Variable Rate Irrigation Research Articles

Agronomic Basis and Strategies for Precision Water Management: A Review

Agriculture faces the challenge of feeding a growing population with limited or depleting fresh water resources. Advances in irrigation systems and technologies allow site-specific application of irrigation water within the field to improve water use efficiency or reduce water usage for sustainable crop production, especially in arid and semi-arid regions. This paper discusses recent development of variable-rate irrigation (VRI) technologies, data and information for VRI application, and impacts of VRI, including profitability using this technology, with a focus on agronomic factors in precision water management. The development in sprinkler systems enabled irrigation application with greater precision at the scale of individual nozzle control. Further research is required to evaluate VRI prescription maps integrating different soil and crop characteristics in different environments. On-farm trials and whole-field studies are needed to provide support information for practical VRI applications. Future research also needs to address the adjustment of the spatial distribution of prescription zones in response to temporal variability in soil water status and crop growing conditions, which can be evaluated by incorporating remote and proximal sensing data. Comprehensive decision support tools are required to help the user decide where to apply how much irrigation water at different crop growth stages to optimize water use and crop production based on the regional climate conditions and cropping systems.

Maximizing water productivity of winter wheat by managing zones of variable rate irrigation at different deficit levels

To determine the specific application rate for each management zone of a variable rate irrigation system, the yield and water use efficiency (WUE) of winter wheat were evaluated during two growing seasons at different deficit levels in the alluvial flood plain of the North China Plain. One 1.64-ha quadrant irrigated by a variable rate center pivot system was delineated into four management zones with available soil water holding capacity, and varied soil profiles were detected in these zones. Each zone was divided into several subzones to be irrigated at different deficit levels. In the 2016 season, each subzone was managed individually with irrigation trigger points of 55%, 65%, 75%, and 80% of field capacity along with a rain-fed treatment. In the 2017 season, all subzones were irrigated simultaneously with 0%, 33%, 67%, 100%, and 120% of the base application depth. For the two-season study, the rain-fed treatment produced significantly lower yield and WUE than the irrigated treatments, and both the maximum yield and the maximum WUE were obtained in zone 2, where a more uniform soil profile was detected. A linear crop water production function was determined for zones 1 and 3 in the 2017 season, while a quadratic equation fit the crop water production function well for other zones in the two seasons. The relationship between WUE and crop water use in the three zones can be represented by a curvilinear equation for both seasons. Taking the optimal application rate of maximizing WUE in zone 1 as a basis, 89% and 94% of the rate in zone 1 was recommended for zones 2 and 3, respectively, to achieve the maximum WUE in the entire field. Our results also suggested that the existing layered-textural soil profile can greatly influence crop productivity and should therefore be considered in mapping irrigation prescriptions.

Variable Pulsed Irrigation Algorithm (VPIA) to Reduce Runoff Losses under a Low-Pressure Lateral Move Irrigation Machine

Due to restrictions and limitations on agricultural water worldwide, one of the most effective ways to conserve water in this sector is to reduce the water losses and improve irrigation uniformity. Nowadays, the low-pressure sprinkler has been widely used to replace the high-pressure impact sprinklers in lateral move sprinkler irrigation systems due to its low operating cost and high efficiency. However, the hazard of surface runoff represents the biggest obstacle for low-pressure sprinkler systems. Most researchers have used the pulsing technique to apply variable-rate irrigation to match the crop water needs within a normal application rate that does not produce runoff. This research introduces a variable pulsed irrigation algorithm (VPIA) based on an ON–OFF pulsing technique to conserve irrigation water through (1) decreasing the runoff losses by considering the soil infiltration rate, surface storage capacity, and sprinkler wetting diameter; and (2) ensuring a high level of water distribution uniformity in the direction of machine movement. From a wide range of pulse numbers and widths tested applying a certain water depth to a sandy loam soil, the best solution that gives the lowest runoff and highest uniformity while delivering an acceptable water depth was selected. A MATLAB code was written to simulate the soil infiltration rate, the sprinkler application rate, and to apply the proposed algorithm. The simulation results showed a runoff reduction of at least 90.7% with a high level of distribution uniformity in the direction of movement while delivering the highest possible irrigation depth using the lowest number of pulses.

