calculating olive tree water needs for irrigation, but one commonly used formula is the Penman-Monteith equation. The equation considers various factors such as temperature, humidity, wind speed, solar radiation, and crop characteristics to estimate the amount of water lost through evapotranspiration, which is the combined water loss from soil and plant surfaces. The formula is as follows:

ETc = Kc x ETo

Where:

• ETc = Crop evapotranspiration (mm/day)
• Kc = Crop coefficient (dimensionless)
  ET0 = Reference evapotranspiration (mm/day)

Reference evapotranspiration is the amount of water that would be lost from a well-watered, uniformly growing grass cover, which is determined by meteorological data.

Crop coefficient represents the amount of water needed by the specific crop, and varies depending on the crop growth stage, canopy cover, and other factors.

By multiplying the reference evapotranspiration with the crop coefficient, the formula gives an estimate of the water needs of olive trees for irrigation. The specific values for Kc and ET0 may vary depending on the location, soil type, and other factors.

Therefore, it is important to consult with local agricultural experts to determine the appropriate values for the specific conditions in the olive orchard.

4.6. Irrigation Methods

Several irrigation methods can be used for olive trees, depending on water availability, soil type, tree spacing, topography, and other factors. The most popular irrigation techniques for olive trees are listed below.

Irrigation Method	Conditions to Use It
Sprinkler Irrigation	– Low Humidity – High Temperatures – Relatively Flat Topography
Drip Irrigation	– Limited Water Availability – Uneven Topography – Widely Spaced Trees
Subsurface Irrigation	– Limited Water Availability – Saline Soil Conditions – Uneven Topography
Flood Irrigation	– Relatively Flat Topography – Uniform Soil Type – Low Water Costs

4.6.1. Sprinkler Irrigation

With sprinkler irrigation, water is dispersed using either stationary or mobile overhead sprinklers. 

This method suits large olive orchids with uniform tree spacing and relatively flat topography. The sprinkling technique can adapt to any soil conditions, move and transfer materials efficiently, and intervene quickly.

The downside of sprinkler irrigation compared to other methods is inefficiency which may cause water loss from evaporation and wind drift. Also, they typically come at a high cost.

When to Use Sprinkler Irrigation?

Sprinkler irrigation can be used for olive trees during periods of low humidity or high temperatures when the water demand of the trees is high. It can also be used in orchards with relatively flat topography, where applying water using other irrigation methods is difficult.

However, sprinkler irrigation may not be suitable for all olive orchards, especially those with higher tree densities or sloping terrain. Overhead sprinkler systems can result in water loss from evaporation or runoff and may also increase the risk of fungal diseases.

4.6.2. Localized Irrigation

Drip irrigation is a localized irrigation type that supplies water directly to the root zone through a network of emitters or drippers. 

This method works well in regions with poor water supply, sloping topography, or inconsistent tree spacing. 

Drip irrigation can be more efficient than sprinkler irrigation, reducing water loss from evaporation and wind drift.

Localized drip irrigation, which enables water distribution evenly in sloping terrain, is a technique that provides the opportunity to intervene during some biologically important olive tree phases (flowering, fruit setting, pit hardening, etc.), resulting in a significant reduction in water consumption of about 25–30%. Additionally, it enables shorter shifts and more consistent water distribution throughout time, increasing irrigation efficiency by up to 90% while preventing losses from evapotranspiration, runoff, etc.

With sprinkling irrigation, the volumes are calculated for the entire surface. The localized irrigation on the wet surface equals 10% with drip irrigation and 25% with microjets. Microjets make it possible to irrigate a sizable area to fulfill the different needs of the olive tree as it grows. Still, they also limit the tillage and mechanical weed removal and increase water loss through evapotranspiration.

When to Use Drip Irrigation?

Drip irrigation is suitable for olive trees in areas with limited water availability or in orchards with uneven topography or widely spaced trees. Drip irrigation systems deliver water directly to the root zone of the trees, reducing water loss from evaporation and runoff.

Drip irrigation also benefits olive trees during the fruiting stage, when water stress can reduce fruit quality or yield. However, it is essential to ensure that the system is designed correctly and maintained to prevent clogging of the emitters and ensure uniform water distribution throughout the orchard.

4.6.3. Subsurface Irrigation

Subsurface irrigation distributes water beneath the soil’s surface using buried drip lines or porous pipes. 

This method can be more efficient than surface irrigation and reduce water loss from evaporation and wind drift. 

However, subsurface irrigation may require more management and maintenance than other methods.

When to Use Subsurface Irrigation?

Subsurface irrigation suits olive trees in areas with limited water availability, saline soil conditions, or uneven topography. It involves the placement of buried perforated pipes or drip tape beneath the soil surface to deliver water directly to the root zone of the trees.

Subsurface irrigation is a low-pressure system that reduces water loss from evaporation, runoff, and drift. It also minimizes weed growth and increases nutrient uptake by the trees. However, subsurface irrigation can be more expensive to install and maintain than other irrigation systems and may not be suitable for all soil types or orchard layouts.

4.6.4. Flood Irrigation

Flood irrigation uses furrows or basins to distribute water to the soil’s surface. 

This method works well in olive orchards with flat topography and affordable labor. 

However, flood irrigation may not be as efficient as other methods and may cause soil erosion and water loss from runoff.

When to Use Flood Irrigation?

Flood irrigation is suitable for olive trees in orchards with relatively flat topography and a uniform soil type, where water can be applied uniformly across the entire orchard. It can also be used in areas with relatively low water costs and ample water availability.

However, flood irrigation can be less efficient than other irrigation methods and may result in water loss from evaporation and runoff. It can also increase the risk of soil erosion and the leaching of nutrients from the soil.

4.7. Deficit Irrigation Strategies

Numerous studies have tried to adapt deficit irrigation techniques to olive trees in recent years. It is a water management strategy that involves intentionally reducing the water applied to olives to increase water use efficiency and reduce water consumption.

The controlled irrigation deficit is a water management technique during the growing season that only partially satisfies all the tree’s water needs. It results in a brief and controlled water deficit during a particular phenological stage.

When applied during the pit-hardening phase, the olive oil yield is unaffected while the water usage efficiency (WUE) is enhanced.

On the other hand, when the controlled irrigation deficit is used from fruit set through harvest, the water usage efficiency and several other aspects of the olive oil’s quality increase while the oil yield decreases.

As the productive tree responses are not affected by moderate levels of water stress, irrigation deficit strategies are recommended in arid and semi-arid areas to save scarce conventional water resources (source).

Final Thoughts

In summary, selecting the appropriate irrigation method for olive trees is essential in achieving optimal tree growth, fruit production, and water use efficiency. The choice of irrigation method will depend on several factors, including soil type, tree age, tree spacing, topography, water availability, and management practices.

Drip, localized, and subsurface irrigation are among olive trees’ most efficient irrigation methods as they deliver water directly to the root zone and reduce water loss from evaporation and runoff. Sprinkler and flood irrigation may be suitable for specific conditions but can result in higher water loss and lower efficiency.

It is also important to consider implementing deficit irrigation strategies to reduce water consumption while maintaining fruit yield and tree health.

Ultimately, selecting an appropriate irrigation method and management strategy for olive trees requires careful consideration of site-specific factors and may need a consultation with local experts. Regular monitoring and adjustment of irrigation practices can help optimize water use efficiency and maintain the health and productivity of olive trees.

Read Next

Cultivating Olive Trees: Soil – Part 1

Cultivating Olive Trees: Soil Management – Part 2

Cultivating Olive Trees: Nutrition – Part 3

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