Managing irrigation and drainage systems in arid areas in the presence of shallow groundwater: case studies

Two field studies were conducted on the west side of the San Joaquin Valley of California to demonstrate the potential for integrated management of irrigation and drainage systems. The first study used a modified cotton crop coefficient to calculate the irrigation schedule controlling the operation of a subsurface drip system irrigating cotton in an area with saline groundwater at a depth of 1.5 m. Use of the coefficient resulted in 40% of the crop water requirement coming from the groundwater without a loss in lint yield. The second study evaluated the impact of the installation of controls on a subsurface drainage system installed on a 65 hectare field. As a result of the drainage controls, 140 mm less water was applied to the tomato crop without a yield loss. A smaller relative weight of tomatoes classified as limited use, was found in the areas with the water table closest to the soil surface.     

Farm-level and district efforts to improve water management during drought

The fifth year of drought in California brought reductions in surface water deliveries to many water districts. In the central San Joaquin Valley, water deliveries to Broadview Water District were reduced by 50% in 1990 and by 75% in 1991. The district increased the level of service provided to farmers during these years by providing accurate water use data, increasing the flexibility allowed in scheduling water deliveries, and managing water transfers and purchases when water was available. Farmers in the district implemented new irrigation practices and increased the efficiency of water applications. Several crops were irrigated more frequently than usual, but the amount of water applied during each irrigation event was reduced. The total amount applied during pre-irrigations and seasonal irrigations was also reduced. More than 38% of district land was idled in 1991, with the largest proportional reductions in melon, sugarbeet, and grain plantings. Field application efficiencies increased for all crops in 1990 and 1991 and the district-wide field application efficiency increased from 0.73 in 1989, to 0.77 in 1990, and 0.81 in 1991.     

Simulated crop production under saline high water table conditions

Summary Many irrigated lands in semi-arid regions of the world are underlain with saline high water tables. Water management is critical to maintain crop productivity under these conditions. A multi-seasonal, transient state model was used to simulate cotton and alfalfa production under various irrigation management regimes. The variables included in-season water application of 1.0 or 0.6 potential evapotranspiration (PET), and 18 or 33 cm pre-irrigation amounts for cotton. The water table was initially at a 1.5m depth and a 9 dS/m salinity. A impermeable lower boundary at 2.5 m depth was imposed. Irrigation water salinity was 0.4 dS/m. Climatic conditions typical to the San Joaquin Valley of California were used for PET and precipitation values. The simulations were for no-lateral flow and also lateral flow whereby the water table was raised to its initial level prior to each irrigation event. Uniform application of 1.0 PET provided for relative cotton lint yields and alfalfa yields of 95% or more for at least 4 years. In-season irrigation of cotton with 0.6 PET had higher yields when associated with a 33 cm rather than an 18 cm pre-irrigation. Lateral flow provided for higher cotton lint yields production than the no-lateral flow case for each pre-irrigation treatment. The beneficial effects of lateral flow diminished with time because of the additional salt which accumulated and became detrimental to crop production. Substantial alfalfa yield reductions occurred after the first year when irrigation was set at 0.6 PET regardless of other conditions. Evaporation losses from the soil during the cotton fallow season were higher when the soil water content entering the fallow season were higher.Research was supported by the University of California Salinity/ Drainage Task Force     

Salinity, drainage and selenium problems in the Western San Joaquin Valley of California

Highly productive, irrigated agriculturecan be found in California's Central Valleymade up of the Sacramento, San Joaquin, andTulare Lake basins. High water tablesthroughout much of the San Joaquin andTulare Lake portions of the Valley threatenthis highly productive region. Due totopographic and environmental constraintsmuch of the region is currently withoutdrainage. In 1990 State and Federalgovernment agencies combined to produce areport that outlined possible steps to dealwith the drainage issue. These stepsincluded: 1) Source control (practices toreduce the amount of drainage water); 2)Drainage reuse; 3) Evaporation systems; 4)Land retirement (cease irrigation); 5)Groundwater management; and 6) Discharge tothe San Joaquin River. General backgroundinformation to the history and hydrology ofthe Valley as well as a discussion of thefeasibility and constraints of providingdrainage by discharging drain water to theSan Joaquin River are presented. Inaddition a general discussion of thetechnical and political limitations ofproviding drainage in the River arediscussed.     