Fuzzy control system for variable rate irrigation using remote sensing

Variable rate irrigation (VRI) is the capacity to spatially vary the depth of water application in a field to handle different types of soils, crops, and other conditions. Precise management zones must be developed to efficiently apply variable rate technologies. However, there is no universal method to determine management zones. Using speed control maps for the central pivot is one option. Thus, this study aims to develop an intelligent fuzzy inference system based on precision irrigation knowledge, i.e., a system that can create prescriptive maps to control the rotation speed of the central pivot. Satellite images are used in this study because remote sensing offers quick measurements and easy access to information on crops for large irrigation areas. Based on the VRI-prescribed map created using the intelligent decision-making system, the pivot can increase or decrease its speed, reaching the desired depth of application in a certain irrigation zone. Therefore, considering the spatial variability in the crop has made the strategy of speed control more realistic than traditional methods for crop management. The intelligent irrigation system pointed out areas with lower leaf development, indicating that the pivot must reduce its speed, thus increasing the water layer applied to that area. The existence of well-divided zones could be observed; each zone provides a specific value for the speed that the pivot must develop for decreasing or increasing the application of the water layer to the crop area. Three quarters of the total crop area had spatial variations during water application. The set point built by the developed system pointed out zones with a decreased speed in the order of 50%. From the viewpoint of a traditional control, the relay from pivot percent timer should have been adjusted from 70% to 35% whenever the central pivot passed over that specific area. The proposed system obtained values of 37% and 47% to adjust the pivot percent timer. Therefore, it is possible to affirm that traditional control models used for central-pivot irrigators do not support the necessary precision to meet the demands of speed control determined by the developed VRI systems. Results indicate that data from the edaphoclimatic variables when well-fitted to the fuzzy logic can solve uncertainties and non-linearities of an irrigation system and establish a control model for high-precision irrigation.

Potential Water Conservation Using Site-Specific Variable Rate Irrigation

Abstract. Site-specific variable-rate irrigation (VRI) systems can be used to spatially manage irrigation within sub-field-sized zones and optimize spatial water use efficiency. The goal of the research is to provide farmers and consultants a tool to evaluate the potential benefits of implementing VRI. The specific objective of this research is to evaluate the potential water savings using VRI management compared with uniform irrigation management to maintain soil water holding capacity above 50% depletion using two irrigation scenarios: 1) a standard 12.5 mm irrigation per application; and 2) an application to refill the soil profile to field capacity. A 21-year simulation study was carried out on a selected field with varying degrees of soil and topographic variability. The simulated field had 12 soil mapping units with water holding capacities in the top 0.30-m ranging from 42 to 70 mm. The 21-year simulation covering all weather conditions for each soil produced only two significantly different irrigation management zones for scenario 1, and for scenario 2 only one management zone. However, when the 21-year period was divided into periods with different ratios of rainfall to reference evapotranspiration, the simulations identified 1 to 5 management zones with significantly different irrigation requirements. These results indicate that variable rate irrigation system design and management should not be solely based on long term average weather conditions. Years with differing weather conditions should be used for potentially identifying management zones for VRI systems. Irrigation application depths between management zones ranged from 17 to 38 mm. However, when the actual soil areas of the study field were utilized to calculate the total volume of irrigation water applied, it resulted in an increase in water usage in the 2 and 4 management zones ranging from -1.2% to 5.8%. Water usage with VRI over uniform irrigation was greater by -1.6% to 6.8% in the 12.5 mm irrigations and by -1.2% to 2.2% for the field capacity irrigations Keywords: Management zones, Precision farming, Variable-rate irrigation, Water conservation.

Identifying Advantages and Disadvantages of Variable Rate Irrigation: An Updated Review

Abstract. Variable rate irrigation (VRI) sprinklers on mechanical move irrigation systems (center pivot or lateral move) have been commercially available since 2004. Although the number of VRI, zone or individual sprinkler, systems adopted to date is lower than expected there is a continued interest to harness this technology, especially when climate variability, regulatory nutrient management, water conservation policies, and declining water for agriculture compound the challenges involved for irrigated crop production. This article reviews the potential advantages and potential disadvantages of VRI technology for moving sprinklers, provides updated examples on such aspects, suggests a protocol for designing and implementing VRI technology and reports on the recent advancements. The advantages of VRI technology are demonstrated in the areas of agronomic improvement, greater economic returns, environmental protection and risk management, while the main drawbacks to VRI technology include the complexity to successfully implement the technology and the lack of evidence that it assures better performance in net profit or water savings. Although advances have been made in VRI technologies, its penetration into the market will continue to depend on tangible and perceived benefits by producers. Keywords: Center pivots, Crop water use efficiency, Irrigation, Management zones, Moving sprinkler irrigation systems, Precision irrigation, Sensor based systems.

