Almonds

Summary of University of California research on irrigation management for almond trees under drought conditions

For maximum growth, yield, crop quality and orchard longevity almonds trees should be supplied with water to meet their full water requirement. There are some disease concerns with hull rot under full water conditions which can be addressed with moderate water stress during hull split. (Teviotdale et al. 2001) If water availability is limited, growers can react by applying irrigation water when trees are most sensitive to stress and by taking measures to minimize water losses that occur during irrigation events. Supplying less water than the trees can potentially use reduces soil water availability, causes tree water deficits, and reduces transpiration. Cover crops, depending on the coverage and the time of the season in which they are grown can increase the orchard water use by up to 30% . Cover crops should be removed when water is in limited supply (Prichard et al. 1989).

Water deficits affect almond orchards not only in the year in which stress occurs, but also in the following seasons. Generally, nut size is reduced in the first season of significant water stress. Because water stress also reduces vegetative growth and potentially decreases productivity per unit canopy volume, nut load can be reduced in subsequent years (Lampinen et al. 2007). Recent research indicates some stages of almond fruit growth are more sensitive to water stress than others. Understanding these stages permits growers to withhold water while minimizing damage to trees and to current and subsequent crops.

Early season stress

Water stress affects more tree and crop development processes during the early season - from leaf out through shoot growth and development of terminal and lateral buds. During this period, rapid vegetative development is necessary for canopy development and fruiting positions for the following season. (Goldhamer et al. 2006) (Prichard et al. 1994) In addition, orchard water use during this time is low compared to summer demand, reducing potential water savings from an early-season deficit irrigation strategy.

Fruit growth and development

Nuts undergo a rapid growth phase early in the fruit growth and development period and are sensitive to water deficits during this time. However, trees can tolerate drought stress fairly well during the two months prior to harvest, allowing for the successful use of deficit irrigation strategies during this period. (Shackel et al. 2004). Providing less than the full water requirement to cause moderate water stress during this period, will have little influence on kernel weight. However, severe water stress in the months leading up to hull split will reduce kernel weight and significantly reduce hull splitting. A one-inch irrigation prior to hull split will mitigate the water stress impacts and will improve hull split and reduce the number of hull-tights. (Prichard et al. 1994) If drip irrigation is used, possibly less irrigation can provide the same benefit, but this has not been proven in the field.

Post harvest stress

The effect of water deficits during the postharvest period are substantially affected by 1) pre harvest water deficits and 2) the quantity of water use over the remainder of the season. Bud differentiation can continue through mid-September. Moderate stress during this period will have little effect on subsequent year's nut numbers, but severe stress during bud differentiation has been found to dramatically reduce fruit set the following spring (Goldhamer et al. 2006). In early harvest (early August) districts, particularly with early varieties, more of the high water use season remains after harvest. This increases the necessity for postharvest irrigation. Later harvest (north State) districts and later varieties have a slightly shorter postharvest period which occurs at a time of lower crop water demand. These factors reduce the chance of moderate water deficits causing bud differentiation problems (Prichard et al. 1994).

Tree response to postharvest stress can be influenced by the type of irrigation system used, and the previous irrigation management. Low volume systems with limited soil water reserves can result in severe water deficits very quickly after irrigation cut off. In the southern San Joaquin Valley where harvest is earlier than in the north, or with drought-sensitive varieties, postharvest irrigation is a necessity. Deep rooted, surface irrigated trees may have enough pre-harvest deep moisture remaining to carry them through the critical period of bud differentiation. This all depends on the irrigation management occurring pre-harvest.

Developing an Deficit Irrigation Strategy

Crop Water Use

Almond water use begins when the leaves develop and shoot growth begins. Concurrent with canopy development, the climatic demand increases, driven by longer days and higher temperatures and low humidities as the season progresses. Both of these factors result in a seasonal water use starting at a low level, peaking in mid-season and falling as season ends. Sources of water available to trees include: soil-stored moisture (including frost protection water applications if the root zone is less than field capacity when applications are made), any in-season rainfall absorbed by the soil, and applied irrigation water. These all combine to determine the total seasonal water available to the orchard.

Mature conventionally spaced almond trees in the Southern Sacramento Valley can use about 41- 44 inches of water in an average year of unrestricted water use. High-density orchards, long pruned orchards, or those with a cover crop can have even higher use. . Soil moisture monitoring demonstrations in more than 40 almond orchards in Kern County indicate that seasonal water use in the southern San Joaquin Valley may be as high as 50 - 54 inches (Sanden 2007). Figure 1 shows a typical water use pattern for fully irrigated and a deficit irrigation regime for almond in the Manteca area.