Infrared canopy temperature of early-ripening peach trees under postharvest deficit irrigation

Canopy temperature measurements with infrared thermometry have been extensively studied as a means of assessing plant water status for field and row crops but not for fruit trees such as peaches. Like in many regions of the world, the lack of water is beginning to impact production of tree fruit such as peaches in the San Joaquin Valley of California. This is an area where irrigation is the only source of water for agricultural crops in the summer growing season. A two-year field study was conducted to assess plant water stress using infrared canopy temperature measurements and to examine its feasibility for managing postharvest deficit irrigation of peach trees. Twelve infrared temperature sensors were installed in a mature peach orchard which received four irrigation treatments: furrow and subsurface drip irrigation with or without postharvest water stress. During the two-year period, measured midday canopy to air temperature differences in the water-stressed postharvest deficit irrigation treatments were in the 5-7 °C range, which were consistently higher than the 1.4-2 °C range found in the non-water-stressed control treatments. A reasonable correlation (R² = 0.67-0.70) was obtained between stem water potential and the canopy to air temperature difference, indicating the possibility of using the canopy temperature to trigger irrigation events. Crop water stress index (CWSI) was estimated and consistently higher CWSI values were found in the deficit irrigation than in the control treatments. Results of yield and fruit quality assessments were consistent with the literature when deficit irrigation was deployed.     

Crop coefficients for drip-irrigated processing tomato

Drip irrigation of processing tomato is increasing in the San Joaquin Valley of California (USA), a major tomato production area. Efficient management of these irrigation systems requires reasonable estimates of crop evapotranspiration (ET_c) between irrigations. A common approach for estimating ET_c is to multiply a reference crop evapotranspiration (ET_o) by a crop coefficient. However, a review of literature revealed mid-season crop coefficients for processing tomato to range from 1.05 to 1.25. Because of this variability, uncertainty exists in the crop coefficients appropriate for drip irrigation in the San Joaquin Valley. Thus, a study was initiated to determine the ET_c of processing tomato for drip irrigation in commercial fields and then calculate crop coefficients from the ET_c and ET_o data for the west side of the San Joaquin Valley. Crop ET_c was determined at five locations using the Bowen Ratio Energy Balance Method (BREB). Canopy coverage was also measured using a digital infrared camera. Average crop coefficients ranged from about 0.19 at 10% canopy coverage to 1.08 for canopy coverage exceeding about 90%. A second order regression equation reasonably described a relationship between crop coefficient and canopy coverage. Generic curves describing crop coefficient versus time of year were developed for various planting times.     

Nitrate leaching and soil nitrate content as affected by irrigation uniformity in a carrot field

High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO₃-N) and ammonium (NH₄-N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO₃-N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO₃-N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity (K_s), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO₃-N and NH₄-N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.     

Characterizing ground water use by safflower using weighing lysimeters

Decades of irrigation on the west side of the San Joaquin Valley without sufficient drainage have created large areas where shallow ground water (<1.5 m) has become a problem for agriculture. Because drainage outflow is restricted as a result of environmental concerns, reducing the amount of irrigation applied is a farm management solution for this situation. One option to reduce the amount of irrigation water is to include shallow ground water use as a source of water for crop production when scheduling irrigation. The objective for this study is to describe soil water fluxes in the presence of saline, shallow ground water under a safflower crop. Two weighing lysimeters, one with and one without shallow saline ground water were used to measure crop evapotranspiration of surface drip irrigated safflower. A saline water table (14 dS/m) was maintained in one of the lysimeters. Ground water use as part of crop evapotranspiration was characterized using hourly measurements of the water level in a ground water supply tank (Mariotte bottle). Ground water contribution of up to 40% of daily crop water use was measured. On a seasonal basis, 25% of the total crop water use originated from the ground water. The largest ground water contribution was shown to occur at the end of the growing season, when roots are fully developed and stored soil water in the root zone was depleted. The applied irrigation on the crop grown in the presence of a water table was 46% less than irrigation applied to the crop without a water table. The reduction of irrigation was obtained by using the same irrigation schedule as on the lysimeter without ground water, but through smaller applied depths per irrigation event.     