Targeted, Precision Irrigation for Moving Platforms: Selected Papers from a Center Pivot Technology Transfer Effort

Abstract. This article is an introduction to an ASABE Special Collection of ten articles concerning center pivot (CP) technology transfer that appears in this issue of and in . The year 2018 marked the 40th anniversary of research and development of low-energy precision application (LEPA) for use with CP sprinkler irrigation systems. Since 1978, researchers, extension specialists, and industry have continued development of multiple technologies that are suitable for mobile lateral irrigation platforms. A two-year technology transfer effort with funding from the USDA-ARS Ogallala Aquifer Program (OAP) was initiated in January 2017 to promote adoption of advanced and efficient irrigation technologies and to highlight recommended practices for mobile irrigation platforms, including both CP and lateral-move (LM) systems. The articles in this Special Collection address five major topic areas: in-canopy and near-canopy irrigation application from moving platforms, site-specific variable-rate irrigation (VRI) and associated sensor technologies for moving platforms, the use of variable-frequency drives (VFDs) with CP systems, and uniformity evaluations for CP systems. While these ten articles are not inclusive of all the important advances in irrigation from moving platforms since 1978, they illustrate that continued improvement in irrigation management occurs by combining engineering and agronomic sciences. Pressurized irrigation from moving platforms is increasing in the U.S. and will be an important tool for meeting global food and water challenges. In addition to introducing and summarizing the Special Collection, this article provides additional rationale for the CP technology transfer effort. Keywords: Center pivot, Irrigation uniformity, Lateral-move sprinkler, LEPA, Linear-move sprinkler, Low-energy precision application, Site-specific irrigation, Sprinkler irrigation, Variable-frequency drives, Variable-rate irrigation, VFD, VRI.

Response of Drought-Tolerant Corn to Varying Irrigation Levels in the Texas High Plains

Abstract.Corn ( L.) for grain continues to be an important crop for livestock feed in the Texas High Plains (THP) region despite lackluster prices. It offers greater crop water productivity compared with other crops grown in the region but also has a relatively high water requirement, which must be met by irrigation. The sole water resource in the region is the Ogallala Aquifer, which is declining because withdrawals exceed recharge, and this is of major concern. Producers are interested in the performance of drought-tolerant (DT) corn, but data on DT crop production functions are limited. From 2015 to 2017, studies of DT corn response to different irrigation treatments were conducted in the THP at Bushland, Texas. Results showed that grain yields, seasonal evapotranspiration (ETc), and crop water use efficiency (WUE) varied significantly between seasons and among different DT hybrids. Comparisons between a mid-season (MS) and an early-maturing (EM) hybrid showed: (1) at the severe deficit irrigation treatment level, grain yields were low, but the EM hybrid produced 400% more grain; (2) at the moderate deficit irrigation treatment level, grain yields and ETc were similar; and (3) at the full irrigation treatment level, the EM hybrid required 75 mm less water, but it produced 24% less grain. Non-hail damaged MS DT corn produced grain yields that were numerically greater than conventional corn grown in the THP in an optimal year. However, during drought seasons, DT hybrid response was not improved over conventional hybrids under severe deficit irrigation. This study demonstrated that MS DT corn hybrid P1151AM, irrigated at a level that fully met evapotranspiration demand, resulted in grain yield and WUE levels that were near the upper limits for corn produced in the THP. Further research is needed to determine the constancy of response among different DT hybrids under favorable and drought conditions. Keywords: Center pivot, Deficit irrigation, Early-maturing corn, Hail damage, Mid-season corn, Variable-rate irrigation, Water use efficiency.

