Variable Rate Irrigation Management Research Articles

Irrigation Zone Delineation and Management with a Field-Scale Variable Rate Irrigation System in Winter Wheat

Understanding spatial and temporal dynamics of soil water within fields is critical for effective variable rate irrigation (VRI) management. The objectives of this study were to develop VRI zones, manage irrigation rates within VRI zones, and examine temporal differences in soil volumetric water content (VWC) from irrigation events via soil sensors across zones. Five irrigation zones were delineated after two years (2016 and 2017) of yield and evapotranspiration (ET) data collection. Soil sensors were placed within each zone to give real time data of VWC values and assist in irrigation decisions within a 23 ha field of winter wheat (Triticum aestivum ‘UI Magic’) near Grace, Idaho, USA (2019). Cumulative irrigation rates among zones ranged from 236 to 298 mm. Although a statistical comparison could not be made, the irrigation rates were 0.6 to 21% less than an estimated uniform grower standard practice (GSP) irrigation approach. Based on soil sensor data, crop water stress was avoided with VRI management in all but Zone 3. Thus, this simple approach to VRI zone delineation and VWC monitoring has the potential to reduce irrigation, such as this study, on average by 12% and should be evaluated in other site-years to assess its viability.

Development and Evaluation of a Variable-Rate Irrigation Management Method in the Mississippi Delta

HighlightsWe developed and evaluated a variable-rate irrigation (VRI) management method for five crop years in the Mississippi Delta.VRI management significantly reduced irrigation water use in comparison with uniform-rate irrigation (URI). There was no significant difference in grain yield and irrigation water productivity between VRI and URI management.Soil apparent electrical conductivity (ECa) was used to delineate irrigation management zones and generate VRI prescriptions.Sensor-measured soil water content was used in irrigation scheduling.Abstract. Variable-rate irrigation (VRI) allows producers to site-specifically apply irrigation water at variable rates within a field to account for the temporal and spatial variability in soil and plant characteristics. Developing practical VRI methods and documenting the benefits of VRI application are critical to accelerate the adoption of VRI technologies. Using apparent soil electrical conductivity (ECa) and soil moisture sensors, a VRI method was developed and evaluated with corn and soybean for five crop years in the Mississippi Delta. Soil ECa of the study fields was mapped and used to delineate VRI management zones and create VRI prescriptions. Irrigation was scheduled using soil volumetric water content measured by soil moisture sensors. A center pivot VRI system was employed to deliver irrigation water according to the VRI prescription. Grain yield, irrigation water use, and irrigation water productivity in the VRI treatment were determined and compared with that in a uniform-rate irrigation (URI) treatment. Results showed that the grain yield and irrigation water productivity between the VRI and URI treatments were not statistically different with both corn and soybean crops. The VRI management significantly reduced the amount of irrigation water by 22% in corn and by 11% in soybean (p = 0.05). Adoption of VRI management could improve irrigation water use efficiency in the Mississippi Delta. Keywords: Soil electrical conductivity, Soil moisture sensor, Variable rate irrigation, Water management.

Comparative analyses of variable and fixed rate irrigation and nitrogen management for maize in different soil types: Part II. Growth, grain yield, evapotranspiration, production functions and water productivity