Economic analysis of integrated on-farm drainage management

The goal of integrated on-farm drainage management (IFDM) is to eliminate the discharge of subsurface drainage water from farms into waterways or evaporation ponds. Components of a typical IFDM system include improved irrigation practices, irrigation of salt-tolerant plants with drainage water, and on-farm disposal of drainage water using a solar evaporator. Costs of an IFDM system include initial investments, operation and maintenance, and the opportunity costs of land used for the solar evaporator and for irrigation of nonmarketable, salt-tolerant plants. The farm-level cost of an IFDM system increases with the proportion of farmland used to irrigate salt-tolerant plants. A conceptual framework for evaluating the farm-level costs of IFDM is presented, along with empirical analysis from California’s San Joaquin Valley.     

Drip Irrigation of Tomato and Cotton Under Shallow Saline Ground Water Conditions

Artificial subsurface drainage is not an option for addressing the saline, shallow ground water conditions along the west side of the San Joaquin Valley because of the lack of drainage water disposal facilities. Thus, the salinity/drainage problem of the valley must be addressed through improved irrigation practices. One option is to use drip irrigation in the salt affected soil.A study evaluated the response of processing tomato and cotton to drip irrigation under shallow, saline ground water at depths less than 1 m. A randomized block experiment with four irrigation treatments of different water applications was used for both crops. Measurements included crop yield and quality, soil salinity, soil water content, soil water potential, and canopy coverage. Results showed drip irrigation of processing tomato to be highly profitable under these conditions due to the yield obtained for the highest water application. Water applications for drip-irrigated tomato should be about equal to seasonal crop evapotranspiration because yield decreased as applied water decreased. No yield response of cotton to applied water occurred indicating that as applied water decreased, cotton uptake of the shallow ground water increased. While a water balance showed no field-wide leaching, salinity data clearly showed salt leaching around the drip lines.     

Growth and yield responses of almond (Prunus amygdalus) to trickle irrigation

Growth and yield responses of developing almond trees (Prunus amygdalus, Ruby cultivar) to a range of trickle irrigation amounts were determined in 1985 through 1987 (the fifth through seventh year after planting) at the University of California's West Side Field Station in the semi-arid San Joaquin Valley. The treatments consisted of six levels of irrigation, ranging from 50 through 175% of the estimated crop evapotranspiration (ET_c), applied to a clean-cultivated orchard using a line source trickle irrigation system with 6 emitters per tree. ET_c was estimated as grass reference evapotranspiration (ET₀) times a crop coefficient with adjustments based upon shaded area of trees and period during the growing season. Differential irrigation experiments prior to 1984 on the trees used in this study significantly influenced the initial trunk cross-section area and canopy size in the 50% ET_c treatment and 125% ET_c treatment. In these cases, treatment effects must be identified as relative effects rather than absolute. The soil of the experimental field was a Panoche clay loam (nonacid, thermic, Typic Torriorthents). The mean increase in trunk cross-sectional area for the 3-year period was a positive linear function (r ² = 0.98) of total amounts of applied water. With increases in water application above the 50% ET_c treatment, nut retention with respect to flower and fertile nut counts after flowering, was increased approximately 10%. In 1985 and 1987, the nut meat yields and mean kernel weights increased significantly with increasing water application from 50% to 150% ET_c. Particularly in the higher water application treatments, crop consumptive use was difficult to quantify due to uncertainty in estimates of deep percolation and soil water uptake. Maintenance of leaf water potentials higher than –2.3 MPa during early nut development (March through May) and greater than –2.5 MPa the remainder of the irrigation season (through August) were positively correlated with sustained higher vegetative growth rates and higher nut yields.     

