Tree water requirements & regulated deficit irrigation


1 production TREE WATER REQUIREMENTS & REGULATED DEFICIT IRRIGATION David A. Goldhamer the water budget and involves knowledge of the he goal of irrigation scheduling is to supply T soil, plant and climate. An additional scheduling the proper amount of water to the orchard on approach, now primarily in the research phase, a timely basis. Good irrigation management uses plant-based measurements of water status or insures that an adequate supply of soil moisture is some parameter that is related to water status. maintained throughout the season. Informed These measurements are made with specialized irrigation decision making requires knowledge of equipment, such as the pressure chamber, to the water use requirements of the trees and signal when irrigation is needed. irrigation system performance, including the This chapter will focus on the water budget uniformity of application and delivery rates. approach which we believe is the most cost Information on how productivity is affected by effective, comprehensive water management suboptimal irrigation practices also can be used technique currently in use. Successful utilization to guide irrigation decision making. We believe of the water budget enables us to answer the two that our studies on pistachio tree water use, questions necessary for effective irrigation: 1) response to season-long deficit irrigation, and when to irrigate, and 2) how much water to regulated deficit irrigation provide growers with apply. While most of California's pistachio the agronomic information necessary to orchards are drip or microsprinkler irrigated, we successfully manage irrigation, both in normal also cover water budget approach issues in this water years and in drought periods. chapter that relate primarily to surface irrigation Pistachio trees have a reputation of being (border strip and furrow). These include soil drought tolerant, for being able to survive and water holding capacity, allowable depletion, and even produce modest crops with very little water. rooting depth. However, drought tolerance does not mean that pistachio trees require little water for optimal performance. Indeed, we have found that mature THE WATER BUDGET pistachio trees can utilize water at a fast rate; Since the orchard water budget is essentially an much faster than other fruit and nut trees. If ter additions and losses exercise in balancing wa pistachio trees are under-irrigated during stress- from the orchard, the irrigation requirement is the sensitive periods, important tree processes can be difference between the water losses and effective adversely affected. Thus, one must recognize that rainfall. Thus, the well-known drought tolerance of pistachio refers to its ability to survive under severe water Irrigation requirement = ETc - Effective rainfall + Irrigation system losses. stress conditions; it doesn't mean that the trees will grow rapidly or have high productivity if Crop evapotranspiration (ETc) is the sum of they are poorly irrigated. ) and transpiration from evaporation from the soil (E There are two fundamentally different leaves (T). Effect ive rainfall is the total rainfall approaches used today for scientific irrigation amount stored in the profile; an amount which can scheduling: 1) monitoring soil moisture levels by be difficult to estimate. Rainfall, however, can hand or with various instruments and supply a significant part of the seasonal crop needs replenishing soil water in a timely fashion, and 2) in some areas of California and, therefore, should estimating how much water the crop is using and not be ignored. While the amount of rainfall stored applying water to match this crop water in the soil depends on its intensity and duration, requirement. This latter technique is known as 103