Comparisons of variable rate irrigation (VRI) and variable rate fertigation (VRF) with fixed rate irrigation (FRI), no irrigation (NI) and conventional fertilizer management for maize (Zea mays L.) under three soil types (S1, S2 and S3) were made for three years (2015, 2016 and 2017). The research quantified and compared maize growth and development [leaf area index (LAI) and plant height], grain yield, crop evapotranspiration (ETc), irrigation-yield production functions (IYPF), evapotranspiration-yield production functions (ETYPF) and crop water productivity (CWP) under VRI, FRI and NI at fixed rate fertigation (FRF), VRF and pre-plant nitrogen (PP) management in the same environment and under the same agroomic management practices. The VRF treatment used 20% less fertilizer as compred with PP and FRF treatment without significantly (P > 0.05) reducing the grain yield. In the higher elevation soil S1, the grain yield was not significantly different (P > 0.05) between FRI and VRI treatments. However, in S2 and S3 which have lower elevation, yield in FRI was 43% and 55% greater than the yield in VRI, respectively. On average, under VRI management total irrigation amount 24% lower than FRI in S1, with only 4% reduction in yield as compared with FRI. Soil type impacted the response of maize grain yield to ETc and the responses also varied between FRI and VRI. For all soil types and years, higher ETc was observed in FRI treatment, except in 2015 for S1 and S2 where highest ETc was observed in VRI. FRI had greater productivity per unit of ETc than VRI. Observing the linear relationship of the pooled data for each soil type, a 25.4 mm of ETc (beyond the intercept) resulted in 0.48 and 0.35 ton ha−1 maize yield for FRI and VRI treatment, respectively, in S1; 0.75 and 0.45 ton ha−1 for FRI and VRI, respectively, in S2; and 0.71 and 0.43 ton ha−1 for FRI and VRI, respectively, in S3. VRI strategy increased the variability in grain yield, ETc and CWP as compared with FRI management. S1 had the lowest grain yield variability as compared with S2 and S3. The coefficient of variation (CV) of grain yield ranged from 8% in S1-FRI to as high as 35.3% in S3 VRI. FRI treatment in all soil types and years had less variation in yield as compared with VRI. Soil type had impact on CWP. On a three-year average basis, CWP was 2% higher in VRI in S1 whereas 11% lower in S2 and S3 than FRI CWP. Stronger grain yield response to irrigation and ETc (IYPF and ETYPF) were observed for FRI than VRI and NI at all nitrogen levels. The results of this research indicated that, in most cases, FRI had superior performance in terms of maintaining optimum crop yield and reducing yield variations than VRI. VRI management based on soil water status has potential of maintaining maize grain yield and improving CWP as compared with FRI in certain conditions or soil types such as in S1. However, further research is needed to validate/justify its adoption for the fields with significant spatial soil heterogeneity (both in horizontal and vertical domains) and to understand the economics of VRI-VRF systems. These results would be beneficial for maize growers and their advisors in terms of understanding the productivity responses to water under VRI and FRI management strategies with VRF, FRF and PP nitrogen application.

Mapping Management Zones Based on Soil Apparent Electrical Conductivity and Remote Sensing for Implementation of Variable Rate Irrigation—Case Study of Corn under a Center Pivot

Climate change, especially the trend towards global warming, will significantly affect the global hydrological cycle, leading to a general reduction of the water available for agriculture. In this scenario, it is essential that research should focus on the development of ‘water saving’ techniques and technologies. This work summarizes the methodology followed in a project for large scale implementation of variable rate irrigation (VRI) systems using center pivots in corn crop. This is based on technologies for monitoring (i) soil electrical conductivity (ECa) and altimetry, (ii) soil moisture content, (iii) vegetation indices (Normalized Difference Vegetation Index, NDVI) obtained from satellite images, and automatic pivot travel speed control technologies. ECa maps were the basis for the definition of first homogeneous management zones (HMZ) in an experimental corn field of 28 ha. NDVI time-series were used to establish the subsequent HMZ and the respective dynamic prescription irrigation maps. The main result of this study was the reduction of spatial yield variability with the VRI management in 2017 compared to the conventional irrigation management. This study demonstrates how a relatively simple approach could be designed and implemented on a large scale, which represents an important and sustainable contribution to the resolution of practical farmer issues.

Economic comparisons of variable rate irrigation and fertigation with fixed (uniform) rate irrigation and fertigation and pre-plant fertilizer management for maize in three soils

