Almond Drought Management: 2021 Update
Drought conditions are forecasted to persist into 2021 for California. Although almonds are relatively tolerant to drought from a survival standpoint, yields are impacted when water applications are reduced. To minimize this impact, the water use strategy that accounts for available water resources must be developed and applied to the orchard operation. These resources include contributions from stored soil moisture, rainfall, wells, and surface water resources.
Impact of water stress on almonds.
Reduced water applications affect both in-season and future yields. In-season water stress reduces gas exchange, affecting the amount of energy that can be directed into kernel development. This often leads to reduced kernel size and weight, an increase in shriveled kernels, and minimal growth. Typically, nut set is unaffected. Although not exactly clear, the relationship between in-season crop loss and water stress is around 1 to 0.7-1.0, meaning that for every percent of reduced water application leads to the same percentage of crop loss (i.e. a 10% water deficit leads to 7-10% crop loss). Please note that this relationship isn't exactly defined, and these numbers are estimates.
Moderate to severe deficits will also affect next season’s crop. This crop loss is due to the reduced spur positions from the lack of growth and the reduced carbohydrate reserves going into floral bud development. This leads to reduced nut set. Nut weight and size will only be affected if in-season curtailments continue. . Field observations suggest that water shortage in a given year will have a greater impact on crop yields the following year, where the relationship of water stress (from the previous season) and crop loss approaches 1:2. This means that for every percent of reduced water application in year one, year two’s crop will be reduced by about 2% (i.e. a 10% water deficit leads to 20% crop loss). This “hangover” effect occurs even if the tree’s full water demand is met for the second year.
It will take two years of full irrigation to return to the orchard’s production potential. For example, severe water deficits accruing in 2021, will lead to reduced yields in 2022, with near-to-normal yields expected in 2023. This yield recovery would require that future water applications meet the orchard’s needs. If drought conditions and water curtailments continue, yields will be reduced until the second season after full irrigation requirements have been met.
To minimize these effects, the on-farm water resources must be applied as efficiently as possible. Much of the strategy to mitigate the effect of drought can be found in the UC ANR’s publication “Drought Management for California Almonds.” Below, however, provides an update to the publication that takes into account irrigation start-timing, and a further elaboration of the proportional deficit irrigation strategy.
When to start irrigating.
A growing amount of data within perennial nut crops have shown that most farm operations start irrigation too early in the spring. Most farmers begin to irrigate around mid-March, and typically make this decision based on the weather, sometimes using soil moisture sensors, but rarely measurements of tree water status. Using the pressure chamber to determine when to begin the irrigation season is highly recommended. Although this work is ongoing, it is currently advised to begin irrigation when trees reach 2 bars more negative than baseline (Baseline in the spring ranges from (-6) - (-8) bars, irrigation would occur at ~(-9) - (-10) bars). Delaying the start timing to this point can reduce spring water applications by several inches.
Soil types and depths will influence the start of the irrigation season. Soils with higher water-holding capacities will be able to support the tree longer in the spring without irrigation. Shallow soils have less stored soil moisture capacity, which means an earlier start may be needed. Lastly, all of this depends on both dormant and in-season rains. If minimal rain has fallen, stored soil moisture reserves may not have been refilled, leading to an earlier start timing. Spring rains, however, may delay irrigation applications. Typically, rains over a ½” (12 mm) are needed to refill soil moisture levels.
As described in a previous article, taking into account the pressure chamber readings, soil moisture levels, and in-season rainfall, the first irrigation may be delayed to as late as early May. Previous studies in Merced County for Butte/Padre and current research in Stanislaus and Butte Counties for Nonpareil have observed start dates ranging from early to late April. Earlier leafing dates, due to variety and location, seem to be the largest influencer on start timing.
Although not as effective as the pressure chamber, soil moisture probes can be useful in determining start timing. Soil moisture levels should show some level of decline at 24-36” (60-90 cm) before beginning to irrigate. Furthermore, research has shown that if the deeper root zone has some level of moisture “drawdown” prior to the first irrigation, root health will be improved, leading to a more stress resilient tree. If using soil moisture measurements as a guide, I typically have seen that moisture levels begin to decline around mid-March. In most years, these reserves provide about 2-3 weeks of moisture due to the lower water demand at this point in the season. This, of course, is dependent on the factors of soil depth, winter recharge, and in-season rain.
