NEBRASKA WATER AND ENERGY FLUX MEASUREMENT, MODELING, AND RESEARCH NETWORK (NEBFLUX)

Surface energy and water vapor fluxes play a critical role in understanding the response of agro‐ecosystems to changes in environmental and atmospheric parameters. These fluxes play a crucial role in exploring the dynamics of water and energy use efficiencies of these systems. Quantification of the fluxes is also necessary for assessing the impact of land use and management changes on water balances. Accomplishing these goals requires measurement of water vapor and energy exchanges between various vegetation surfaces and microclimates for long‐enough periods to understand the behavior and dynamics involved with the flux transfer so that robust models can be developed to predict these processes under different scenarios. Networks of flux towers such as AMERIFLUX, FLUXNET, FLUXNET‐CANADA, EUROFLUX, ASIAFLUX, and CAR‐BOEUROPE have been collecting data on exchange processes between biosphere and atmosphere for multiple years across the globe to better understand the functioning of terrestrial ecosystems and their role in regional and/or continental and global carbon, water, and energy cycles, providing a unique service to the scientific community. Nonetheless, there is an imperative need for these kinds of networks to increase in number and intensity due to the great diversity among ecosystems and agro‐ecosystems in species composition, physiological properties, physical structure, microclimatic and climatic conditions, and management practices. The Nebraska Water and Energy Flux Measurement, Modeling, and Research Network (NEBFLUX) is a comprehensive network that is designed to measure surface energy and water vapor fluxes, microclimatic variables, plant physiological parameters, soil water content, surface characteristics, and their interactions for various vegetation surfaces. The NEBFLUX is a network of micrometeorological tower sites that uses mainly Bowen ratio energy balance systems (BREBS) to measure surface water vapor and energy fluxes between terrestrial agro‐ecosystems and microclimate. At present, ten BREBSs and one eddy covariance system are operating on a long‐term and continuous basis for vegetation surfaces ranging from tilled and untilled irrigated and rainfed croplands, irrigated and rainfed grasslands, alfalfa, to Phragmites (Phragmites australis)‐dominated cottonwood (Populus deltoides var. occidentalis) and willow stand (Willow salix) plant communities. The NEBFLUX project will provide good‐quality flux and other extensive supportive data on plant physiology [leaf area index, stomatal resistance, within‐canopy radiation parameters, productivity (yield and/or biomass), and plant height], soil characteristics, soil water content, and surface characteristics to the micrometeorology, water resources and agricultural engineering, and science community on broad spectrum of agro‐ecosystems. The fundamental premise of the NEBFLUX project is to measure continuous and long‐term (at least ten complete annual cycles for each surface) exchange of water vapor and energy fluxes. In addition to the scientific and research objectives, information dissemination to educate the general public and youth is another important objective and output of the network. This article describes the specific goals and objectives, basic principles, and operational characteristics of the NEBFLUX

Magnitude and Trends of Reference Evapotranspiration Rates in South Central Nebraska: Daily, Monthly, Growing Season Total, and Annual Total

Reference evapotranspiration (ETref ) is used to determine the actual water use (ETa) rate for various crops and is one of the key variables that needs to be determined accurately for effective irrigation management. This extension circular provides information on the magnitude and temporal distribution of daily, monthly, annual, and growing season alfalfa-reference evapotranspiration (ETref)) in south central Nebraska. It also discusses why long-term average ETref values may not represent non-average years or be accurate enough for irrigation management. The most common way to calculate actual crop water use (ETa) for a given agronomic crop is to use the ETref and crop coeffi cient (Kc) approach (i.e., ETa = ETref x Kc). This is described in detail in the UNL Extension NebGuide Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients (G1994). This NebGuide also presents information about sources of local ETref and crop coefficients. ETref can either be measured directly using advanced evaporative flux measurement systems, including atmometers (ETgage™), or calculated from weather variables such as solar radiation, air temperature, relative humidity, and wind speed. (For more information on ET gauges see Using Modified Atmometers (ETgage™) for Irrigation Management, UNL NebGuide G1579.

Magnitude and Trends of Reference Evapotranspiration Rates in South Central Nebraska: Daily, Monthly, Growing Season Total, and Annual Total

Reference evapotranspiration (ETref ) is used to determine the actual water use (ETa) rate for various crops and is one of the key variables that needs to be determined accurately for effective irrigation management. This extension circular provides information on the magnitude and temporal distribution of daily, monthly, annual, and growing season alfalfa-reference evapotranspiration (ETref)) in south central Nebraska. It also discusses why long-term average ETref values may not represent non-average years or be accurate enough for irrigation management. The most common way to calculate actual crop water use (ETa) for a given agronomic crop is to use the ETref and crop coeffi cient (Kc) approach (i.e., ETa = ETref x Kc). This is described in detail in the UNL Extension NebGuide Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients (G1994). This NebGuide also presents information about sources of local ETref and crop coefficients. ETref can either be measured directly using advanced evaporative flux measurement systems, including atmometers (ETgage™), or calculated from weather variables such as solar radiation, air temperature, relative humidity, and wind speed. (For more information on ET gauges see Using Modified Atmometers (ETgage™) for Irrigation Management, UNL NebGuide G1579.

