Usage would best be confined to lemons and in areas where salinity and gummosis may be critical factors. Hybrids of pummelos with other promising root stock cultivars should be considered.Almonds are California’s top agricultural export — 80% of those consumed worldwide are grown here. As water resources become increasingly scarce due to population growth, environmental needs and periodic drought, it will become more difficult both monetarily and politically to obtain sufficient water for crop irrigation. Drought tolerance in almonds has been documented in previous studies, but substantial irrigation is still required to maintain current production levels. Over the last 14 years there has been a steady increase in both bearing acres and yields — about 70 pounds per acre in almond yield improvement annnually , indicating a steady improvement in cultural practices, among them, irrigation. There is a pressing need to reliably maintain current almond production with less water. Surface-water allotments for irrigation during drought are often significantly reduced because precedence is given to other uses . Water reserves in California were low following the droughts of 2007, 2008 and 2009. In fact, spring 2008 was the driest on record . The current basis for estimating the irrigation need of a crop is to combine the water lost from the soil with the water lost through leaves , into an overall loss, the crop evapotranspiration . ETc is calculated by multiplying a weather based reference crop ET , by a crop coefficient , to give the final estimate . Research in the late 1980s and 1990s estimated the average seasonal ETc for almonds at 40 to 42 inches ,square pots with estimated seasonal irrigation requirements of 36 to 38 inches under typical soil and rainfall conditions of the southern San Joaquin Valley . But later field research suggested that almond ETc may average from 48 to 54 inches .
Reasons for the higher recent estimates probably reflect the many changes that have occurred in almond culture over the past two decades. Almond orchards are now intensively managed with pressurized rather than surface irrigation systems, and crop water status can also be monitored directly using midday stem water potential . SWP is measured directly on leaves sampled in the orchard using a pressure chamber, and it indicates the level of physiological water stress that is being experienced by the trees at the time of sampling, much as blood pressure or temperature can be a measure of any physiological stress in humans . Furthermore, nitrogen fertility management is more intensive than it was when the earlier research was conducted, and pruning practices have changed to manage canopy light differently, both producing more foliage and potentially higher ETc. In fact, a higher ETc rate and higher yields may both be responses to more intensive almond management. The ETc method of irrigation scheduling aims to maintain the crop in a non-stressed condition by supplying enough water to satisfy ETc. Alternative methods have been proposed that attempt to reduce unnecessary vegetative growth in orchard and vine crops in order to make water use more efficient; they include deficit irrigation, partial root-zone drying and regulated deficit irrigation . The objective of regulated deficit irrigation is typically to irrigate so that trees experience mild-to-moderate levels of water stress, in order to achieve an optimal horticultural balance between vegetative growth, which is very sensitive to stress, and fruit production, which is less sensitive . Previous studies in almonds and other crops have shown the beneficial effects of regulated deficit irrigation, including control of excessive vegetative growth, reduced hull rot and improved hull split in almonds , increased fruit density in prunes and pears and reduced vegetative growth in peaches . Previous studies of regulated deficit irrigation have created stress by applying a fraction of ETc, but for this 5-year study we used a plant-based indicator of stress and set a target level of mild-to-moderate stress during the hull-split period. We undertook this study to determine whether meaningful reductions in consumptive water use could be achieved with minimal impacts on orchard productivity.
Our study took place in a micro-sprinkler-irrigated, 270-acre almond orchard near Orland in the northern Sacramento Valley, which was planted with ‘Nonpareil’ and ‘Carmel’ trees spaced at 12 feet by 24 feet . The orchard was divided into five approximately equal blocks; two were planted in 1993 and three in 1999. From the first year of the experiment , the canopy shaded area in midsummer at noon was greater than 50% in all blocks, so all blocks were considered to exhibit fully developed crop water requirements . The five blocks were each subdivided into two sections to match the existing irrigation system design, with control and regulated deficit irrigation treatments assigned to the sections on alternating sides. Two rows of ‘Nonpareil’ almond trees in the center of each section were designated as the experimental plots, with two trees from each block used as the monitoring trees for SWP measurements. The rows averaged approximately 69 trees per block, and monitoring trees were positioned approximately one-third and two-thirds of the way into each row . SWP values were initially taken on weekly field visits using a pressure chamber, and were collected biweekly during the hull-split period. Leaves, still on the tree, were covered with an aluminized Mylar bag for a minimum of 10 minutes prior to measurements . Meters were installed on a single lateral line in each irrigation section to measure water applications. In 2004 and 2005, block-specific recommendations for regulated deficit irrigation were communicated to the grower, who was responsible for dayto-day irrigation management. In 2005, the orchard exhibited defoliation due to Alternaria leaf spot, and the grower was reluctant to withhold water from the large regulated deficit irrigation plots. In 2006, a separate irrigation system that could be monitored and controlled via a satellite-linked Internet service was installed for the experimental ‘Nonpareil’ row and the two adjacent Midday SWP and water meter data were collected weekly from early April until the hull-split period. Visual surveys were made weekly starting in mid-June to anticipate the beginning of hull split. Irrigation was reduced once the onset of hull split was observed in blank nuts, generally about a week before the onset of hull split in normal nuts. Before and following the hull-split period,drainage plant pot the water amounts applied to the regulated deficit irrigation and grower control treatments were equivalent. During the hull-split period, SWP was measured twice weekly and irrigation was adjusted to achieve a target mild-tomoderate stress level of −14 to −18 bars in each block.
