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Crop nitrogen (N) status is known to affect crop water status and crop water use. To investigate further the N effects on soil water changes and on canopy temperature, three water levels × four N levels were imposed on two growing seasons of maize in west central Nebraska, USA. Soil water changes were measured using a neutron probe, whereas canopy temperature was measured using infrared thermometers on a ground-based mobile platform. At all water levels, soil water losses over month-long intervals were generally greater as N levels increased. Given equal water levels, early afternoon canopy temperatures were usually lower with higher N levels, but no trend or even the opposite trend was occasionally observed. Jointly considering canopy reflectance and soil water depletion shows potential to explain much of the variation in estimated instantaneous water use among plots. However, determining the relative contributions of the canopy and soil factors on a particular day may require season-to-date knowledge of the crop. Further research on assimilating such sensor data for a combined stress coefficient would improve crop modeling and irrigation scheduling when variable water sufficiency and variable N sufficiency are simultaneously significant.

Highlights Within-field variability was larger for individual depths than for the profile average across multiple depths. Distributions of the profile average were approximately normal, with increasing variances as the soil was drying. Probability theory was applied to quantify the effect of sensor set number on irrigation scheduling. The benefit of additional sensors sets may decrease for longer irrigation cycles and for more heterogeneous fields. Abstract. Even when located within the same field, multiple units of the same soil moisture sensor rarely report identical values. Such within-field variability in soil moisture sensor data is caused by natural and manmade spatial heterogeneity and by inconsistencies in sensor construction and installation. To better describe this variability, daily soil water tension values from 14 to 23 sets of granular matrix sensors during the middle part of four soybean site-years in the Mississippi Delta were analyzed. The soil water tension data were found to follow approximately normal distributions, to exhibit moderately high temporal rank stability, and to show strong positive correlation between mean and variance. Based on these observations and the existing literature, a probabilistic conceptual framework was proposed for interpreting within-field variability in granular matrix sensor data. This framework was then applied to investigate the impact of sensor set number (i.e., number of replicates) and irrigation triggering threshold on the scheduling of single-day and multi-day irrigation cycles. If a producer’s primary goal of irrigation scheduling is to keep soil water adequate in a particular fraction of land on average, the potential benefit from increasing sensor set number may be smaller than traditionally expected. Improvement, expansion, and validation of this probabilistic framework are welcomed for developing a practical and robust approach to selecting the sensor set number and the irrigation triggering threshold for diverse soil moisture sensor types in diverse contexts. Keywords: Irrigation scheduling, Probability, Sensors, Soil moisture, Soil water tension, Variability, Watermark.

The interpretation of optical canopy sensor readings for determining optimal rates of late-season site-specific nitrogen application to corn (Zea mays L.) can be complicated by spatially variable water sufficiency, which can also affect canopy size and/or pigmentation. In 2017 and 2018, corn following corn and corn following soybeans were subjected to irrigation × nitrogen fertilizer treatments in west central Nebraska, USA, to induce variable water sufficiency and variable nitrogen sufficiency. The vegetation index-sensor combinations investigated were the normalized difference vegetation index (NDVI), the normalized difference red edge index (NDRE), and the reflectance ratio of near infrared minus red edge over near infrared minus red (DATT) using ACS-430 active optical sensors; NDVI using SRS-NDVI passive optical sensors; and red brightness and a proprietary index using commercial aerial visible imagery. Among these combinations, NDRE and DATT were found to be the most suitable for assessing nitrogen sufficiency within irrigation levels. While DATT was the least sensitive to variable water sufficiency, DATT still tended to decrease with decreasing water sufficiency in high nitrogen treatments, whereas the effect of water sufficiency on DATT was inconsistent in low nitrogen treatments. A new method of quantifying nitrogen sufficiency while accounting for water sufficiency was proposed and generally provided more consistent improvement over the mere averaging of water effects as compared with the canopy chlorophyll content index method. Further elucidation and better handling of water-nitrogen interactions and confounding are expected to become increasingly important as the complexity, automation, and adoption of sensor-based irrigation and nitrogen management increase.