Extensive field research for data collection to conduct economic comparisons of variable rate irrigation (VRI) with fixed (uniform) rate irrigation (FRI) and no irrigation (NI) in combination with three nitrogen application strategies of fixed (uniform) rate fertigation (FRF), variable rate fertigation (VRF) and pre-plant nitrogen (PP) management for maize (Zea mays L.) were conducted. Research was conducted in three soil types [(i) Crete silt loam (S1); (ii) Hastings silty clay loam (S2); and (iii) Hastings silt loam (S3)] for three growing seasons (2015, 2016 and 2017) in Nebraska, USA. For the economic analyses, the average initial investment of the irrigation system and necessary VRI technology, salvage value of the system, total capital investment, total fixed cost, net present value (NPV) and internal rate of return (IRR) were quantified by considering numerous factors/variables, including interest rate, production input cost, longevity of the system, insurance cost, ownership cost and salvage value. Soil types and irrigation management strategies (treatments) had significant impact on grain yield and thus on profitability, NPV, IRR and irrigation system payback period. Net income from FRI management was significantly higher than VRI management in all soil types. The nitrogen treatments did not affect net income in any of the growing seasons. The FRI management strategy had a positive NPV in all soil types whereas VRI management in S2 and S3 had negative NPVs. The negative NPV indicates that the present value of the costs exceeds the present value of future profits at the assumed discount rate (5%). Averaging all three years and three soils, FRI had a substantially higher net income than VRI in most cases. The maximum NPV of $4,882.07 per ha and maximum IRR of 18 % was observed in FRI-FRF treatment. A payback period of ≤10 years was determined for all FRI management treatments while the payback period for VRI management, in most cases, was more than 27 years. While the pay-back period in VRI irrigation system was less than ten years in S1, it was still longer than the corresponding FRI management treatments. Results suggest that the VRI and VRF strategies are not economically feasible in current conditions. For these variable irrigation and fertilizer technologies to be competitive with the FRI and FRF, the cost of the VRI and VRF technology will need to be significantly lower than the current investment costs. While there could be some environmental benefits of VRI and VRF technology, with the current high investment cost of VRI technology and the fact that the grain yields are not improved sufficiently to offset the investment cost, it is not possible for VRI technology to be an economically viable technology for profitable economic net return. This may explain, in part, extremely limited adoption of these technologies by producers currently in large scale production fields.

Evaluation of a Decision Support System for Variable-Rate Irrigation in a Humid Region

HighlightsAn Irrigation Scheduling Supervisory Control and Data Acquisition (ISSCADA) system was tested against a soil electrical conductivity (EC) based method for variable-rate irrigation (VRI).Soil EC was used to create irrigation prescription in EC-based VRI.ISSCADA generated VRI prescriptions using canopy temperature, soil water content, and weather data.ISSCADA-based VRI reduced irrigation water use and increased irrigation water productivity.Abstract. Use of variable-rate irrigation (VRI) technology has the potential to improve irrigation water use efficiency (IWUE). VRI hardware is commercially available and can be implemented in any center pivot or lateral move irrigation system. However, practical methods and algorithms for creating VRI prescriptions have become the bottleneck in accelerating the adoption of VRI. An Irrigation Scheduling Supervisory Control and Data Acquisition (ISSCADA) system for VRI was evaluated for two years in a humid region in the Mississippi Delta. The ISSCADA system was used to manage irrigation of soybeans for two seasons. In field practice, the ISSCADA system scanned the field for canopy temperature and collected soil water data from time domain reflectometers and weather data from a nearby weather station. The ISSCADA system automatically generated VRI prescription maps. The maps were modified to include plots managed using soil electrical conductivity (EC) based VRI prescriptions. Test results indicated that there was no difference in crop yield between EC-based VRI and ISSCADA-based VRI management. However, ISSCADA-based VRI management reduced irrigation water use and increased irrigation water productivity in comparison with EC-based VRI. There is great potential for the use of ISSCADA for VRI in humid regions. Keywords: Canopy temperature, Soil electrical conductivity, Soil moisture sensor, Soil water sensor, Soybean, Variable rate irrigation.