2 experience has shown that this value usually ranges INFILTRATION between 50 and 70 percent of total winter rainfall. Water applied to a field infiltrates into the soil Rather than use estimates, the best way to evaluate quickly at first and then slows down as irrigation effective rainfall is to measure the depth of wetted continues. With surface irrigation methods soil at the beginning of the season. As for losses soil infiltration rate (flood, furrow, border), the other than ETc, deep percolation of water below the controls the amount of water that infiltrates and, root zone and surface runoff can occur when using therefore, may dictate the length of time you flood, furrow, basin or border irrigation, even with should irrigate. With sprinklers or localized the best management. While this waste can be irrigation (drip, low-volume sprinklers), the minimized, it is usually not economically feasible application rate determines the amount of water to eliminate these losses entirely. infiltration, assuming the minimum soil intake The water budget procedure for drip and low- rate is not exceeded. Because the goal of efficient volume sprinkler-irrigated orchards allows you to surface irrigation is to store a specific quantity of use information on ETc, rainfall and irrigation water in the soil, the infiltration rate is of system losses to determine your irrigation fundamental importance with surface methods. program. For surface irrigated orchards, you need These intake rates are greatest early in the season additional information on soil water holding and usually decrease with each successive capacity, root zone depth, and allowable irrigation. While reasons for this are not well depletion levels to schedule irrigations. These understood, it seems to be associated with data allow you to estimate the size of the soil changes in soil surface chemistry and structure. It water reservoir, and by knowing the rate that can make the irrigator's task much more difficult water is being used, you can determine when to and will be discussed later. irrigate next and how much water will be needed As water is applied to a field with surface to refill the reservoir. Figure 13a illustrates this irrigation methods, it almost completely fills the analogy, but it should be emphasized that storing pore space in the profile's upper level. As soil water is not as simple as holding water in a irrigation continues, the depth of the zone at or reservoir. The problems of replenishing the root near saturation increases. If the soil profile has zone profile and the ability of trees to extract been dry, a distinct boundary exists between wet stored water will be discussed later. and dry soil. When irrigation stops, some water moves out of the wetted zone and partially wets the dry soil below it. If the subsoil is already moist, the water passes below the root zone, and is not available for tree uptake. Soil is a complex matrix composed of solid particles, void space and small amounts of organic material. Its water-holding capacity depends on the relative volume of void space (porosity) and the size of the pores. There is a direct relationship between soil particle size (texture) and pore space. Coarse textured soils total pore (sands) have a smaller percentage of space , and fine textured soils (clays and clay loams) have a greater percentage. On the other hand, even though clays have a larger porosity, is small compared with sands; average pore size thus, water moves much more readily in sandy Figure 13a . Analogy of irrigation scheduling soils. using the water budget approach to replacement After irrigation, soil water drains rapidly at of water in a tank. first. As the large pores empty, the soil conducts water much less readily. After 3 or 4 days, the rate of water movement slows and can be 104

3 affect water holding capacity. The University of neglected for our purposes. At this point, the California Cooperative Extension and the USDA remainder of the soil water can be considered Natural Resources Conservation Service have stored. The water content of the soil at this point developed specific AWC information for many is called field capacity (FC) and is the upper limit areas of California. This information can be of water storage. A practical lower limit of soil obtained from the local offices of each water content below which crop growth is organization. Precise determinations of AWC are severely reduced by water stress has been defined generally not necessary for irrigation scheduling. as the permanent wilting point (PWP). Although Indeed, the expressions FC, PWP and AWC are pistachio trees do not visibly wilt, the PWP is as actually concepts and should not be thought of as significant for them as for plants that show absolute, fixed amounts; they are practical wilting. estimates used to represent what are actually The difference between FC and PWP is continuously changing water contents in the termed the available water content (AWC). profile. Table 1 shows the range and average AWC of various soil types in units of inches of water per foot of soil. While several terms are used to ALLOWABLE DEPLETION express soil water contents, inches of water per While it is useful to compare soil water storage foot of soil is a practical unit and can be with a reservoir, it is not entirely accurate. As visualized as the depth of water obtained if all soil water content decreases, it becomes more available water were extracted from a one-foot difficult for roots to extract the remaining water, depth of soil. As shown in Table 1, sands, with even though it is well above the PWP. This is their relatively small total pore space, do not because after the large soil pores give up their store large amounts of water. What water is held water, the smaller pores must assume two is easily removed by plant roots. Clay soils, important functions: 1) to store water, and 2) to because of their large total pore space, have a conduct water moving between the soil and plant large AWC. However, their small water-filled roots. Small pores not only hold water tightly, but pores have attractive forces that tend to resist water travels exceedingly slowly through small water extraction by plants. Intermediate textured pores. These factors combine to limit water soils, the loams, have good water-holding uptake as soils dry out. Thus, crop growth properties, and because of their wide range of decreases before the entire root zone reaches the particle sizes, are readily able to release their PWP. For this reason, you should usually irrigate water for plant use. Once the AWC is known, the before the root zone water content reaches a level total water-holding capacity of the profile is that restricts growth below its maximum easily determined by multiplying the AWC by potential. the depth of the root zone. Unfortunately, no single soil water level can be recommended for all situations. The safe amount of depletion [ca lled allowable depletion Table 1 . Estimates of available water content (AD) or yield threshold depletion (YTD) and . for different soil types usually referred to as a percentage of the total available water in the root zone] depends on Available water content Range Average numerous factors, including rooting density, soil (in/ft) (in/ft) Soil type texture, and the weather. Figure13b illustrates the relationship between AD and plant growth for Coarse textured sand 0.50 - 1.25 0.90 two extreme situations. One example is a low 1.50 Sandy loams 1.25 - 1.75 root density crop on a clay soil grown under hot, 1.90 Silty clay loams 1.50 - 2.30 windy conditions. Here a depletion of more than Clay 2.10 1.60 - 2.50 30 to 40 percent of the available root zone moisture may affect crop growth. On the other The wide range of AWC for each soil type hand, a densely rooted crop grown on a sandy demonstrates the uncertainty of estimates and soil under mild conditions may be able to tolerate also indicates that factors other than particle size an AD of 70 to 80 percent before growth rate 105