A Water Transfer and Agricultural Land Retirement in a Drainage Problem Area

The increasing scarcity of water in California and the rising cost of compliance with environmental regulations are motivating some farmers in the San Joaquin Valley to sell their land and water, and discontinue production of irrigated crops. In the summer of 2004, all landowners in the 3,700-ha Broadview Water District decided to sell their land to Westlands Water District. The land sales have been completed and Westlands has acquired Broadview's water supply contract. Farmland in Broadview will no longer be irrigated. We describe what motivated the purchase and sale of land and water in Broadview and discuss the potential gains to participants. We describe also the potential public benefits that include an increase in economic activity and environmental enhancement in the San Joaquin Valley. Farm workers displaced by land retirement in Broadview will find employment in the Westlands Water District. Tenant farmers in Broadview will need to find other land on which to continue farming after the land sales are completed. The challenge they face is caused partly by a regional trend toward greater production of perennial crops that is leaving less land available for annual leases.Formerly Manager of Broadview Water District, Firebaugh, California     

Water use efficiency of narrow row cotton

Summary A field experiment was conducted on the west side of the San Joaquin Valley in California to determine water use, crop growth, yield and water use efficiency of Acala (SJ-2) cotton (Gossypium hirsutum L.) grown in 0.5 m spaced rows on a Panoche clay loam soil (Typic Torriorthents). Evapotranspiration was determined by water balance techniques utilizing neutron soil moisture measurements. All neutron measurements were made within a 3 m soil profile in 0.20 m increments. The measured evapotranspiration was compared to climatic estimates of potential evapotranspiration, and to calculations using a one-dimensional soil water balance model that separately computed soil water evaporation and plant transpiration. Crop growth was determined by weekly destructive plant sampling. Leaf area was determined along with dry matter components of leaves, stems, fruiting parts (flowers and squares) and bolls. Final yield was determined by machine harvesting (brush stripper) 720 m² from each plot. Lint yields and fiber quality were determined by sample ginning and fiber analysis at the U.S. Cotton Research Station at Shafter, California. Three irrigation regimes were established that resulted in an evapotranspiration range from a high deficit condition to full irrigation at the calculated atmospheric demand.The measured evapotranspiration of narrow row cotton under a full irrigation regime was 778 mm, 594 mm under a limited irrigation regime and 441 mm under a regime with no post-plant irrigation. The evapotranspiration from these irrigation treatments was accurately simulated by a water balance model. that used inputs of potential evapotranspiration, leaf area index, soil water holding capacity and root development.The average lint yield from narrow row cotton with a full irrigation regime was 1583 kg/ha, the average lint yield from a limited irrigation regime was 1423 kg/ha and the average lint yield from a treatment with no postplant irrigation (fully recharged soil profile at planting) was 601 kg/ha. The full irrigation regime resulted in a dry matter production of approximately 16 t/ha while the limited irrigated regime produce 11 t/ha and the no-postplant irrigation regime produced 7 t/ha of dry matter. The fiber quality results indicated significant (0.05 level) differences only in 50% span length and micronaire, with the 2.5% span length, uniformity index, elongation and strength indicating no difference.Cotton lint yield was found to be directly related to total evapotranspiration although the relationship was slightly non-linear while dry matter yield was found to be linearly related to evapotranspiration. Both lint and dry matter yield were found to have a linear relationship to estimated transpiration from the water balance model calculations.Contribution from the Unived States Department of Agriculture, Agricultural Research Service, Western Region and the University of California     

Temporal persistence of spatial soil-water patterns under trickle irrigation

Summary Trickle irrigation of perennial crops results in local wetting near trees and vines. Methods to measure soil-water content or storage within the root zone generally require intensive instrumentation to characterize spatial patterns of soil water adequately. The goals of this research were to determine if spatial patterns of soil-water storage under trickle irrigation are temporally presistent which may make it feasible to use less intense sampling to characterize total storage. Soil-water storage from the 0 to 1.5 m soil depth was measured at 23 sites on one side of trickle-irrigated almond trees using a neutron probe over three years in the San Joaquin Valley of California. Measurements were made on two trees in each of five different irrigation treatments. The persistence of spatial patterns with time was evaluated using Spearman rank correlation and relative differences from mean values. Spatial patterns were different for each tree and irrigation treatment but remained fairly persistent with time during a season. In many cases, temporal changes in soil-water storage were adequately estimated from a single location. Single sampling locations identified during one year gave estimates of mean storage during the following year with some increase in error. However, use of the same sampling locations for more than two years increased the error in storage estimates. Soil-water content or storage in trickle-irrigated orchards may be monitored by intense sampling during the early part of the irrigation season in order to identify locations giving mean soil-water storage. Only these locations may then be sampled to monitor changes in soil water.     