ARSPivot, A Sensor-Based Decision Support Software for Variable-Rate Irrigation Center Pivot Systems: Part A. Development

HighlightsThe ARSPivot software seamlessly integrates site-specific irrigation scheduling methods with weather, plant, and soil water sensing systems in the operation of variable-rate irrigation (VRI) center pivot systems.ARSPivot embodies an Irrigation Scheduling Supervisory Control and Data Acquisition (ISSCADA) system that incorporates site-specific irrigation scheduling methods and automates the collection and processing of data obtained from sensing systems supporting them.ARSPivot incorporates a friendly graphical user interface (GUI) that assists in the process of setting up a computerized representation of a coupled ISSCADA VRI center pivot system and simplifies the review of irrigation prescriptions automatically generated based on sensor feedback.ARSPivot’s GUI includes a geographic information system (GIS) that relates sensed data and imported GIS data to specific field control zones.Abstract. The commercial availability of variable-rate irrigation (VRI) systems gives farmers access to unprecedented control of the irrigation water applied to their fields. To take full advantage of these systems, their operations must integrate site-specific irrigation scheduling methods that in turn should be supported by a network of sensing systems. An Irrigation Scheduling Supervisory Control and Data Acquisition (ISSCADA) system patented by scientists with the USDA-Agricultural Research Service (ARS) at Bushland, Texas, incorporates site-specific irrigation scheduling methods informed by weather, plant, and soil water sensing systems. This article introduces a software package, ARSPivot, developed to integrate the ISSCADA system into the operation of VRI center pivot systems. ARSPivot assists the operation and integration of a complex network of sensing systems, irrigation scheduling methods, and irrigation machinery to achieve this end. ARSPivot consists of two independent programs interacting through a client-server architecture. The client program is focused on automatically collecting and processing georeferenced data from sensing systems and communicating with a center pivot control panel, while the server program is focused on communicating with users through a friendly graphical user interface (GUI) involving a geographic information system (GIS). The GUI allows users to visualize and modify site-specific prescription maps automatically generated based on sensor-based irrigation scheduling methods, and to control and monitor the application of irrigation amounts specified in these recommended prescription maps using center pivots equipped for VRI zone control or VRI speed control. This article discusses the principles and design considerations followed in the development of ARSPivot and presents tools implemented in the software for the virtual design and physical operation of a coupled ISSCADA VRI center pivot system. This article also illustrates how the ISSCADA system and ARSPivot constitute a comprehensive sensor-based decision support system (DSS) for VRI management that is accessible to users without in-depth knowledge of sensing systems or irrigation scheduling methods. Keywords: Center pivot irrigation, Decision support system, Precision agriculture, Sensors, Site-specific irrigation scheduling, Software, Variable rate irrigation.n

Integrating SEBAL with in-Field Crop Water Status Measurement for Precision Irrigation Applications—A Case Study

The surface energy balance algorithm for land (SEBAL) has been demonstrated to provide accurate estimates of crop evapotranspiration (ET) and yield at different spatial scales even under highly heterogeneous conditions. However, validation of the SEBAL using in-field direct and indirect measurements of plant water status is a necessary step before deploying the algorithm as an irrigation scheduling tool. To this end, a study was conducted in a maize field located near the Venice Lagoon area in Italy. The experimental area was irrigated using a 274 m long variable rate irrigation (VRI) system with 25-m sections. Three irrigation management zones (IMZs; high, medium and low irrigation requirement zones) were defined combining soil texture and normalized difference vegetation index (NDVI) data. Soil moisture sensors were installed in the different IMZs and used to schedule irrigation. In addition, SEBAL-based actual evapotranspiration (ETr) and biomass estimates were calculated throughout the season. VRI management allowed crop water demand to be matched, saving up to 42 mm (−16%) of water when compared to uniform irrigation rates. The high irrigation amounts applied during the growing season to avoid water stress resulted in no significant differences among the IMZs. SEBAL-based biomass estimates agreed with in-season measurements at 72, 105 and 112 days after planting (DAP; r2 = 0.87). Seasonal ET matched the spatial variability observed in the measured yield map at harvest. Moreover, the SEBAL-derived yield map largely agreed with the measured yield map with relative errors of 0.3% among the IMZs and of 1% (0.21 t ha-1) for the whole field. While the FAO method-based stress coefficient (Ks) never dropped below the optimum condition (Ks = 1) for all the IMZs and the uniform zone, SEBAL Ks was sensitive to changes in water status and remained below 1 during most of the growing season. Using SEBAL to capture the daily spatial variation in crop water needs and growth would enable the definition of transient, dynamic IMZs. This allows farmers to apply proper irrigation amounts increasing water use efficiency.