Multispectral satellite imagery to quantify in-field soil moisture variability

As pressure on available water resources increases, the need to exploit technology in order to produce more food with less water becomes crucial. The technological hardware requisite for precise water delivery methods, such as variable rate irrigation, is commercially available. Despite that, techniques to formulate a timely, accurate prescription for those systems are inadequate. Spectral vegetation indices are often used to gauge crop vigor and related parameters (e.g., leaf nitrogen [N] content). However, research heretofore rarely addresses the influence of soil moisture on the indices. The objectives of this study were to determine (1) if vegetation indices derived from multispectral satellite imagery could assist in quantifying soil moisture variability in irrigated maize (<i>Zea mays</i> L.) production, and (2) the period of time that a single image is representative of soil moisture. A variable rate irrigation pivot was used to form six water treatment zones. Each was equipped with tensiometers installed in the center of the plots at 20, 45, and 75 cm depths to individually monitor conditions in the water treatment zones. Water was applied for each treatment as a percentage of the estimated evapotranspiration (ET) requirement: 40%, 60%, 80%, 100%, 120%, and 140%. Data collected from tensiometers was paired with the image pixels corresponding to its ground location. Statistical analysis was performed separately to assess whether vegetation indices are representative of soil moisture at several crop growth stages. Findings from this study indicate that Red Edge Normalized Difference Vegetation Index could quantify soil moisture tension variability at V6 (six leaf; <i>r</i>² = 0.850, <i>p</i> = 0.009) and V9 (nine leaf; <i>r</i>² = 0.913, <i>p</i> = 0.003) crop growth stages. Results suggest that satellite-derived vegetation indices may be useful for creating time-sensitive characterizations of soil moisture variability. Further study is necessary to investigate additional crop growth stages, more crops, and other sources of multispectral imagery.

Variable Rate Precision Irrigation Technology for Deficit Irrigation of Processing Tomato

AbstractCentre pivot machines, with a variable rate irrigation (VRI) package that provides individual sprinkler control systems (VRI iS), are not common. This research focuses on how uniform the water distribution is under VRI control, and on the size of over‐ and under‐irrigated areas and volumes. Moreover, we wanted to know how wide transition zones are. We conducted different uniformity measurements using common rain gauges for measuring water application depths. Besides grid shape and radial direction measurements for uniformity, perpendicular measuring lines to borders were set to evaluate transition zones. In a processing tomato deficit irrigation experiment, we investigated how different plant properties react to different water depths. Results showed that very good uniformity is achievable both in the IR100 and IR50 application rates, but the former showed higher uniformity. CUC ranged from 91.8 to 92.9% and 88.8 to 90.8% in the IR100 and IR50 rates respectively. The highest RMSE was 2.65 mm. The amount of over‐ and under‐irrigation was not significant. Transition zones were not equally wide; 9 m was enough for transition widthways between the IR100 and IR50 rates, but was wider longitudinally. This technology is suitable for conducting deficit irrigation experiments, but consideration of transition zones is important at plant sampling. © 2018 John Wiley &amp; Sons, Ltd.

An iris mechanism for variable rate sprinkler irrigation

Variable rate irrigation intends to supply water at the right time and place in order to improve water-use efficiency, save energy and decrease nutrient leaching. The concept of an adjustable orifice sprinkler using an iris mechanism actuated by a stepper motor for use in variable rate irrigation was proposed and developed. A prototype was manufactured using additive manufacturing techniques. Its operational characteristics were evaluated. To predict the flow rate, a deterministic model that represented the operation of the sprinkler was proposed and validated. The observed discharge coefficients varied according to the operating conditions, these being: 0.530 when the orifice was fully opened; and an average of 0.636 when the iris mechanism reduced the orifice section. For the condition of a partially opened orifice, the phenomenon of jet inversion occurred causing the jet to become asymmetrical. The proposed concept allows flowrates closer to the required values to be obtained and consequently provides greater flexibility and accuracy to applying the target irrigation depth.

Prediction of plant water status in almond and walnut trees using a continuous leaf monitoring system