Environmental decision support system development for seasonal wetland salt management in a river basin subjected to water quality regulation

Seasonally managed wetlands in the Grasslands Basin on the west-side of California’s San Joaquin Valley provide food and shelter for migratory wildfowl during winter months and sport for waterfowl hunters during the annual duck season. Surface water supply to these wetlands contain salt which, when drained to the San Joaquin River (SJR) during the annual drawdown period, can negatively impact water quality and cause concern to downstream agricultural riparian water diverters. Recent environmental regulation, limiting discharges salinity to the SJR and primarily targeting agricultural non-point sources, now also targets return flows from seasonally managed wetlands. Real-time water quality management has been advocated as a means of continuously matching salt loads discharged from agricultural, wetland and municipal operations to the assimilative capacity of the SJR. Past attempts to build environmental monitoring and decision support systems (EDSS’s) to implement this concept have enjoyed limited success for reasons that are discussed in this paper. These reasons are discussed in the context of more general challenges facing the successful implementation of a comprehensive environmental monitoring, modelling and decision support system for the SJR Basin.     

Temporal variability in water quality of agricultural tailwaters: Implications for water quality monitoring

Accurate assessments of non-point source pollution and the associated evaluation of mitigation strategies depend on effective water quality monitoring programs. Intensive irrigation season water quality monitoring was conducted on three agricultural drains (6 h to daily sampling) along with analysis of decade long records from two larger agricultural drains (biweekly to monthly sampling) in the San Joaquin Valley, California. Analyses revealed significant temporal variability in concentrations of nutrients, salts, and turbidity over short time-scales (<1 day), as well as significant differences in monthly and annual mean concentrations. Statistical techniques were used to evaluate the sampling intensity required to meet rigorous confidence and accuracy criteria, as well as to evaluate the efficacy of different sampling strategies (e.g. grab samples versus composite samples). The number of samples required to determine mean constituent concentrations within 20% of the mean at a 95% confidence level ranged from 2 to 39 samples per month (SPM) for total phosphorus, 1-16 SPM for total nitrogen, 5-25 SPM for turbidity, and 1-3 SPM for electrical conductivity. Using a daily composite sample (4 subsamples per composite) instead of discrete samples was shown to maintain the same accuracy and confidence standards, while reducing the required sample number by up to 50%. This study emphasizes the value of a statistical approach for evaluating water quality monitoring strategies, and provides a framework through which cost-benefit analysis can be implemented in the development of monitoring plans.     

Changes in spatial and temporal variability of SAR affected by shallow groundwater management of an irrigated field, California

In the irrigated western U.S. disposal of drainage water has become a significant economic and environmental liability. Development of irrigation water management practices that reduce drainage water volumes is essential. One strategy combines restricted drainage outflow (by plugging the drains) with deficit irrigation to maximize shallow groundwater consumption by crops, thus reducing drainage that needs disposal. This approach is not without potential pitfalls; upward movement of groundwater in response to crop water uptake may increase salt and sodium concentrations in the root zone. The purposes for this study were: to observe changes in the spatial and temporal distributions of SAR (sodium adsorption ratio) and salt in a field managed to minimize drainage discharge; to determine if in situ drainage reduction strategy affects SAR distribution in the soil profile; and to identify soil or management factors that can help explain field wide variability. We measured SAR, soil salinity (EC_1:1) and soil texture over 3 years in a 60-ha irrigated field on the west side of the San Joaquin Valley, California. At the time we started our measurements, the field was beginning to be managed according to a shallow groundwater/drainage reduction strategy. Soil salinity and SAR were found to be highly correlated in the field. The observed spatial and temporal variability in SAR was largely a product of soil textural variations within the field and their associated variations in apparent leaching fraction. During the 3-year study period, the percentage of the field in which the lower profile (90-180 cm) depth averaged SAR was above 10, increased from 20 to 40%. Since salinity was increasing concomitantly with SAR, and because the soil contained gypsum, sodium hazard was not expected to become a limiting factor for long term shallow groundwater management by drain control. It is anticipated that the technology will be viable for future seasons.     