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.

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.

Determining placement criteria of moisture sensors through temporal stability analysis of soil water contents for a variable rate irrigation system

Developing placement criteria for soil moisture sensors is crucial in increasing the practical functionality of a variable rate irrigation (VRI) system. In this field study, the temporal stability pattern of soil water content was compared between VRI and uniform rate irrigation (URI) treatments during growing seasons of winter wheat and summer maize to determine the placement criteria of soil water sensors. The 1.64-ha experimental site located in a highly variable alluvial flood plain was divided into four management zones according to the available water holding capacity ranging from 152 to 205 mm within the 0.6 m soil profile. In each zone, two sub-zones were created to represent VRI and URI treatments. A temporal stability analysis of soil moisture was conducted by regularly measuring soil water contents at 62 locations in the field during the growing seasons. Results showed that the VRI management changed the overall similarity of soil moisture spatial patterns when crop water consumption was provided mainly by irrigation water rather than precipitation. In each management zone, every measuring position was a time-stable location with respect to the mean soil water content. Significant linear regressions were detected between the mean clay percentile in each management zone and the clay percentile representing the mean soil water content sites, and a nearly equivalent value of fitted equation coefficient was obtained for winter wheat (1.15) and summer maize (1.19). These results demonstrated that the temporal stability of soil water content spatial patterns still existed in each management zone with the VRI management, and the clay percentile supplied a priori identification for placement of soil moisture sensors.

Soil water content monitoring for irrigation management: A geostatistical analysis

With the increasing attention to site-specific or variable rate irrigation management, it is helpful to reconsider the quantity and placement of soil water monitoring locations in this context. Volumetric soil water content (θv) was monitored using a neutron probe (NP) at 72 locations in a center pivot irrigated field in eastern Nebraska. Variance reduction and temporal stability analyses were performed on θv from shallow (∼top 46cm) and full profile (∼122cm) readings for four monitoring cycles in the 2015 growing season and 2016 preseason. Eleven additional cycles were included for a subset of the data for the temporal stability analysis. The spatial correlation scale for θv was found to be less than the closest spacing of monitoring locations in the study (i.e. <37m). For this field site, approximately three neutron probe monitoring locations were required to determine mean soil water depletion (±2cm) for the field or for a management zone. Little economy would be gained in variance reduction for areal mean θv from using a stratified network for management areas of reasonable size in a center pivot irrigated field. Temporally stable monitoring locations were identified. However, relatively low-cost spatial predictor variables, including elevation, deviation from mean elevation, apparent electrical conductivity, and mean relative difference of interpolated cosmic ray neutron probe surveys, were not consistent predictors of NP mean relative difference. The small range of variability of θv within the study field is thought to be a contributing factor. It is possible that for fields with similar variability, or for site-specific irrigation where zones have been selected to reduce within-zone variance, that sensor quantity is more important than sensor placement in quantifying the areal mean θv for irrigation management.

Field Study of Variable Rate Irrigation Management in Humid Climates

AbstractComparison of variable rate irrigation (VRI) management with uniform rate irrigation (URI) management in soybean and corn crops was studied for 2 years in Stoneville, Mississippi, USA. The experiments were conducted on two 6.7 ha fields. Each of them was equally split into two sectors. VRI management was performed in one sector and URI management in the other. A centre pivot VRI system was used for delivering irrigation water. Soil apparent electrical conductivity (EC) of the fields was used to delineate VRI management zones and create a VRI prescription map. The VRI treatment used 25% less irrigation water and produced 2.8% more yield in soybean and 0.8% more yield in corn than the URI treatment. Irrigation water productivity (WP) of soybean under VRI management was 0.84 kg m−3 which is 31.2% higher than URI. The WP of corn under VRI management was 1.69 kg m−3, 27.1% higher than URI. Yield of the rainfed treatment was significantly lower than the VRI and URI treatments for both soybean and corn in the 2015 season (p &lt; 0.05). Results in this study demonstrated that VRI management was superior to URI in terms of water use efficiency. Copyright © 2017. This article has been contributed to by US Government employees and their work is in the public domain in the USA. Copyright © 2017. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Addressing Groundwater Declines with Precision Agriculture: An Economic Comparison of Monitoring Methods for Variable-Rate Irrigation