4 drops. Allowable depletion determination is, WATER REQUIREMENTS AND METHODS therefore, not easily made, but precise estimates OF ESTIMATING ETc are not required. The particular sensitivity to The water budget procedure can be used water stress during bloom, rapid shell and kernel successfully only if the ETc rate is known. ETc development, and shoot initiation and growth depends on climatic, plant, soil and orchard periods suggests irrigating pistachio trees at management factors, each of which will be relatively small depletion levels during these briefly discussed. periods. At other times, 50 to 75 percent depletion has been used successfully in Weather conditions California orchards. Weather conditions largely determine ETc rates. Because both processes that make up ETc involve vaporizing water, the energy status of the atmosphere is of primary importance. Components of the energy balance include solar radiation (sunlight intensity), air temperature, humidity and wind speed. Additionally, if an orchard is bordered upwind by bare ground, advective energy can cause ETc rates to increase drastically and must be taken into account in estimating crop water use. Evaporation Evaporation is important only when the soil surface is wet. After an irrigation, water evaporates from the soil at the same rate that trees transpire. As the soil surface dries out, upward water flow is drastically reduced, and Figure 13b Relationship between allowable . surface evaporation decreases rapidly. Thus, the depletion of soil water and plant growth for two total amount of water evaporated depends on the extreme situations. area of the orchard floor that is wetted and the number of irrigations. Because both evaporation Remember: The objective of irrigation is to and transpiration require energy, speculation has keep adequate moisture in the soil. Estimates of focused on whether excessive evaporation will allowable depletion allow use of the maximum reduce the available energy and thus lower the amount of soil water (consis tent with optimal tree crop's transpiration rate. The answer is unclear, performance) between irri gations. This approach although it is certain that evaporation does not means growers can irrigate the fewest times reduce transpiration on a one-to-one basis. possible and, because there are fixed costs associated with each irrigation, this is usually the Plant factor most economical practice. For example, if 50 The most significant affecting ETc is plant factor percent of the total AWC can be safely removed the size of the total leaf area intercepting solar by the trees between irrigations, irrigating when radiation. Thus, the size of the tree canopy, the only 25 percent of the available water in the root tree spacing and the stage of leaf development zone has been depleted will require twice as during the season all influence crop water use. The AD concept is important many irrigations. Rather than trying to measure leaf area, research only for surface irrigation and sprinklers; it is indicates that the degree of plant cover (shade) of Indeed, irrelevant for drip and microsprinklers. the orchard floor correlates well with sunlit leaf the goal of these high frequency systems is to area. The relationship between plant cover and maintain soil moisture levels relatively constant ETc is important in determining irrigation over the season as opposed to conventional schedules for young orchards. Field studies show methods that involve wetting and drying cycles. 106

5 the irrigator can control both factors and limit that ETc reaches its maximum when 50-60 evaporation loss. With furrow irrigation of young percent of the ground is shaded by tree canopies orchards, one furrow can be used on either side at midday. Figure 13c shows the relationship of the tree rows rather than completely wetting between percent ground cover and ETc for the ground with multiple furrows. Studies have almonds. The relationship for pistachio trees has shown that localized irrigation, including drip, not been established, but since the canopy can significantly decrease the amount of architectures are similar, the almond research is evaporation and, therefore, save water in young most likely applicable to pistachio trees. Note orchards. In mature orchards, little, if any, that in Figure 13c the relationship between reduction of surface evaporation is achieved by percent shade and ETc varies considerably from a using localized irrigation methods. one-to-one basis. Presumably, the area of the orchard floor receiving direct sunlight transfers Although cover crops can have many energy to the tree canopies by microadvection, benefits, an undesirable consequence is that they thereby increasing the ETc rate. use considerable amounts of water. Cover crops or uncontrolled weeds can increase seasonal orchard ETc by 20-25 percent in deciduous orchards and by much more in young trees. Therefore, the cost and availability of water should be taken into account when considering cover crops. Since climatic conditions have the greatest influence on ETc, many mathematical formulas have been developed to estimate ETc based on meteorological measurements. Reference crop ETc (ETo) values are derived from these empirical equations, ETo values approximating evapotranspiration from a closely cut grass crop. Another index of evaporative demand is evaporation from a free water surface (Epan). But since pan evaporation is strictly a physical mechanism and transpiration is biologically controlled by the leaf stomata, ETo has been . Figure 13c Relationship for developing shown to correlate better with actual crop ETc pistachio trees between ET and percent shaded than Epan. Therefore, crop water use estimates area of the orchard floor. made with ETo values are potentially more accurate. Long-term, historical average daily ETo While most factors affecting ETc cannot be has been compiled for all locations in California manipulated by growers, irrigation system and and is available in UC Publication #21454 orchard floor management can influence water ("Irrigation Scheduling--A Guide for Efficient usage. As noted earlier, the frequency of On-Farm Water Management). Bi-monthly ETo irrigation and size of the wetted surface area values for Parlier are shown in Table 2. influence evaporation. With good management, 107