Persistent drought conditions in the Central valley of California demands efficient irrigation scheduling tools such as precision or variable rate irrigation (VRI). To assist VRI scheduling, an experiment was conducted in almond and walnut orchards using a sensor system called ‘leaf monitor’, which was developed at UC Davis to detect plant water status. A Modified Crop Water Stress Index (MCWSI) was calculated to quantify plant water status using leaf temperature and environmental data collected by the leaf monitor. This technique also took into account spatio-temporal variability of plant water status. Stem water potential (SWP), which is considered a standard method for determining plant water stress (PWS), was also measured simultaneously. Relationships between measured deficit stem water potential (DSWP), which is the difference between SWP and the saturated baseline, and MCWSI were developed for both crops based on data collected during the 2013 and 2014 growing seasons. A linear relationship was found in the case of walnut crop with a coefficient of determination (r2) value of 0.67. A quadratic relationship was found in the case of almonds with a coefficient of multiple determination (R2) value of 0.75. Moreover, these results highlighted that at lower PWS of below 0.5 MPa of DSWP, almonds crops did not show any decrease in transpiration rate. However, when the stress level exceeded 0.5 MPa of DSWP, transpiration rate tended to decrease. On the other hand, walnut crop showed decrease in transpiration rate even at low PWS of below 0.5 MPa of DSWP. Temporal variability was noticed in PWS as it was found that coefficients of saturation baseline used for MCWSI method changed significantly throughout the season. MCWSI values estimated before an irrigation event was used to calculate the irrigation amount for low frequency variable rate irrigation (VRI) based on the relationship found between MCWSI and DSWP, and VRI led to an average 39% reduction in water usage as compared to the fixed 100% ET replacement irrigation method for all trees. Based on the results, leaf monitor showed potential for use as an irrigation scheduling tool.

Smart Agricultural Machine with a Computer Vision-Based Weeding and Variable-Rate Irrigation Scheme

This paper proposes a scheme that combines computer vision and multi-tasking processes to develop a small-scale smart agricultural machine that can automatically weed and perform variable rate irrigation within a cultivated field. Image processing methods such as HSV (hue (H), saturation (S), value (V)) color conversion, estimation of thresholds during the image binary segmentation process, and morphology operator procedures are used to confirm the position of the plant and weeds, and those results are used to perform weeding and watering operations. Furthermore, the data on the wet distribution area of surface soil (WDAS) and the moisture content of the deep soil is provided to a fuzzy logic controller, which drives pumps to perform variable rate irrigation and to achieve water savings. The proposed system has been implemented in small machines and the experimental results show that the system can classify plant and weeds in real time with an average classification rate of 90% or higher. This allows the machine to do weeding and watering while maintaining the moisture content of the deep soil at 80 ± 10% and an average weeding rate of 90%.

Evaluation of variable rate irrigation using a remote-sensing-based model

Improvements in soil water balance modeling can be beneficial for optimizing irrigation management to account for spatial variability in soil properties and evapotranspiration (ET). A remote-sensing-based ET and water balance model was tested for irrigation management in an experiment at two University of Nebraska-Lincoln research sites located near Mead and Brule, Nebraska. Both fields included a center pivot equipped with variable rate irrigation (VRI). The study included maize in 2015 and 2016 and soybean in 2016 at Mead, and maize in 2016 at Brule, for a total of 210 plot-years. Four irrigation treatments were applied at Mead, including: VRI based on a remote sensing model (VRI-RS); VRI based on neutron probe soil water content measurement (VRI-NP); uniform irrigation based on neutron probe measurement; and rainfed. Only the VRI-RS and uniform treatments were applied at Brule. Landsat 7 and 8 imagery were used for model input. In 2015, the remote sensing model included reflectance-based crop coefficients for ET estimation in the water balance. In 2016, a hybrid component of the model was activated, which included energy-balance-modeled ET as an input. Both 2015 and 2016 had above-average precipitation at Mead; subsequently, irrigation amounts were relatively low. Seasonal irrigation was greatest for the VRI-RS treatment in all cases because of drift in the water balance model. This was likely caused by excessive soil evaporation estimates. Irrigation application for the VRI-NP at Mead was about 0 mm, 6 mm, and –12 mm less in separate analyses than for the uniform treatment. Irrigation for the VRI-RS was about 40 mm, 50 mm, and –98 mm greater in separate analyses than the uniform at Mead and about 18 mm greater at Brule. For maize at Mead, treatment effects were primarily limited to hydrologic responses (e.g., ET), with differences in yield generally attributed to random error. Rainfed soybean yields were greater than VRI-RS yields, which may have been related to yield loss from lodging, perhaps due to over-irrigation. Regarding the magnitude of spatial variability in the fields, soil available water capacity generally ranked above ET, precipitation, and yield. Future research should include increased cloud-free imagery frequency, incorporation of soil water content measurements into the model, and improved wet soil evaporation and drainage estimates.