Irrigated row-crop agriculture is contributing to declining groundwater in areas such as the Mississippi Delta region of eastern Arkansas. There is a need to move toward sustainable levels of groundwater withdrawal. Recent improvements in remote monitoring technologies such as wireless soil moisture sensors and unmanned aerial vehicles offer the potential for farmers to effectively practice site-specific variable-rate irrigation management for the purpose of applying water more efficiently, reducing pumping costs, and retaining groundwater. Soil moisture sensors and unmanned aerial vehicles are compared here in terms of their net returns per acre-foot and cost-effectiveness of aquifer retention. Soil moisture sensors ($9.09 per acre-foot) offer slightly more net returns to producers than unmanned aerial vehicles ($7.69 per acre-foot), though costs associated with unmanned aerial vehicles continue to drop as more manufacturers enter the market and regulations become clear.

Crop Yield and Water Productivity Responses in Management Zones for Variable-Rate Irrigation Based on Available Soil Water Holding Capacity

Abstract. Effective management of variable-rate irrigation (VRI) is a critical factor for maximizing the benefit of a VRI system. In this study, the influences of soil properties on winter wheat and summer maize were studied to verify whether differences in soil available water holding capacity (AWC) had an influence on crop growth parameters, yield, and water productivity (WP). A center-pivot VRI system was employed to deliver irrigation water across the field in an alluvial flood plain in China, and AWC was used to delineate VRI management zones. Three management zones with substantial differences in AWC were created, with AWC varying from 152 to 161 mm, from 161 to 171 mm, and from 171 to 185 mm for zones 1, 2, and 3, respectively. All zones were managed using the same allowed depletion. In the two-year study, the seasonal irrigation amount was basically equivalent among management zones for both winter wheat and summer maize. Differences in crop growth parameters were detected in plant height and leaf area index for winter wheat. The maximum plant height and leaf area index observed in zone 2 were 5 cm and 2.1 greater, respectively, than in the other zones. For both winter wheat and summer maize, the highest yield and WP were observed in zone 2, except for summer maize WP in the 2014 season. Compared with the average value for this field, the yields in zone 2 were 27% and 23% greater for winter wheat and 4% and 11% greater for summer maize in the 2014 and 2015 seasons, respectively. We demonstrate that AWC is an effective parameter for zone identification in VRI management, and differences in AWC and the layered-textural soils in a field may influence the crop growth parameters, yield, and WP of winter wheat and summer maize. Keywords: Center-pivot irrigation, Critical soil moisture deficit, Management zone, Summer maize, Variable-rate irrigation, Winter wheat.

Pumpage Reduction by Using Variable-Rate Irrigation to Mine Undepleted Soil Water