6 Table 2. Long-term, historical average ETc for mature pistachio trees under clean cultivation in San Joaquin Valley, CA 1 Date ETc ETc ETc Kc ETo 2 (in/period) (in/day) (gal/tree/day) (in) 0.07 0.011 2.0 0.16 Apr 1-15 0.17 0.43 1.16 0.077 Apr 16-30 0.18 13.8 0.21 2.14 0.143 25.7 May 1-15 0.68 0.93 3.57 0.223 40.1 May 16-31 0.24 0.25 1.09 4.09 0.273 49.1 Jun 1-15 0.26 Jun 16-30 4.56 0.304 54.6 1.17 0.27 1.19 0.321 57.7 Jul 1-15 4.82 0.26 4.95 0.309 55.5 Jul 16-31 1.19 0.24 1.19 4.28 0.285 51.2 Aug 1-15 Aug 16-31 0.22 3.94 0.246 44.2 1.12 0.19 0.188 2.82 Sep 1-15 33.8 0.99 Sep 16-30 0.87 2.09 0.139 25.0 0.16 Oct 1-15 0.12 0.67 1.21 0.081 14.6 Oct 16-31 0.09 0.045 8.1 0.50 0.72 0.35 0.32 3.8 0.06 Nov 1-15 0.021 40.80 TOTAL 1 Crop coefficient for grass reference crop (Eto). ETc = (Kc)(ETo) 2 Based on 17 x 17 ft tree spacing. The following equation can be used to calculate individual tree ETc for other spacings: gal/tree/day = 2 2 ETc(in/day) x spacing(ft ) x .622(gal/in.ft ) Long-term average ETc data can be The relationship between ETc and ETo, successfully used for irrigation scheduling even when expressed as a ratio (ETc/ETo) is called the though a normal year seldom occurs. Common crop coefficient (Kc). The Kc varies with the sense should be used to modify irrigation crop and its stage of growth, but it is assumed to schedules based on long-term averages, if the be independent of location for most areas in season has drastically higher or lower California. Thus, we believe that a single set of temperatures or winds than normal. If you want pistachio tree Kc values can be used throughout to be more accurate, current (real time) ETo the state, with the exception of coastal areas. estimates are calculated from data collected from Table 2 gives ETc estimates on a bi-monthly the CIMIS network of automated weather basis for pistachio trees grown in the San Joaquin stations, and this data is available from the State Valley under clean cultivated conditions during a of California Department of Water Resources. normal weather year. ETc was determined based Also, several newspapers and radio stations in on long-term ETo and research-generated Kc California report reference crop data. You must values (also shown in Table 2) assuming: ETc = be careful to recognize whether the data are (Kc) (Eto) and is shown as inches per each estimates of Epan, ETo, or some other reference bimonthly period, inches per day, and gallons per value, in that they each represent different tree per day assuming a 17 x 17 ft tree spacing. measurements, and the appropriate set of crop Note that the peak pistachio ETc rate (early July coefficients must be used to avoid errors. through mid- August) is approximately 0.32 inches per day, which is significantly greater than The CIMIS network can be accessed published values for other deciduous trees. For through the Internet ( ). example, peak almond ETc in the San Joaquin For further information on the CIMIS network, Valley is approximately 0. 25 inches per day. This contact: high pistachio ETc is reflected by a peak Kc of State of California, Department of Water Resources 1.19 vs. 0.96 for almond. In other words, Water Use Efficiency Office pistachio trees can use water at a rate 19% greater 901 P Street, Third Floor than a well-watered grass crop, while almonds P.O. Box 942836 use 4% less. Sacramento, CA 94236-0001 108