Center-Pivot-Mounted Sensing System for Monitoring Plant Height and Canopy Temperature

Abstract. Easy-to-use data acquisition methods are required for variable-rate irrigation (VRI) decision support systems. Plant canopy temperature is related to plant water stress. Plant height is useful as an indicator of plant health conditions and can be used to estimate yield potential. Therefore, measurements of plant canopy temperature and plant height coupled with spatial information in the field can be used for determining VRI water application depths. A center-pivot-mounted wireless data acquisition (WDAQ) system was developed to collect plant canopy temperature and plant height data in the field. Each WDAQ unit consisted of a GPS receiver, programmable data logger, infrared temperature sensor, ultrasonic distance sensor, solar power supply, and wireless data transmitter/receiver. The system included two WDAQ units installed on a four-span center-pivot VRI system. One unit was mounted at the middle of the third span, and the other was mounted at the middle of the fourth span from the pivot. The infrared temperature sensors were used to detect the canopy temperature, while the ultrasonic distance sensors were used to measure plant height. The WDAQ system was designed to continuously and simultaneously measure plant canopy temperature and plant height and record the spatial coordinates at each measurement location as the center pivot moved around the field. Data collected were wirelessly transferred to a receiver for data processing. This WDAQ system has been tested and evaluated in the field for two years. Test results indicated that the WDAQ system was able to record approximately 3,200 measurements from each sensor in one pivot circle (360°). The measurement error of the ultrasonic distance sensor was 0.2 to 3 cm in a measurement range of 14 to 209 cm, and the sensor-measured plant heights were strongly correlated with manually tape-measured plant heights in soybean and cotton crops (r2 = 0.97). Combined with the spatial information, measurements of plant height and crop canopy temperature were used to generate plant height and crop canopy temperature maps. Spatial variabilities of plant height and canopy temperature across the field could be identified from the maps and used in irrigation research. The WDAQ system has great potential for automatic creation of VRI prescription maps and plant-based irrigation scheduling. Keywords: Canopy temperature, Irrigation scheduling, Plant height, Sensors, Variable-rate irrigation.

Crop Yield and Water Use Efficiency as Affected by Different Soil-Based Management Methods for Variable-Rate Irrigation in a Semi-Humid Climate

Abstract. To improve the management of variable-rate irrigation (VRI) systems in semi-humid climates, three different soil-based irrigation management methods were evaluated on their potential for reducing irrigation water use and maximizing crop yield and water use efficiency (WUE) during the 2016 and 2017 growing seasons of summer maize in the North China Plain. The three irrigation management methods evaluated were soil water balance modeling (SWB), measured soil water content (SWC), and a combination of SWB and the rain forecast for the next three days (RF). The experiments were implemented on four management zones delineated by available soil water holding capacity of a center-pivot VRI system. A similar irrigation trigger point (70% of field capacity) was used for the three irrigation management methods in the four management zones. In the two seasons, the total water application in the SWC treatments varied in a larger range among the management zones, and the irrigation water applied was 22% and 21% less than in the SWB and RF treatments, respectively. Similar yields were obtained among the irrigation management methods in both seasons. The maximum WUE was always observed with the SWC treatments for the four management zones in the 2017 season. The WUE with the SWC treatments was 36% and 23% higher than with the SWB and RF treatments, respectively. Considering the amount of irrigation water applied, yield, and WUE, our results demonstrated that the SWC method was more suitable for VRI management than the SWB and RF methods in this semi-humid climate. Keywords: Center pivot, Soil water balance, Soil water content, Rain forecast, Summer maize, Yield.

Evaluation of a Hybrid Reflectance-Based Crop Coefficient and Energy Balance Evapotranspiration Model for Irrigation Management