The presence of clay particles increases the specific surface area of a soil and can affect the calibration of electromagnetic soil water sensors including reflectometers. To quantify and correct for this effect in two relatively new reflectometers, three TDR315 and three CS655 sensors were installed in each of five soils with clay content ranging from 5 to 49%. As the soils were dried in a temperature controlled room, sensor reported soil volumetric water content (θv) according to the factory calibration was compared against reference θv determined by weighing the soils. Sensor reported θv was similar to reference θv in the sand soil (root mean square difference (RMSD) 0.93) for both sensors. By estimating calibration coefficients based on clay content alone to recalculate sensor θv from sensor reported apparent relative permittivity, RMSD from reference θv was reduced by approximately 36% for both sensors as compared with using the factory calibration. Applying the same procedure to independent literature data tended to improve θv accuracy of both sensors increasingly as clay content increased above 20%. The findings suggest that a simple, user-friendly correction for clay content effects may provide initial practical improvement over the factory calibration of a reflectometer in clayey soils.

Diversity in the knowledge, mindset, strategies, and tools that growers use to manage irrigation and fertilizer nitrogen culminates in diversity in profitability and environmental impact among farms. As growers, academia, and industry strive together to tackle the technological and non-technological challenges impeding better irrigation and fertilizer nitrogen management, a science-based evaluation of grower input amount and timing becomes an important initial step in the process. Providing such valuable feedback to growers is a high-priority objective for the University of Nebraska-Lincoln Testing Ag Performance Solutions (UNL-TAPS) farm management competition. In this competition, each team of mostly growers made management decisions for field corn in three replicated plots within the same field at the West Central Research and Extension Center in North Platte, NE, and vied for maximum profitability and most judicious input management. The 2017 dataset affirmed existing theory predicting that many efficiency indices strictly decrease in value with increasing seasonal input amount and thus would fail to point towards an appropriate input level. Furthermore, grain yield was heavily affected by cultivar choice, so efficiency indices that depend on yield actually obscured rather than facilitated the evaluation of irrigation and fertilizer nitrogen management. An alternate evaluation approach is to compare a grower’s seasonal input amount or input temporal distribution against an appropriate range enveloped by university recommendations on the high end and observed yield-limiting thresholds on the low end. Where irrigation and fertilizer nitrogen are relatively inexpensive and where producing near-maximum yield is optimal, this approach is suitable for analyzing an input in isolation and for analyzing multiple inputs simultaneously.

Corner watering attachments, specifically end guns and corner extensions, attached to center-pivot systems are a common means to irrigate the corners of square fields. The pumping plant is designed to supply water to all sprinklers when the center pivot is irrigating the point of maximum elevation. Most designs include powering the irrigation pump at a fixed speed, which can lead to excessive energy use. Variable-frequency drives (VFDs) were introduced to adjust the pump speed in response to changes in pressure requirements. In this study, three scenarios were analyzed: scenario 1, end gun attached at end of center-pivot lateral; scenario 2, corner extension attached at end of center-pivot lateral; and scenario 3, corner extension attached at end of center-pivot lateral and end gun attached at the end of the corner extension. This study was conducted on 1000 randomly selected center-pivot irrigation systems located in ten counties of Nebraska using GIS-based tools. High-resolution digital elevation maps were used to investigate the impact of flow rate changes and topographical variations on energy requirements. Reductions in the energy required, obtained through varying the pump speed, and annual monetary savings were calculated for each scenario. Average payback periods for VFDs were calculated for each county. The major factors contributing to energy savings and annual returns include large topographical changes, duration of corner attachment operation, hours of operation, and electricity cost. Scenario 3 resulted in the maximum energy reduction, maximum annual monetary savings, and minimum payback period compared to the other scenarios. This approach allows investigation of the potential savings associated with installation of a VFD for center pivots equipped with end guns and corner arm extensions. Keywords: Center-pivot irrigation systems, Corner watering attachments, End gun, Energy savings, GIS, Pumping, Topography, Variable-frequency drive.

Accurate continuous measurements of temperature (T), apparent electrical conductivity (ECa), apparent dielectric permittivity (era), and volumetric water content (θv) are extremely valuable to irrigation management and other agronomic decisions. The performance of eight electromagnetic (EM) sensors (TDR315, CS655, HydraProbe2, 5TE, EC5, CS616, Field Connect, AquaCheck), were analyzed through a field study in a loam soil. T, ECa, and era were compared in reference to overall average among all sensors, and θv in reference to a neutron moisture meter (NMM). The reported T and ECa difference among the sensors were within 1 °C and 1 dS m−1, respectively, at 0.15 and 0.76 m depths. Among the single-sensor probes, the range of depth-combined (0.15 and 0.76 m) RMSD for factory calibration varied from 0.039 m3 m−3 (5TE) to 0.157 m3 m−3 (CS616). In comparison to single-sensor probes, RMSD of Field Connect at combined depths (0.30 and 0.51 m) was moderate (0.083 m3 m−3), and RMSD of AquaCheck at combined depths (0.30 and 0.61 m) was high (0.163 m3 m−3). Regression calibrations improved θv accuracy substantially beyond factory calibrations, as RMSD of the evaluated sensors except Field Connect was below 0.025 m3 m−3 using regression calibrations. The betterment in θv accuracy gained by using offset calibrations was smaller and less consistent than the improvements gained by using regression calibrations. The lower and upper bounds of the 95% confidence interval for mean RMSD of most sensors were below 0.02 and 0.04 m3 m−3, respectively, when using depth-specific offset calibrations. The relative success of offset calibrations for certain sensors in this study is encouraging and may signal new opportunities. Because much of the uncertainty in sensor-reported θv for the sensors under evaluation was systematic, future work should aim to develop universal calibrations or facilitate site-specific calibrations.

