Irrigation and water use in almonds crops

When water supplies are limited, growers must minimize water losses normally associated with irrigation and apply water when the trees are most sensitive to stress.

Certain stages of tree crop development are more sensitive to water stress than others:

Early season (bud break through to fruit set)

Early season water stress compromises a range of growth processes including bud break, bloom, flowering, fruit set, and shoot growth:

• Without adequate shoot development and growth, the fruiting positions for the following year’s crop are affected.
• A reduced ability to supply the carbohydrates required by the developing fruit limits yield potential.
• Slowed leaf growth – and poor photosynthesis – also restricts seasonal tree and nut development.

As a result, common practice is to start the season with a near-full soil water profile, and irrigation is used to maintain a good water supply.

Fruit growth and development

As a general rule, plant water stress should be avoided during rapid fruit growth. Thus, common practice is to minimize water deficits at this stage, when the crop is most susceptible to drought.

Nut tree growth is generally less sensitive to water stress than fruit trees at this stage. However, other factors, such as hull split in almonds, shell splitting in pistachios, and kernel sunburn in walnuts are affected by large water deficits.

Post harvest

The post-harvest period is generally when trees can tolerate most water-stress. Irrigation can be safely reduced (not omitted) in most species.

The rule of thumb is that as long as the trees do not defoliate, the stress can be tolerated with little impact on production in the following years. The notable exception to this rule is almonds (see below) where water stress post-harvest reduces fruiting in the following season.

Typical almond requirements

In almonds, full irrigation early in the season is important for good growth of the fruiting spurs and therefore necessary for long-term tree productivity. In early to mid-season cultivars, irrigation can be curtailed between mid-June and harvest (for around a two-month period).

Later-harvested cultivars can also tolerate a two-month period of severe water deprivation before harvest. This reduces kernel weight by around 10% and reduces hull splitting; resuming full irrigation two weeks before harvest will increase hull splitting.

In almonds, on shallow or drip-irrigated soils, post-harvest water management is crucial for the following year’s crop. Research in California shows that postharvest water deprivation reduced fruit set and, to a lesser extent, bloom.

On deep soils that are surface or sprinkler-irrigated close to harvest, post-harvest irrigation may not be as important.

Premature defoliation of trees indicates inadequate irrigation.

Pre-harvest defoliation followed by post-harvest irrigation leads to a stronger leaf canopy in the following spring. This practice is now also thought to benefit the tree by improving tree water status during the critical period of flower bud formation.

Fertigation strategy

Effective fertigation is a key means of producing optimum yield, without adversly affecting the environment. Research by UC Davis, California shows that fanjet irrigated trees can outperform drip irrigated trees.

Average yield increases were between 3-7% but, where potassium was continuously fertigated, fanjet irrigation raised yields by 14% compared to drip irrigation.

The most efficient irrigation systems also ensure better utilization of nutrients e.g. nitrogen.

When managed correctly, continuous fertigation will reduce the risk of deep nitrate leaching by maintaining the nitrogen in the root zone.

It also provides greater flexibility to adjust in season fertilizer rates than less frequent irrigation use.

Research at UC Davis shows that applying the nitrogen in the final two hours of the irrigation schedule results in less leaching. Allow sufficient time for the fertilizer to be flushed out of the lines before the irrigation terminates.

For further information on effective fertigation, consult your Yara specialist and ask for Yara's Fertigation Manual.

Water quality

Growers need to assess the salinity of irrigation water – particularly levels of chloride and boron - and adjust practices accordingly.

Irrigation water with a total salt status, or ECw (dS/m) of 1.0 will have little effect on almond yield. However, when salinity increases to 2.8dS/m, yields can be halved.

If saline water must be used because of drought, applying more water to increase leaching can lessen the effects of salinity on plant growth.

Consult with your local Yara agronomist for specific recommendations when dealing with saline or sodic soils or irrigation water.