Abstract. Accurate generation of spatial soil water maps is useful for many types of irrigation management. A hybrid remote sensing evapotranspiration (ET) model combining reflectance-based basal crop coefficients (Kcbrf) and a two-source energy balance (TSEB) model was modified and validated for use in real-time irrigation management. We modeled spatial ET for maize and soybean fields in eastern Nebraska for the 2011-2013 growing seasons. We used Landsat 5, 7, and 8 imagery as remote sensing inputs. In the TSEB, we used the Priestly-Taylor (PT) approximation for canopy latent heat flux, as in the original model formulations. We also used the Penman-Monteith (PM) approximation for comparison. We compared energy balance fluxes and computed ET with measurements from three eddy covariance systems within the study area. Net radiation was underestimated by the model when data from a local weather station were used as input, with mean bias error (MBE) of -33.8 to -40.9 W m-2. The measured incident solar radiation appeared to be biased low. The net radiation model performed more satisfactorily when data from the eddy covariance flux towers were input into the model, with MBE of 5.3 to 11.2 W m-2. We removed bias in the daily energy balance ET using a dimensionless multiplier that ranged from 0.89 to 0.99. The bias-corrected TSEB ET, using weather data from a local weather station and with local ground data in thermal infrared imagery corrections, had MBE = 0.09 mm d-1 (RMSE = 1.49 mm d-1) for PM and MBE = 0.04 mm d-1 (RMSE = 1.18 mm d-1) for PT. The hybrid model used statistical interpolation to combine the two ET estimates. We computed weighting factors for statistical interpolation to be 0.37 to 0.50 for the PM method and 0.56 to 0.64 for the PT method. Provisions were added to the model, including a real-time crop coefficient methodology, which allowed seasonal crop coefficients to be computed with relatively few remote sensing images. This methodology performed well when compared to basal crop coefficients computed using a full season of input imagery. Water balance ET compared favorably with the eddy covariance data after incorporating the TSEB ET. For a validation dataset, the magnitude of MBE decreased from -0.86 mm d-1 (RMSE = 1.37 mm d-1) for the Kcbrf alone to -0.45 mm d-1 (RMSE = 0.98 mm d-1) and -0.39 mm d-1 (RMSE = 0.95 mm d-1) with incorporation of the TSEB ET using the PM and PT methods, respectively. However, the magnitudes of MBE and RMSE were increased for a running average of daily computations in the full May-October periods. The hybrid model did not necessarily result in improved model performance. However, the water balance model is adaptable for real-time irrigation scheduling and may be combined with forecasted reference ET, although the low temporal frequency of satellite imagery is expected to be a challenge in real-time irrigation management. Keywords: Center-pivot irrigation, ET estimation methods, Evapotranspiration, Irrigation scheduling, Irrigation water balance, Model validation, Variable-rate irrigation.

Editorial ‐ December 2017

Seventy years ago in April 1947, the British Society of Soil Science had its inaugural ordinary meeting. The eminent soil physical chemist, R.K. Schofield, gave a presentation entitled ‘A New Approach to Problems of Soil’. However, one chooses to place the emphasis, it is clear that Schofield considered soils provided problems, which required resolution. In the first three issues of the Society's journal, then aptly named ‘Journal of Soil Science’, soil-based problems were identified under crop transpiration (H.L. Penman); fresh groundwater laying over saline water (E.C. Childs); soil aggregation, soil organic matter and the influence of earthworms (R.J. Swaby); and the evaluation of soils and the definition of quality classes from studies of the physical properties of the soil profile in the field (G.R. Clarke). In this issue, we highlight two invited papers that both help celebrate the 70th anniversary of the Society and provide an update on the state of the art of soil science understanding and application in solving practical problems. The first by Hallett et al. focuses on developments in the capability of land information systems (LIS). Combining database and metadata allows modern LIS to offer management options at a variety of scales. In the second paper, McDowell poses and answers the question ‘Does variable rate irrigation decrease nutrient leaching losses…?’. Benefits are reported in terms of N species and reactive and total P downstream of areas under variable rate irrigation compared with flood or uniform rate irrigation. Included in this issue are the last three papers of the series looking at the application electromagnetic techniques to digital soil mapping, with John Triantafilis and Denis Corwin as Guest Editors. The studies described look at the potential for use of these techniques to provide maps of differing scales and report on a web-based geographic information system for producing maps and making them available to interested parties. We will continue to publish new studies in this field of research under the same subheading in future issues, but I would like to thank John and Denis for the wide-ranging scope of the papers they have provided. We currently have a call for papers on ‘Soil information sharing and knowledge building’ (see our website). These papers are intended to provide a single issue dedicated to this subject and be published early in 2019. Lisa Lobry de Bruyn (University of New England, Armadale) and Julie Ingram (University of Gloucester) will be the Guest Editors for this special issue. I would also like to draw attention to the papers and the correspondence that has resulted from the publication of the paper by Huijsmans et al. in the special supplement dedicated to the late Brian Chambers in 2016. I would like to think that Soil Use and Management has provided a forum for a proper scientific evaluation of a storm that has raged in the Netherlands over the past couple of years. I have been delighted with the broad range of topics and the significant developments reported in the journal over this anniversary year. The next five years, culminating in the British Society of Soil Science celebrating 75 years of service by hosting the 2022 International Union of Soil Science (IUSS) meeting in Glasgow is likely to see more major changes in the publication of scientific advances. Soil Use and Management will continue to provide a reliable platform for the peer assessment of reports on the application of soil science to the management of land, and we expect to be publishing many of the papers presented at the meeting in Rio de Janeiro next year as well as those from other contributors across the globe.

