Impacts Of Drought On Almond Production

David Doll, University of CaliforniaCalifornia is in an extreme drought. It has been difficult for many farmers to find water to maximally produce the 400-plus commodities grown in California, which represent nearly half of the fruits, nuts, and vegetables produced in the U.S. Lack of snow and rainfall has reduced the amount of water stored, decreased the leaching of salts from soils, and reduced groundwater recharge. These issues are compounded by increased plantings of perennial crops, such as tree nuts, which cannot be fallowed. As a result, less surface water is available, forcing growers to either rely on groundwater, or to reduce the amount of water applied to fields.

Owing to their high profitability in recent years, most of the expansion in perennial crop acreage has been in almonds. Almonds are drought-tolerant, but high yields require considerable water application. For example, although almonds can survive on as little as 7.6 inches of water annually, mature, high-yield orchards in Kern County require as much as 56 inches of applied water.

As a consequence of the drought, water availability will be limited in many major production areas. When determining how to manage the reduced water resource, compare the amount of water available to the amount traditionally applied to an orchard in a normal, non-drought year. This percentage of available water should be applied at each scheduled irrigation timing in order to spread the water out across the season. For example, if the water allocation is 50% of normal, apply 50% of the amount of water that would normally be applied at each scheduled irrigation. This “spread out evenly” strategy is recommended because it reduces extreme tree stress events. However, if the allocation is only reduced by only 15% or less (that is, 85% of normal), the strategy of managed deficit irrigation should be used.

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Growth, Yield Reduced
While the above “spread it out evenly” strategy will reduce the effects of drought-related stress, reductions in tree growth and yield should be expected. Stress imposed on the tree from reduced irrigations will close stomatal openings, reducing gas exchange and photosynthesis. Stoma, or the “holes” on the leaf surface, close when water loss from transpiration exceeds the amount of easily accessible water. Without CO2, photosynthesis stops, reducing the production of carbohydrates for the anabolic processes of vegetative growth, as well as fruit and kernel development.

Responses to water stress depend on when the stress is imposed. Impacts on vegetative growth, fruit and kernel development, and floral bud development are outlined below.

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2013-09-06: Almond trees exhibiting leaf drop due to severe stress. Periods of extreme stress should be avoided to reduce the chance of total tree defoliation.  (Photo Credit: David Doll)

2013-09-06: Almond trees exhibiting leaf drop due to severe stress. Periods of extreme stress should be avoided to reduce the chance of total tree defoliation.
(Photo Credit: David Doll)

Vegetative Growth: Moderate to severe water stress applied long enough at any point in the growing season will reduce vegetative growth. Lackof canopy growth due to pre-harvest irrigation deficits leads to a reduction of fruiting spurs, which can reduce future yields. One year of reduced spur production won’t necessarily lead to a dramatic decrease in next year’s yield, but the effect can be cumulative. If the viable spur pool is already limited due to a previous year’s droughts, future yields will drop. This phenomenon has been observed in trials in Spain and California, where fruit loads were unaffected by applied water stress in the first two trial years, but were reduced in the final two years of the four-year California study.

Fruit Bud: Severe stress from deprivation of postharvest irrigation has been found to decrease next year’s crop yield more than pre-harvest water deficits. Studies have shown that significant postharvest stress caused a 52% reduction in bloom density and a 94% reduction in fruit set, resulting in a 73.6% reduction in the following year’s yield. This reduction is attributed to stress impacts on the late season floral bud development. It is important to note that trees in this study were under severe stress, reaching -40 bars pre-dawn leaf water potential, enough stress to cause defoliation. Therefore it is important to minimize stress during this period in order to maximize next year’s production.

In-Season Kernel Development: Water stress also impacts almond production in the current season. This reduction is not as severe as what to expect from next year’s crop, but is noticeable by impacts on kernel size and quality. Impacts on kernel yields are dependent on stress timing. Severe tree stress rarely occurs during the initial period of Stage 1 fruit growth (petal fall through late April/May) due to stored soil moisture, but if it does, smaller fruit and kernel size will be observed because of reduced cell division and expansion. Stress during Stage 2 fruit development (late April/May through late May/June) leads to a reduction in carbohydrates being allocated for kernel fill, resulting in smaller kernel size. Stress imposed during Stage 3 (June through harvest) will reduce kernel dry weights, and produce textured or shriveled almond kernels. The kernel weight loss during this stage is due to reduced carbohydrate production as well as a redirection of carbohydrates away from the kernel due to an early, accelerated hull split.

Kernel shriveling can occur from water stress applied during the beginning (May through mid-June) or the end (between hullsplit and harvest) of stage three. The “texturing” observed in this photo was from a water deficit applied between hullsplit and postharvest. (Photo Credit: David Doll)

Kernel shriveling can occur from water stress applied during the beginning (May through mid-June) or the end (between hullsplit and harvest) of stage three. The “texturing” observed in this photo was from a water deficit applied between hullsplit and postharvest.
(Photo Credit: David Doll)

There are, however, periods where almonds are tolerant to water stress. Stress applied in the latter half of Stage 3 fruit development has shown to have minimal impact on kernel yield and quality. A study used reduced irrigations to achieve and maintain moderate stress levels of -15 to -18 bars mid-day stem water potential for four weeks, between the initiation and completion of hull split. Prior to and after this period, trees were irrigated fully. This applied and regulated deficit did not impact kernel yields, but did reduce kernel weight by 2%-3% compared to the full irrigation treatment. A 10%-15% reduction in the season’s water budget was observed with the application of the deficit treatment. This strategy of managed deficit irrigation is recommended for situations in which there is a 15% reduction in allocated water.

What can a farmer expect if water applications are reduced? In 2009, a severe water shortage was experienced by several almond growers along the Interstate 5 corridor in California on the west side of the San Joaquin Valley. In one case, the growersr was only able to apply 12 inches of water to a mature, full canopy, high producing (less than 3,000 pounds per acre) orchard. Excluding soil moisture, this application was equivalent to only 25% of his historical water application. Irrigation was initiated after his trees hit -12 bars mid-day stem water potential, and water was applied at 25% of normal at each irrigation (the “spread it out evenly” strategy). Although crop yields and quality dropped that year, he still had a harvestable yield. The next year, he was able to fully meet the trees’ water demands, but crop yields were dramatically reduced. It took another year of full irrigation to bring the trees back to full production. This response was expected by researchers, and colleagues in Australia reported a similar response from their almond orchards during their historic drought of 1995-2007.

Should a farmer just apply any water that could be found? Decreased availability of surface water will have many farmers turning to groundwater in order to maintain orchard productivity. If groundwater is to be applied, a water analysis must be performed to determine suitability for almond trees. Water that has an electrical conductivity (ECw) above 1 dS/m must be applied with a leaching fraction to reduce accumulation of salts, which can accumulate to toxic levels. Water with an ECw above 3 dS/m will cause a severe growth and yield reduction, and alternatives should be found. If no alternatives are available, a properly calculated leaching fraction needs to applied, along with a winter leaching program, to reduce the risk of sodium toxicity. It is important to remember that it may take years to reclaim saline ground from just a single season of irrigation with poor quality water. In some cases, the accumulated salts will kill the trees — in which it may have been better not to apply the water at all!

To maintain the highest-possible-producing orchard, periods of severe stress should be reduced. In managing orchards with limited water, applications should spread out the available water at the percentage of ETc available. This strategy will reduce the extremes in stress level, minimizing the impacts of the water deficit. Both the current and next year’s yield will be reduced, with greater reductions in next year’s yield due to impacts on vegetative growth and fruit bud development, but the trees will survive.

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