7 general, sprinkler and drip/microsprinkler IRRIGATION EFFICIENCY systems can be operated with higher efficiencies far can be used to The information discussed so than surface methods, since soil intake properties estimate when to irrigate and the amount of water are of minor importance. Moreover, estimating needed to refill the soil water reservoir (net application efficiency with these systems is much irrigation requirement). When water is applied to easier than with surface methods. an orchard, however, some losses are Because application efficiencies vary, each unavoidable, especially with surface irrigation situation must be evaluated for Ea. Cooperative methods, and they must be considered in Extension, the Natural Resources Conservation calculating the actual amount of water to be Service, and private consultants offer assistance applied (gross irrigation requirement). The type in evaluating systems. The following list shows of irrigation used, soil and climatic conditions, gross estimates of Ea associated with different and water management practices largely irrigation methods. determine irrigation efficiency. Water applied to a field can be lost by runoff, System Ea (%) zone, evaporation and, percolation below the root 70-80 Basin with sprinklers, spray evaporation and drift. The 70-80 Border strip goal of good on-farm water management is to Furrow 65-75 minimize these losses. For instance, runoff can Sprinkler 75-85 be minimized by using an irrigation system Drip, microsprinkler 85-95 design that prevents it or reuses the collected tailwater. Application efficiency (Ea) is a term commonly used to describe how well growers DEVELOPING IRRIGATION SCHEDULES irrigate. It is defined as the percentage of applied Each component of the water budget approach irrigation water that is available for crop use. has been discussed; it is a simple procedure to determine an actual irrigation schedule. For Ea = Water used by plant surface irrigated orchards, this means the water Water applied loss through ETc is totaled until it exceeds a predetermined percentage of the total available In general, Ea is directly related to how water in the tree root zone. At that time, you need uniformly water can be applied over the surface. to irrigate and should apply the amount equal to Therefore, the method of irrigation is of prime the ETc loss plus the unavoidable loss associated importance. with each irrigation. For drip/microsprinkler Most pistachio orchards use drip and irrigated orchards, you must decide upon an microsprinkler irrigation, rather than the surface irrigation frequency and then apply water to meet methods (basin, furrow and border strip) ETc and system losses due to application commonly used with other deciduous trees. Each nonuniformity. The following examples using method differs in how uniformly water can be long-term, historical ETo show actual applied. With surface irrigation, the intake development of irrigation schedules for a mature properties of the soil and the rate that water pistachio orchard; first for a microsprinkler moves over the field determines the uniformity of system, and then for a border strip system. infiltration. The faster the water moves to the bottom of a basin or run, the smaller the General Information difference in the opportunity time for infiltration Location: San Joaquin Valley, California between the top and bottom of the field. Soil: Sandy loam Distribution of water under sprinkler irrigation Rooting depth: 6 ft depends mostly on systems, design, including Tree spacing: 17 x 17 ft spacing, nozzle type and size, riser height, Available water storage capacity (AWC): 1.5 in/ft and operating pressure. With drip and Allowable depletion: 50% of total AWC microsprinklers, design and maintenance determine the uniformity of application. In 109

8 EXAMPLE 1 . Microsprinkler irrigation system. Assume: application efficiency: 90% application rate: 11 gallons/sprinkler/hour (one sprinkler per tree) irrigation frequency: every 3 days Step 1. Calculate the water use rate (July 1-15, for example) Orchard Etc = (Kc)(ETo) = (1.19)(0.27 inches/day) = 0.32 inches/day Individual tree ETc = (orchard ETc) (tree spacing) (conversion factor) 2 = (0.32 inches/day) (17 x 17 ft) (.622 gal/in.ft ) = 58 gal/tree/day Step 2. Calculate the irrigation amount. amount to apply = Etc application efficiency = 58 gal/tree/day 0.90 = 64 gal/tree/day = 193 gals every 3 days Step 3. Calculate the set time (duration of water application). Set time = Amount to apply application rate = 193 gals 11 gals/hr = 17.5 hrs every 3 days 110