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.

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.

Interreplicate variability—the spread in output values among units of the same sensor subjected to essentially the same condition—can be a major source of uncertainty in sensor data. To investigate the interreplicate variability among eight electromagnetic soil moisture sensors through a field study, eight units of TDR315, CS616, CS655, HydraProbe2, EC5, 5TE, and Teros12 were installed at a depth of 0.30 m within 3 m of each other, whereas three units of AquaSpy Vector Probe were installed within 3 m of each other. The magnitude of interreplicate variability in volumetric water content (θv) was generally similar between a static period near field capacity and a dynamic period of 85 consecutive days in the growing season. However, a wider range of variability was observed during the dynamic period primarily because interreplicate variability in θv increased sharply whenever infiltrated rainfall reached the sensor depth. Interreplicate variability for most sensors was thus smaller if comparing θv changes over several days that excluded this phenomenon than if comparing θv directly. Among the sensors that also reported temperature and/or apparent electrical conductivity, the sensors exhibiting the largest interreplicate variability in these outputs were characterized by units with consistently above or below average readings. Although manufacturers may continue to improve the technology in and the quality control of soil moisture sensors, users would still benefit from paying greater attention to interreplicate variability and adopting strategies to mitigate the consequences of interreplicate variability.

Accurate spatial characterization of field capacity (FC) and root zone available water capacity (R) can enhance site-specific management practices—such as variable rate irrigation—to lower input costs, reduce contaminant leaching, and/or improve crop yield. Measuring the volumetric water content after wet soils drain following substantial precipitation can provide a field estimate of FC. The average FC (FCa) for the managed root zone was determined at thirty-two locations in a topographically variable field in south central Nebraska. The difference between FC and permanent wilting point estimates—computed using a pedotransfer function—yielded values for R for the observation locations. Sampling locations were too sparse for reliable interpolation across the field. Therefore, relationships between a surrogate, or predictor, variable and soil water properties were used to provide spatial distributions of FC and R for the field. Field estimates of FCa and R were more strongly correlated to elevation (correlation coefficient, r = -0.77 and -0.76, respectively) than to deep soil apparent electrical conductivity (r = -0.46 and -0.39, respectively). Comparing maps of FCa and R from gSSURGO to maps from field characterization yielded a root mean squared difference of 0.031 m3 m-3 for FCa and 34 mm for R. Sampling seven locations across the elevation range in this field produced FCa and R prediction functions that achieved 95% and 87%, respectively, of the reduction in the standard error achievable with a larger number of sampling locations. Spatial characterization of FCa and R depends on identifying a suitable predictor variable(s) based on field knowledge and available spatial data. Well-chosen variables may allow satisfactory predictions using several sampling locations that are distributed over the entire field. Ultimately, the costs and benefits of spatial characterization should be considered when evaluating site-specific water management.

Much of the previous research quantifying the potential benefits of variable rate irrigation (VRI) consists of case studies with simulations using data from small numbers of intensely sampled fields. In this study, an indicator of the amount of root zone available water capacity that is unutilized by uniform rate irrigation was calculated for 49,224 center pivot irrigated fields in Nebraska using publicly available data exclusively. Based on the values of this indicator, potential seasonal irrigation reductions from increasing precipitation utilization with VRI were estimated to be high for a small fraction of analyzed center pivots but low on a regional scale. At current VRI and energy prices, pumping cost savings alone may fail to justify VRI adoption for most analyzed center pivots. Although the prevalence of center pivots with high indicator values differed among counties and among soil associations, ruling out with reasonable confidence the occurrence of either low or high indicator values in a county or soil association might be difficult. The study hopes to inform producers considering VRI and other entities interested in the potential impact of this particular application of VRI.