Conventional irrigation schedules are typically based on portions of the field where root zones hold the least available soil water. This leaves undepleted available water in areas with larger water holding capacities. The undepleted water could be used through variable-rate irrigation (VRI) management; however, the benefits of VRI without in-field mapping are unexamined. In this research, the field-averaged amount of undepleted available soil water in the root zone was calculated from the NRCS Soil Survey Geographic database for 49,224 center-pivot irrigated fields in Nebraska. Potential reductions in pumpage from mining undepleted available water were then estimated. Results of the analysis show that widespread adoption of zone control VRI technology based only on the pumping savings from mining undepleted available water may be unwarranted for current VRI costs and average pumping energy expenses in the Central Plains ($0.0026 m -3 to $0.0947 m -3 ). Pumpage reductions exceeded 51 mm year -1 for only 2% of the fields and exceeded 25 mm year -1 for 13% of the fields; thus, reductions may be small compared to annual pumpage requirements. If VRI were implemented on all fields with a potential pumpage reduction greater than 51 or 25 mm year -1 , the volume of pumpage reduction would be approximately 0.35% or 1.3%, respectively, of the total irrigation pumpage in Nebraska. These data may be a conservative estimate of pumpage reduction in fields where the measured variability in soil properties exceeds that described by the NRCS Soil Survey, or if undepleted water is mined early in the season and the soil water profile is refilled by precipitation, allowing undepleted water to be mined again. Adoption of zone control VRI for mining undepleted available water is most feasible for fields where the pumpage reduction from VRI is large and pumping costs are above normal. Pivot fields with high undepleted water were sparsely distributed across Nebraska and were often located along streams and or in associated alluvial areas. The prevalence of fields with large quantities of undepleted water differed among and within soil associations. We were unable to assign feasibility of VRI based on the soil association, as the occurrence of undepleted water varied significantly within a soil association. These findings should assist producers and other entities interested in VRI technology; however, pumpage reduction through use of undepleted soil water is only one benefit of VRI technology and management. Producers are encouraged to consider all potential benefits when analyzing VRI investments.

Site-Specific Variable-Rate Irrigation as a Means to Enhance Water Use Efficiency

<abstract> The majority of irrigated cropland in the U.S. is watered with sprinkler irrigation systems. These systems are inherently more efficient in distributing water than furrow or flood irrigation. Appropriate system design of sprinkler irrigation equipment, application methods, and farming practices (e.g., furrow diking) enhance crop water use efficiency (WUE) by minimizing irrigation losses and improving soil water storage. For years, the paradigm for best irrigation management practices included uniform application over an entire field, even though abiotic (soils, slope, aspect, etc.) and biotic (insect pressure, plant disease) factors often cause spatial variations in water use and yield potential. However, emerging technologies such as wireless communication coupled with soil water and plant sensors, commercially available variable-rate irrigation (VRI) equipment, and the development of algorithms for computational data processing are shifting this paradigm toward variable-rate management as a means to enhance crop WUE. This article focuses on the potential of site-specific VRI management (SS-VRIM) as a tool for enhancing WUE and the challenges encountered.

Variable-rate irrigation management using an expert system in the eastern coastal plain

Variable-rate irrigation (VRI) systems have the potential to conserve water by spatially allocating limited water resources. However, when compared to traditional irrigation systems, VRI systems require a higher level of management. In this 3-year study, we evaluated spatial irrigation management of a peanut crop grown under a VRI system using an expert system (Irrigator Pro). The irrigation management treatments evaluated were: (1) using Irrigator Pro (IP) to manage irrigation uniformly in plots with varying soils; (2) using Irrigator Pro to manage irrigation in plots based on the individual soils (IPS); (3) a treatment based on maintaining soil water potential (SWP) above −30 kPa (approximately 50 % depletion of available water) in the surface 30 cm of each soil within a plot; and (4) a non-irrigated treatment. Over the 3-year study, all irrigated treatments had significantly higher yields (4,230, 4,130, and 4,394 kg ha−1 for the IP, IPS, and SWP treatments, respectively) than the non-irrigated treatment (3,285 kg ha−1), yet the yields of the three irrigation treatments were not significantly different. Averaged over the 3-year experiment, the three treatments did not differ significantly in water usage. In the 2007 and 2009 growing seasons with below normal rainfall, the IP and IPS treatments required significantly greater total water than the SWP treatment. Overall, water use efficiency was significantly higher for the non-irrigated and SWP treatments (9.4 and 8.9 kg ha−1 per mm, respectively). The lower water use efficiency for the IP and IPS irrigation treatments (7.8 kg ha−1 per mm) was attributed to greater water applications mainly due to earlier growing season initiation of irrigation applications. However, the IP and IPS treatments maintained soil water potentials at the 30- and 60-cm depths at higher levels throughout most of the season. The two Irrigator Pro expert system treatments functioned as well as the SWP-based treatment. The Irrigator Pro expert system can be effectively used for site-specific management where management zone soils do not greatly differ. Further refinement of the expert system may be needed to improve its application in spatial irrigation applications.