9 EXAMPLE 2. Border strip irrigation system. Assume: system delivery: 500 gallons/minute/acre application efficiency: 80% Step 1. Estimate available moisture in the root zone. Total available water (AW) = AW x rooting depth = (1.5 in/ft) (6 ft) = 9.0 inches Step 2. Calculate the amount of depletion allowed between irrigations. allowable depletion (AD) = (total AW) (depletion %) = (9.0 inches) (0.50) = 4.5 inches Step 3. Estimate normal orchard water use. Historical ETc data from Table 2 for a clean cultivated or chard are plotted as cumulative ETc vs. time in Figure 4. Step 4. Decide when to irrigate. Assuming that the soil water profile is full as the season begins (from a combination of winter rainfall and postharvest irrigation), deciding when to irrigate is simply a matter of periodically determining when the cumulative ETc equals the amount of depletion allowe d (from Step 2). This procedure is illu strated in Figure 4 and results in a total of 9 irrigations, beginning on May 20, then June 8 and so on. NOTE : If the root zone is only partially wet at the beginn ing of the season, the initial total available stored water can be estimated by soil probing. Calculate the irrigation amount. Step 5. Amount of apply = Depletion amount Application efficiency = 4.5 inches 0.80 = 5.6 inches x 27,100 gal/acre-inch = 152,000 gals/acre Step 6. Calculate the set time. Set time = amount to apply delivery rate = 152,000 gals/acre 500 gals/min = 300 mins/acre = 5 hrs/acre 111

10 Industry Annual Report for detailed results ( ). We found that virtually all tree processes or performance parameters were affected by water stress. The magnitude of the response depended on the process/parameter and the intensity of the stress. To illustrate the effects, we've ranked the important tree processes or parameters in Figure 13e based on their sensitivity to the range of water stress imposed in our study. The response ing average tree response bars were generated us values over the second and third years of stress relative to 100% ETc . Note the sensitivity Figure 13d Graphical means of estimating . ranking of the important yield components (most irrigation dates using the water budget approach rst): blanking and nut stress sensitive listed fi and assuming long-term, historical ET. abortion > shell splitting > nuts per tree > harvestability > individual nut weight and size. Marketable , the integrator of all the yield yield VERIFICATION OF THE WATER BUDGET components, was more affected by the range of The water budget procedure is based on sound deficit irrigation levels imposed than any of the agronomic principles. However, even if the most yield components. individual accurate information is used, it is a good idea to check soil moisture levels periodically. This can Regulated deficit irrigation be done with a soil probe or auger based on how The bulk of California's pistachio orchards are in the soil sample behaves when squeezed in the high water cost areas; commonly $100/acre-ft. In hand. Or, soil-based instruments can be used. a drought year, not only is water expensive, but it This monitoring is necessary because of the simply may not be available. Regulated deficit uncertainties in the water budget associated with irrigation (RDI) is a technique that purposely 1) the amount of water applied and the depth of stresses trees during certain stages of tree/fruit penetration, 2) estimating AWC and AD due to growth in order to reduce ETc (save water) while spatial variability of soils, and 3) estimating minimizing or eliminating negative impacts of application efficiency. stress on fruit yield or quality. This approach differs from season-long stress in that the deficit ADDITIONAL CONSIDERATIONS irrigation is restricted to stress-tolerant periods. Season-long deficit irrigation First applied on stone fruit in Australia and New To evaluate the consequences of not supplying Zealand, successful RDI depends primarily on mature trees with all the water they could use, we inducing stress during periods of slow vegetative conducted a three-year study where various and reproductive growth. We began testing the percentages of full ETc (0, 25, 50, 75 and 100%) RDI concept for pistachios in 1989 and proposed were applied uniformly over the season. Frequent two stress-tolerant periods: 1) from mid-May and intensive measurements of all important tree through early July (growth stage 2), just after full performance parameters were made. Please refer shell development and before the onset of rapid to our paper in the 1987 California Pistachio kernel growth; and 2) postharvest. 112

