Irrigating with a Limited Water Supply
Roger Duncan, UCCE Pomology Advisor, Stanislaus County
Afull-canopy almond orchard in the North San Joaquin Valley uses about 48 inches of water in a typical season using local CIMIS weather data. Peaches generally use six or seven inches less than almonds. Currently, the Modesto and Turlock Irrigation Districts have capped allotments at 30 and 27 acre-inches, respectively, for the 2022 season. Clearly orchards that rely on irrigation district water will experience significant water stress this year.
Even though winter rain was less than average this year, we began the season with a full soil profile. Sandy loam to clay loam soils hold an average of 1.5-1.9 inches of water per foot, but only about half is easily removed by the trees (allowable depletion). Assuming a three-foot root depth, that means roughly 2.2 – 2.9 inches of water can be used in the spring before trees begin to stress excessively, depending on soil type.
We are still researching how much water almond trees NEED vs. how much they will USE, but clearly irrigating with only 75% of full ET will lead to excessive water stress, which generally results in yield loss in the current year and likely the next year. Exactly how much your current and future yield will be reduced depends on how the water is allocated through the season.
Drought Irrigation Strategy for Almonds. The first thing that growers who rely on district water have to accept is that their trees will experience stress. All we can do is manage when and how much stress the trees will experience. Fortunately, University of California researchers, including David Goldhamer and Ken Shackel, have conducted some important drought irrigation experiments on almond trees. In Dr. Goldhamer’s experiment, he imposed three levels of deficit irrigation, 34 inches, 28 inches, and 23 inches of seasonal irrigation water. He also experimented with three strategies of when to use the water: 1) Using most of the water early in the season and stressing the trees late in the season; 2) Stressing the trees early and saving the water for later, including post-harvest; and 3) Spacing the irrigations out through the year to maintain an even level of stress throughout the season. The experiment was conducted for four consecutive years.
The results showed the lower the amount of water applied, the lower the yield. However, the timing of the deficit made a difference in how much yield was reduced. Trees that experienced an even deficit throughout the season always had higher yields than the other irrigation strategies. This means that almond growers with limited irrigation water should try to spread the pain and not use the water too early or save it until the end. Research conducted by the University of California showed that almond flower buds begin initiation around the first of July the previous year, or even a few weeks earlier in a warm year. The process of flower bud differentiation continues through early October. Therefore, severe stress during any of this time can significantly affect the next year’s crop.
Almonds, by nature, are drought tolerant. They are farmed in many countries without irrigation. However, yield is very low under dry-farmed conditions. In a 2009 experiment led by Dr. Ken Shackel at UC Davis, almond trees in a previously irrigated orchard survived the season without any applied irrigation. He also saw that unirrigated trees utilized deep soil moisture more than irrigated trees. Yield was reduced significantly compared to irrigated trees. Yield was impacted even more severely the year after severe deficit, even though trees were returned to full irrigation. The good news is that all irrigation treatments regained full yields by the second year of full irrigation.
Deficit Irrigation Strategy in Peach. There are three phases of peach fruit growth. Phase 1 begins after the flower is fertilized during bloom and is characterized by rapid initial fruit growth, primarily due to cell division. While this stage of fruit development is sensitive to drought stress, it occurs early in the season when soil moisture is usually plentiful, and transpiration demand is low. Because trees don’t begin using water until leaves are present, significant water stress is not likely to occur for many weeks after bloom. Therefore, it is not likely that peach trees will experience significant water stress during most of Phase 1.
Phase 2 is generally characterized by seed development, and there is very little fruit growth during this period. Phase 2 is the least sensitive stage to water stress for peaches. Growers can recognize the beginning of Phase 2 by the earliest onset of pit hardening. Mild water stress applied during this intermediate developmental period of slow fruit growth has little effect on crop yields but can significantly reduce vegetative growth.
Phase 3 is a period of very rapid fruit size and weight increase due primarily to cell expansion, and ends with ripening. During this final growth phase, about 80% of a fruit’s fresh weight is accumulated. Water is primarily what drives cell expansion and therefore this final period of rapid fruit growth is very sensitive to water stress. Withholding water during Phase 3 will result in significant yield reduction.
The overall plan for peach growers should be to avoid irrigating too soon during Phase 1, extend intervals between irrigations during Phase 2 and use the water saved during Phase 3. Significant water savings can also occur postharvest, but growers should be aware that severe water stress during August and September can increase the number of double fruit the following year. Stone fruit growers with limited irrigation water might also want to thin a little more heavily. Peach trees with heavy crops are more sensitive to drought stress. You can watch a short video on using the pressure chamber to monitor tree water stress at https://www.youtube.com/wa tch?v=iDgPG88ie18