A geospatial variable rate irrigation control scenario evaluation methodology based on mining root zone available water capacity

Increasing concern for sustainable water use has the agriculture industry working toward higher efficiency in use of irrigation water. Recent advancements have improved the capabilities of center pivot irrigation systems to vary water application depths across the field, a technology known as variable rate irrigation (VRI). The goal of this study was to provide a geospatial method for potential VRI technology adopters to evaluate control scenarios and potential water savings using freely available datasets. Root zone available water capacity (R) was estimated spatially across two case study fields using the Natural Resources Conservation Service Gridded Soil Survey Geographic Database. The difference in application depth between conventional irrigation (CI) and both sector and zone control VRI was then estimated based on R. Prescription maps were developed to mine undepleted soil water from each irrigation management zone based on a soil water balance approach with a management-allowed depletion of 50%. For CI management, the areal 10th percentile (PCTL) of R for the field was used, while for VRI the 10th PCTL of R for each management zone was used. The highest reduction in irrigation depth was 18 mm where higher values of R were estimated; however, field average reductions ranged from 0 to 12 mm. The greatest improvements in pumpage reduction resulted from converting from sector control to zone control, while increasing the angular resolution only had a minor impact. Energy savings generally increased with higher VRI control resolution. Conclusions support previous notions that VRI may result in small pumping water reductions for some fields; however, improved water distribution may be achieved throughout the field.

Life Cycle Assessment of Two Vineyards after the Application of Precision Viticulture Techniques: A Case Study

Precision viticulture is the application of site-specific techniques to vineyard production to improve grape quality and yield and minimize the negative effects on the environment. While there are various studies on the inherent spatial and temporal variability of vineyards, the assessment of the environmental impact of variable rate applications has attracted limited attention. In this study, two vineyards planted with different grapevine cultivars (Sauvignon Blanc and Syrah) were examined for four consecutive growing seasons (2013–2016). The first year, the two vineyards were only studied in terms of soil properties and crop characteristics, which resulted in the delineation of two distinct management zones for each field. For the following three years, variable rate nutrient application was applied to each management zone based on leaf canopy reflectance, where variable rate irrigation was based on soil moisture sensors, meteorological data, evapotranspiration calculation, and leaf canopy reflectance. Life cycle assessment was carried out to identify the effect of variable rate applications on vineyard agro-ecosystems. The results of variable rate nutrients and water application in the selected management zones as an average value of three growing seasons were compared to the conventional practice. It was found that the reduction of product carbon footprint (PCF) of grapes in Sauvignon Blanc between the two periods was 25% in total. Fertilizer production and distribution (direct) and application (indirect) was the most important sector of greenhouse gas (GHG) emissions reduction, accounting for 17.2%, and the within-farm energy use was the second ranked sector with 8.8% (crop residue management increase GHG emissions by 1.1%, while 0.1% GHG reduction is obtained by pesticide use). For the Syrah vineyard, where the production was less intensive, precision viticulture led to a PCF reduction of 28.3% compared to conventional production. Fertilizers contributed to this decrease by 27.6%, while within-farm energy use had an impact of 2.2% that was positive even though irrigation was increased, due to yield rise. Our results suggest that nutrient status management offers the greatest potential for reducing GHG emissions in both vineyard types. Variable rate irrigation also showed differences in comparison to conventional treatment, but to a lesser degree than variable rate fertilization. This difference between conventional practices and precision viticulture is noteworthy, and shows the potential of precision techniques to reduce the effect of viticulture on GHG emissions.