11 Stage 2 is our stress focus since it is after the initial shoot growth flush, when there is minimal nut growth (Figure 13f); the primary nut growth process during this period is shell wall thickening (lignification), and potential water use is increasing. A typical irrigation schedule for the proposed conservative RDI regime is shown in Table 3. We conducted a large-scale, four-year project (1998-92) which evaluated numerous stress levels during the abovementioned periods in a deep-rooted orchard. We concluded that irrigating at 25-50% of potential ETc during stage 2 and postharvest did not affect production. Figure 13e. Generalized diagram of the sensitivity Stress during stage 3 (early July through harvest) to water stress of selected pistachio plant processes should be avoided. and parameters. Widths of the horizontal bars at given stress levels represent the relative magnitude of the response and were developed by evaluating effects under dry land, 25, 50, 75% ETc relative to 100% ET (dry land illustrated as “severe” stress and 75% ET shown as “mild”). . Table 3 San Joaquin Valley pistachio water use (Etc) for normal and proposed RDI regimes. Irrigation schedule . early in the season must take into account stored winter rainfall Reference Crop Normal Proposed Proposed water use coeff. Etc in RDI RDI Growth Approximate period level Etc (%) (inches) Stage phenology Period (inches) (inches) Bloom Apr 1-15 2.36 0.07 0.17 100 0.17 Leafout Apr 16-30 2.36 0.43 1.01 100 1.01 Stage 1 Shell Expansion May 1-15 3.19 0.68 2.17 100 2.17 Shell Hardening May 16-31 3.40 0.93 3.16 50 1.58 Stage 2 Shell Hardening Jun 1-15 3.84 1.09 4.19 50 2.09 Shell Hardening Jun 16-30 3.84 1.17 4.49 50 2.25 Nut Filling Jul 1-15 4.13 1.19 4.92 100 4.92 Nut Filling Jul 16-31 4.41 1.19 5.25 100 5.25 Stage 3 3.54 1.19 4.21 100 4.21 Nuf Fill./Shell Split. Aug 1-15 Shell Splitting Aug 16-31 3.78 1.12 4.23 100 4.23 Hull Slip Sept 1-15 2.66 0.99 2.63 100 2.63 Harvest Sept 16-30 2.66 0.87 2.31 25 0.58 Postharvest Oct 1-15 1.71 0.67 1.15 25 0.29 Postharvest Postharvest Oct 16-31 1.83 0.50 0.91 25 0.23 Postharvest Nov 1-15 0.80 0.35 0.28 25 0.07 Totals 41.10 31.70 113

12 growers with no closed shell problem may not want to sacrifice fruit size for a small increase in split nuts. There are other impacts of Stage 1 stress on pistachio production. Early season stress has been shown to cause unwanted premature shell splitting in mid stage 3. These nuts are subject to fungal disease and possible aflatoxin-affected nuts at harvest. Additionally, processor data shows that loose shells and kernels are higher with Stage 1 stress. On the positive side, we found that Stage 1 stress with PG1 in an “on” alternate bearing year significantly increased fruit load in the following “off” year, suggesting that this RDI strategy may be useful in mitigating alternating bearing. . Figure 13f Generalized time course We must also point out that irrigating at 25% development of shell and kernel growth for San ETc during stage 2 on a very shallow soil Joaquin Valley pistachios. reduced both shell size and splitting. Where there is a small soil moisture reservoir, we recommend Production scale tests have been conducted irrigating at 50% ETc during stage 2. With since 1992 with cooperating growers under a deeper root zones and consequently larger soil variety of soil conditions. Production scale sites moisture levels at the onset of stage 2, 50% ETc have replicated irrigation regimes, are monitored during the deficit irrigation period is as intensively as research plots, but allow for conservative. commercial evaluation of nut yield and quality. RDI appears to be a promising technique to We have observed no negative effects of save water while maintaining or even enhancing irrigating at 50% ETc during stage 2 or production of marketable product. The key is postharvest. Shell staining due to fungal disease being able to impose the desired stress at the also is occasionally lower apparently due to correct time. The deeper the soil and the greater lower orchard humidity during the imposition of the soil moisture reserves going into stage 2, the deficit irrigation. longer the lag between cutting back on irrigation In some cases, significantly higher shell and the onset of tree stress. RDI is much easier to splitting occurred with deficit irrigation during impose with drip/microsprinkler irrigation which stage 1 (leafout through mid May). We attribute fortunately is used on an estimated 85% of the this to mild stress slightly reducing shell size pistachio acreage. and/or thickness. We believe that this can be due to the combination of a weaker shell and the Young trees developing kernel exerting additional physical Estimating the irrigation requirements of pressure at the shell suture line. On the other immature pistachio trees is difficult for two hand, stage 1 stress can result in slightly smaller reasons. First, researchers have yet to develop nuts at harvest. Ongoing research with both specific information on young pistachio ETc. Atlantica and PGI rootstocks confirms that We must assume that information from other improved shell splitting, albeit with slightly deciduous trees, such as that for almonds smaller nuts, can be achieved with Stage 1 stress. presented in Figure 13c, applies to pistachios. This new work suggests that filled closed shell Successful application of experimental results can be reduced by about 50% with stage 1 stress also depends on accounting for differences in while nut size is reduced by 4 to 8%. Thus, if irrigation regimes. Since the evaporation growers have relatively high closed shell at component accounts for a much larger percentage harvest, Stage 1 stress could be a tool to improve of ETc in young orchards, differences in wetted yield of marketable product. On the other hand, 114