The moderately deficit irrigated orchard used (in a combination of soil supplied and irrigation water) 28 inches of water or about 34 % less than the full potential orchard.

Water Deficits

Water deficits occur when the climatic water demand exceeds the water absorbed by the roots. As the soil becomes depleted of readily available moisture, water uptake by the roots lags behind water use causing plant stress in the mid to late afternoon. This minor crop water deficit has little effect on the crop yield. However, as soil water becomes increasingly difficult to extract water stress increases. One way to measure "tree stress" is to use a portable pressure chamber to measure "midday stem water potential". To use this technique a few leaves from representative trees are first covered with an opaque plastic bag while still on the tree. The covers need to remain on the leaves at least 10 minutes after which they are detached and the the water potential measured using the pressure chamber (Fulton et al. 2001). The pressure chamber measures the amount of pressure needed to force water out of the leaf petiole, indicating trees water status. http://fruitsandnuts.ucdavis.edu/General_Management/The_Pressure_Chamber,_aka_The_Bomb.htm

A Moderate Water Stress Strategy

From the previous discussion it can be concluded that tree water use from leaf out through mid June should not be compromised. From mid June through harvest, reductions up to 50% of full water use have been successfully used to reduce orchard water use with only minimal reductions in kernel weight. It is important to supply the trees with water near hull split to avoid hull-tights.

There are various approaches growers can take to manage limited water supplies depending on what types of irrigations scheduling tools interest or are available to them. A simple method is to reduce irrigation run time or lengthen irrigation intervals to obtain the desired percentage of irrigation reduction in applied water. In a four-year study investigating pre-harvest, post-harvest, and uniform deficit irrigation for the entire season, the best results were achieved when water applications occurred at a uniform deficit rate across the season relative to full potential crop ET. The uniform deficit rate does not mean a uniform irrigation amount across the season (e.g. 1.5 inches each week), but rather a uniform (e.g. 85%) reduction of full ET for each period. Deficit irrigation rates of 55%, 70%, and 85% were tested with the 70% and 85% irrigation reduction treatments showing little yield loss compared to the full ET treatment. (Goldhamer et al. 2006) The 70% and 85% uniform across the season deficit treatments experienced little early season stress, likely because stored soil moisture supplemented the applied irrigations.

Another approach that is likely an improvement over the approach outlined above is to schedule irrigations using periodic pressure chamber readings and irrigate when midday stem water potential reaches a pre-determined threshold stress level (see Figure below). This method effectively extends the irrigation interval, but the interval is determined by tree water status rather than the calendar. Irrigations should be in the volume of a normal set as performed with a full irrigation regime. In a deficit irrigation study conducted on mature almond in the Manteca, CA a just prior to irrigation threshold value of -20 to -22 bars midday stem water potential beginning in June resulted in 34% less tree water consumption and no significant influence on yield for the 4-year measurement period. (Prichard et al. 1994) It should be noted that a reduction in vegetative growth was measured in this treatment, indicating that use of this threshold for a longer-term strategy (more than 4 years) may reduce yields by reducing nut numbers. The impacts of stress on a developing tree canopy is much more detrimental as opposed to the impacts on a canopy that has already reached it's full volume.

A More Severe Water Stress Strategy

A more severe strategy that reduces seasonal tree water use by 50% requires that stress be imposed early as well as mid to late season. Using this strategy, irrigations in April and May are withheld until trees reach a midday stem water potential of -12 to -14 bars. Using conventional sprinklers, a normal set time is used. If lighter applications are made, more water is lost by evaporation. From June 1st through hull split, midday stem water potential values should be allowed to reach -20 to -22 bars just prior to irrigation. This strategy will require a pre-harvest irrigation of about 2 inches with sprinklers-less with micros and drip--to ensure good hull split. Note: this strategy reduces water use significantly but also reduces nut weight the year it is used and the nut number in succeeding years. In the Manteca trial discussed above, it took 2 years of full irrigation for trees to recover. (Prichard et al. 1996)

A "Staying Alive" Drought Strategy

Less is known about this strategy since it is a rarely used option. However, based on past drought conditions, trees may be kept alive with about a foot of applied water. This strategy does not consider growth and yield-just tree survival. This strategy is best conducted using a micro-irrigation system which maximizes water distribution and minimizes evaporative losses from irrigation. Using this strategy no irrigation is applied until water potential reaches -16 bars from leaf out through the end of May. Monitor stem water potential until the threshold is reached again then repeat the cycle. After June 1st, and for the rest of the season allow the stress to climb to -25 bars prior to irrigation. As a guide, try to just retain the leaves on the tree. Good luck, as this is only a guide. Remember that following this severe deficit strategy, it will take at least 2 years of full irrigation for the trees to recover to normal yields.

List of References