Proportional Deficit Irrigation: the preferred strategy.
When managing limited water supplies, the best strategy is to apply the available water as a proportion of water use. Very simply, this means that if only 30% of the water is available, the orchard should be irrigated at 30% of the crop’s evapotranspiration for the entire season. This strategy has been shown to be the easiest and one of the more effective strategies to manage a drought situation.
The first step with this proportional deficit irrigation strategy (PDI) is to determine the amount of water available as a percentage of seasonal water use. This information should be compiled from multiple years of on-farm weather stations, water management services, or California Irrigation Management Information System’s (CIMIS) various weather stations. Data from as many years as possible should be compiled and averaged for the respective weeks/months. For general use, Table 2 provides almond evapotranspiration estimates using long-term CIMIS data. The amount of water from the period after the initiation of irrigation should be totaled. This seasonal water requirement (SWR) should total the water use of April through October (September – April for southern hemisphere).
Once SWR is known, the next step is determining the available water for application. Total on-farm water available for use should be determined from the various sources. These sources should include allocated surface water, ground water, stored water within reservoirs, and any water transfers. The total amount of water available for almond irrigation should be divided by the number of acres of almonds to determine the available water for irrigation (AWI).
Once these two values are known, divide AWI by SWR to determine the proportion of water. For example, there is 36” of available water available for an orchard located in CIMIS zone 14. Based on table 2, 46.67” of water is needed to meet the requirement for April through October. Dividing the AWI by the SWR results in 77%. Based on this calculation, we should apply 77% of the water demands for each irrigation cycle.
If any water resources become available later in the season, the proportion should be adjusted for the new available total. This would lead to an increase in the amount of water applied for the remaining months of the season. Water applications, however, should not exceed 95% of ETc, and any remaining water should be banked for future years or utilized for winter leaching programs. Ideally, it may be best to save this additional water for the postharvest period. Work by David Goldhamer and colleagues, UC Davis, have shown that the impact on next year’s yields is reduced if 8-9” of irrigation water can be applied between mid-August and mid-October.
On-farm water application considerations.
As water supplies are reduced, the next question is how the water should be applied. Generally, it has been stated that water applications should be made in sets greater than 6 hours to reduce the effects of evaporation due to soil cooling. Interestingly, however, lysimeter studies done by Dr. Ken Shackel at UC Davis have found minimal water loss from evaporation within a full canopied orchard under tarping conditions. This suggests that evaporative loss may not be as great as once thought for mature orchards where little light reaches the soil surface under the trees. As such, system run-times do not have a minimal run-time. Too short of run-times, however, lead to issues with system pressurization and irrigation distribution uniformity. Therefore, minimal run times should be long enough to ensure that the proper amount of water and any fertilizers can be applied evenly across the fields.
After determining the appropriate irrigation cycle duration, the next factor is frequency. General observations within Australia have indicated that more frequent irrigation leads to better orchard management. The trade, off, however, is that more frequent irrigations require more attention to detail – line breaks, stuck controller valves, etc. Also, irrigating more then 2-3 times a week may not be feasible within microsprinkler systems due to run times. Frequency, therefore, is an operational decision. The only consideration is to make sure that the duration required for each frequency would not lead to surface water run-off or loss of water due to deep percolation within the soil.
As water applications become limited and use of lower quality of water may occur, water infiltration issues will become more prevalent. These problems must be managed to increase the effectiveness of the type of water available. Observe the orchard for surface sealing, field run-off, puddling, or other symptoms of poor infiltration. Adjust the run time duration to reduce these effects while performing a soil and water analysis to determine the cause. Integrate the appropriate solution to increase water infiltration, thereby increasing the efficiency of the water applied.
Conclusion and referenced works.
It may not be possible to prevent droughts within California, and the effects of the drought will vary depending on water availability. By considering start timing, and applying water as a proportion of availability, on-farm water resources can be stretched. These methods, when utilized with improved irrigation distribution uniformity, will reduce the in-season and long-term effects of the drought.
Works referenced:
UN FAO 66: Crop Yield Response to Water
UC ANR Publication 8515: Drought Management for California Almonds