13 create tree water deficits that may ultimately surface area and frequency will have relatively affect orchard yield more than the disease. large effects on ETc. Fungal diseases, primarily Alternaria, can The second problem in determining the reduce fruit quality due to shell staining. In irrigation requirements of first- or second-year ly permeable soils and orchards that have slow trees is that application efficiency of the applied water ponds on the soil surface for days irrigation systems cannot be measured easily. after an irrigation, or where sprinkler spray This is because of the small size and uncertain patterns are directed into the tree canopies, fungal location of the root zone. Since irrigation systems disease is more prevalent. Higher humidity levels are usually designed to perform optimally in a in such orchards promote disease activity. With mature orchard, a significant amount of water can sprinklers, the best solution is to use low-angle be lost to deep percolation in young orchards just sprinklers. With poor infiltration rate soils, we because water is applied outside of the root zone. have shown that buried dr ip irrigation can reduce Even drip/microsprinkler systems that have high orchard humidity, thus reducing incidence of distribution uniformities and high application fungal diseases. The key is installing the system efficiencies in mature orchards have much lower such that the soil surface is not (or is minimally) application efficiencies in new plantings (values wetted throughout the season. of 30 to 50 percent are typical in first-year trees). Application efficiencies improve as the root zones develop. Poor infiltration or fixed surface Growers can maximize the application w a t e r deliveries efficiency in young orchards by managing their In addition to fungal disease, standing water can irrigation systems to limit the amount of water cause poor root health and can be a logistical applied outside the root zone. With a nightmare. Many growers do not want water drip/microsprinkler system, you can place the standing in their orchards for more than 24 hours. emitters close to the trunk, recognizing that they Because the soil intake rate dictates the quantity will be moved as the trees mature. Another of water entering the soil with surface irrigation, alternative is to operate microsprinklers at the amount that infiltrates in 24 hours is the reduced pressures to decrease the area wetted, maximum application for each irrigation in these although you should check the system to make situations. On low infiltration rate soils, this may sure it maintains a high uniformity of application. be less than the net irrigation requirement With surface systems, small furrows on either calculated by the water budget. For the example side of the tree can improve application presented earlier, assume that the infiltration rate efficiency. averages 0.10 inch per hour. Thus, approximately 2.4 inches of water infiltrate at each irrigation. This value becomes the amount of depletion Irrigation-disease interactions allowed, rather than the 4.5 inches we calculated, Water management can influence the based on AWC, AD and rooting depth. This predisposition of pistachio trees to infection by V. means the grower will have to irrigate more dahliae . Since soil temperature might be a factor frequently than originally scheduled but with less influencing infection, early season irrigation water per application. timing may have to be adjusted in orchards with Similarly, the irrigation schedule can be Verticillium . The experience of cotton growers adjusted, if water deliveries from the irrigation facing this problem can be used as a guide to district occur on a fixed schedule. For example, if develop optimal irrigation strategies. For water is received every two weeks, the irrigator pistachios, refill the soil profile with a winter simply determines the crop water usage over the irrigation, and delay the first post-leafout last two-week period and applies that amount irrigation as long as possible. This practice plus losses due to system inefficiency. So rather allows the soil temperatur e to remain relatively than applying the same amount of water per high during the spring, a time when most irrigation, the quantity applied will change Verticillium infections occur. It should be noted throughout the season. that withholding water for too long a period can 115

14 disastrous consequences of poor decisions. Consultants While all the information necessary to develop The use of irrigation consultants represents a water budget-based irrigation schedules is readily viable option for growers interested in improved available, the "housekeeping" aspects of water management. Growers commonly hire irrigation scheduling can be time-consuming, formation and/or make consultants to provide in making the use of personnel specializing in this decisions for cultural practices where the grower discipline desirable. lacks knowledge, time, and/or recognizes the 116

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