Interaction of applied water amounts and leaf removal in the fruiting zone on grapevine water relations and productivity of Merlot

A study was conducted in the San Joaquin Valley of California on Merlot to determine the interaction of applied water amounts [at 0.4, 0.8, and 1.2 of estimated vineyard evapotranspiration (ET_c)] and leaf removal (at berry set or veraison) in the fruiting zone on productivity. Shaded area was measured beneath the canopy of the 1.2 irrigation treatment at solar noon throughout the study to provide an estimate of seasonal crop coefficients (K _c). Vine water status was assessed across treatments and years by measuring midday leaf water potential (Ψ_l). The maximum K _c determined from the percent shaded area was 0.7 at the row spacing of 3.66?m and canopy type that developed a “California Sprawl.” Irrigation treatment had a significant effect on midday Ψ_l and no such effect for leaf removal. Clusters exposed to direct solar radiation had significantly higher temperatures and lower cluster Ψ than clusters in the shade. Irrigation treatment had a significant effect on berry weight, soluble solids, and titratable acidity. Yields of vines significantly increased as applied water amounts increased. In this wine grape production area, profitability is dependent upon yield. This study provided a reliable estimate of ET_c and applied water amounts to maximize yield.     

Identifying sources of recharge to shallow aquifers using a groundwater model

The poor water quality of sub-surface drainage, hereafter drainage, water generated in the western San Joaquin Valley in California creates management challenges for farmers and water managers. Elevated concentrations of salt and trace elements in agricultural drainage limit the disposal options. In this constrained environment, determining the original source of drainage water is a crucial step in developing appropriate drainage management policies. Numerical modeling results of near-surface water-table fluctuations indicate that the substantial groundwater rise observed in the vicinity of the region's major water supply canal could not be attributed solely to seepage from overlying irrigated fields. An inverse solution approach is used herein to test the theory that seepage from the canal itself and/or that from surface water retention ponds (designed to protect the structure from flash floods) is responsible for an accentuated groundwater mound. The results suggest that canal seepage is the more likely source of non-agricultural aquifer recharge.     

Irrigation scheduling of almond trees with trunk diameter sensors

The use of plant indicators may be the ideal method for irrigation scheduling but it is hampered by the dynamic nature of plant water status and by the lack of suitable indicators, relative to established scheduling methods based on atmospheric and soil observations. A study was conducted in an almond orchard located in the San Joaquin Valley of California during the 2001 season using trunk diameter variations as the only indicator for determining the amount of irrigation. The ratio of the maximum daily shrinkage (MDS) of tree trunks relative to a reference MDS, calculated from a relationship between MDS of fully irrigated trees and atmospheric vapor pressure deficit, was used as a signal for modifying the amount of applied irrigation water. Applied water was increased by 10% each time the MDS signal exceeded the prescribed threshold. When the MDS signal went below the threshold, applied water was reduced by 10% in an interactive manner. Two schedules were tested with signal thresholds of 1.75 and 2.75, which generated mild and moderate water stress, respectively, as indicated by their stem water potential (SWP) values. The two irrigation treatments had SWP that varied over the season from around –0.7 to –1.1 MPa and –0.8 to –1.7 MPa, respectively. The two schedules resulted in seasonal water applications of 860 mm for the 1.75 and 525 mm for the 2.75 signal threshold treatments. The grower/cooperator, who based his schedule primarily on SWP measurements but also considered the water balance, applied 900 mm. Estimated crop evapotranspiration was 1,030 mm. The mean coefficients of variation for the two irrigation treatments during the monitoring period were 0.115 and 0.031 for the MDS and SWP measurements, respectively. The stress produced by the irrigation treatments hastened fruit maturation, as evidenced by accelerated hull splitting. This resulted in lower fruit hydration just prior to harvest; 17.3% and 8.0% for the two irrigation schedules, respectively, compared with 27.3% for the grower/cooperator. Based on harvesting selected trees with the same nut load, fresh and dry nut weights in the 2.75 threshold treatment were 9.0% and 10.7% less than those of the 1.75 threshold, which were not significantly different from the results for the grower cooperator. Our results demonstrate that it is feasible to develop an irrigation schedule for almond trees based solely on MDS signals, which may be tailored to any desired stress pattern and be operated in full automation with appropriate software development.Communicated by R. Evans