By the last year of the study , block-specific irrigation was not necessary because the target SWP could be achieved using about the same level of deficit irrigation in all the treatment blocks. The target levels of midday SWP employed in this field trial were set to achieve mild-to-moderate water stress during the regulated deficit irrigation period. For almonds, Shackel reported about a 50% reduction in midsummer stomatal conductance with SWP values of −14 to −18 bars compared with a non-stressed SWP above −10 bars . Irrigation was returned to normal once visual surveys indicated 90% hull split in each block. The grower commercially harvested entire rows, and a weighing trailer was used to determine gross harvest weight in the field. We collected a 4-pound sub-sample from each of the blocks and used them to convert harvest weights into nutmeat yields. In this field trial, regulated deficit irrigation was limited to the hull-split phase of almond growth and development. ETc is typically highest during midsummer, so the opportunity is greatest at this time to impose crop stress in order to achieve significant irrigation reductions. In addition, Teviotdale et al. reported that both hull split and nut harvestability are improved and hull rot is reduced when regulated deficit irrigation is imposed during the hull-split period. Other stages of almond growth and development have shown greater susceptibility to negative impacts on tree growth and nut production . Crop stress is also difficult to impose from leaf-out through midMay due to rainfall, lower ETc rates and generally sufficient soil moisture.Soil moisture. We installed neutronprobe access tubes to measure the change in stored soil moisture from early spring to late summer, in order to quantify the contribution of soil water to the crop’s water needs . We installed two grids of 16 tubes in a single block, each in the southwest quadrant of a single monitoring tree for both the regulated deficit irrigation and control treatments. The tubes were arranged in 4-by-4 grids with overall dimensions of 6 feet by 12 feet . The grid spacing was measured from the center of the tube, with 2-foot spacing in the north-south direction and 4-foot spacing in the east-west direction. We tried to install the tubes to an overall depth of 60 inches and measure volumetric soil water content at 1-foot intervals, at depths of 8, 18, 30, 42 and 54 inches . However, due to the widespread variability in soils — including areas with significant gravel content, soil stratification and a shallow, temporarily perched water table — we achieved a depth of 54 inches for only 22 of the 32 tubes.
The remaining tubes were installed to a depth of 42 inches . Soil moisture readings were taken two or three times per season, typically around full bloom, in late summer and post harvest. The shallow water table receded during the course of each growing season, especially during the drought years of 2007 and 2008; it did not appear to influence orchard water status significantly during our study. If capillary flow of water from the shallow water table had contributed significantly to crop consumptive use, midday SWP would not have responded to the withholding of irrigation water during hull split. In addition, the gravel content and hard pan appeared to be barriers to deeper root development, so the roots may not have reached the soil water. Soil type. Soil types were variable throughout the orchard, but the majority of acreage consisted of three types: Cortina very gravelly sandy loam, Hillgate loam and Redding gravelly loam . These soils are described by a USDA land capability rating of 3 or 4, which generally groups soil types based on restrictions for field crops. The Redding soil typically has a restrictive layer at 20 to 40 inches , and the other soils extend to below 80 inches . Based on a nominal 60-inch soil profile, all have low available water — approximately 3.5 inches for the Cortina and Redding soils and 8 inches for the Hillgate soil . The two grids of neutron-probe access tubes were positioned in either a Cortina or Redding soil type. Ground cover. Ground cover varied between mowed resident vegetation in spring and winter, and bare ground in summer. Vegetation around the neutron-probe access tubes, where a mower could not be used, was controlled with herbicides each spring to match the surrounding vegetation. Reductions in water use Water savings. An average water balance summary for 5 years of this study showed overall savings of 4.8 inches of applied water in the regulated deficit irrigation regime . The neutron-probe readings showed an average seasonal contribution of approximately 5.0 inches of stored water in the control and 4.5 inches in the regulated deficit irrigation treatment, amounting to about 11% of overall consumptive water use. All in season precipitation was assumed to be an effective contribution. When the savings in applied water were combined with the contribution from soil storage, the regulated deficit irrigation regime resulted in a total average annual consumptive-water-use savings of 5.3 inches over the 5-year period, and yearly savings ranged from 10% to 15%, or 5.2 to 6.1 inches . Yield increases. Yields increased in both treatments during the 5-year study, with no clear trend of any reduction due to regulated deficit irrigation . The orchard’s increasing yields can be attributed to its relatively young age and continuing canopy growth. Canopy growth is typically very sensitive to deficit irrigation, so it is noteworthy that plant-based regulated deficit irrigation did not have a negative impact on yields over time, presumably because the deficit period was after the main period of vegetative growth.