Development and Application of a Performance and Operational Feasibility Guide to Facilitate Adoption of Soil Moisture Sensors

Soil moisture sensors can be effective and promising decision-making tools for diverse applications and audiences, including agricultural managers, irrigation practitioners, and researchers. Nevertheless, there exists immense adoption potential in the United States, with only 1.2 in 10 farms nationally using soil moisture sensors to decide when to irrigate. This number is much lower in the global scale. Increased adoption is likely hindered by lack of scientific support in need assessment, selection, suitability and use of these sensors. Here, through extensive field research, we address the operational feasibility of soil moisture sensors, an aspect which has been overlooked in the past, and integrate it with their performance accuracy, in order to develop a quantitative framework to guide users in the selection of best-suited sensors for varying applications. These evaluations were conducted for nine commercially available sensors under silt loam and loamy sand soils in irrigated cropland and rainfed grassland for two different installation orientations [sensing component parallel (horizontal) and perpendicular (vertical) to the ground surface] typically used. All the sensors were assessed for their aptness in terms of cost, ease of operation, convenience of telemetry, and performance accuracy. Best sensors under each soil condition, sensor orientation, and user applications (research versus agricultural production) were identified. The step-by-step guide presented here will serve as an unprecedented and holistic adoption-assisting resource and can be extended to other sensors as well

Comparative analyses of variable and fixed rate irrigation and nitrogen management for maize in different soil types: Part I. Impact on soil-water dynamics and crop evapotranspiration

Understanding the soil-water dynamics and maize evapotranspiration (ETc) under variable rate irrigation (VRI) and variable rate fertigation (VRF) management with respect to soil spatial variability constitutes the basis for developing effective variable rate water and nitrogen management strategies. This long-term research was designed to quantify and compare the soil-water dynamics, including available water (AW), and ETc during vegetative and reproductive growth periods of VRI, fixed rate irrigation (FRI) and no-irrigation (NI) under fixed rate fertigation (FRF), VRF and pre-plant (PP) nitrogen management in three different soil types [Crete silt loam (S1); Hastings silty clay loam (S2) and Hastings silt loam (S3)] with different topography in the same field under the same environmental and management conditions. The research was conducted in the Irmak Research Laboratory in south central Nebraska, U.S.A., in 2015, 2016 and 2017 maize (Zea mays L.) growing seasons under a variable-rate linear move sprinkler irrigation system. No effect of irrigation and nitrogen fertilizer on AW was observed in the vegetative period. Overall, greater AW was observed in S3 as compared with S1 and S2 due to lower elevation. Maize ETc during the vegetative period was significantly (P \u3c 0.05) impacted by soil type in all three years and by nitrogen treatment in two of the three years. The vegetative ETc in S1 was 27 and 19 mm greater than S2 and S3, respectively, for the pooled 2015, 2016 and 2017 data. During the reproductive period, both ETc and AW were impacted by nitrogen and irrigation treatments, but differently in different soil types and years. Average reproductive ETc for FRI and VRI in 2015, 2016 and 2017 was 175 and 178 mm; 294 and 241 mm; 258 and 206 mm, respectively. Averaged across three years, ETc under FRI was significantly (P \u3c 0.05) greater than in VRI; however, in 2015, no significant difference (P \u3e 0.05) in ETc between FRI and VRI was observed in any soil type. Similarly, in 2017, no significant difference in reproductive ETc was observed between VRI and FRI in S1. During reproductive period, averaged across years, soil types and irrigation treatments, the PP nitrogen treatment had greater ETc and lower AW than VRF and FRF. The results indicate that vegetative period ETc was primarily affected by soil type, weather conditions (evaporative demand and soil wetting) and nitrogen fertilizer application timing. The findings of this research showed that soil-water dynamics is a strong function of not only management practices (irrigation and nitrogen treatments), but also soil type, topography and soil physical properties, which all need to be taken into account for effective management of VRI and FRI under VRF, FRF or PP nitrogen management in different soil types. This research quantified the impact of these management practices on soil-water dynamics and ETc which can be used as a guidance

Evaluation of critical nitrogen and phosphorus models for maize under full and limited irrigation conditions

Proper nitrogen (N) fertiliser application rates and timing of application, coupled with optimum irrigation management can improve the sustainability of maize production and reduce the risk of environmental contamination by nutrients. The impact of full and limited irrigation and rainfed conditions on in-season maize (Zea mays L.) shoot biomass nutrient concentration and critical N and phosphorus (P) indices were evaluated using a combination of measured nutrients and critical N and P models in south central Nebraska in 2009 and 2010. Four irrigation treatments [fully-irrigated treatment (FIT), 75% FIT, 60% FIT and 50% FIT) and rainfed] were imposed. Irrigation regimes impacted the shoot biomass N concentration. The shoot biomass N concentration was above the critical N (Ncrit) concentration throughout the growing season under FIT and 75% FIT and was below the Ncrit value for the most limited irrigation (60% FIT and 50% FIT) and rainfed treatments. Nitrogen nutrient index (NNI) varied from 0.68 to 2.0. Biomass N concentration was below Ncrit [i.e., NNI\u3c1] from 105 days after planting (DAP) to harvest under rainfed and 50% FIT and from 114 DAP to harvest under 60% FIT. Overall, the FIT and the 75% FIT had NNI values greater than 1.0 throughout both growing seasons. Phosphorus concentration, which decreased with biomass accumulation and irrigation amounts, varied from 1.0 to 4.8 g kg–1, with FIT having the highest biomass P concentration. The critical N model combined with NNI can be used to evaluate N and P in maize for in-season nutrient diagnosis under the conditions presented in this research

Soybean crop coefficients under different seeding rates and full and limited irrigation and rainfed management

The effects of soybean seeding rates on evapotranspiration (ETc) and grassand alfalfa-reference crop coefficients (Kco and Kcr) were investigated under five seeding rates (185,250, 247,000, 308,750, 370,500 and 432,250 seeds ha-1) (62, 82, 103, 124 and 144% of the recommended seeding rate of 300,000 seeds ha-1 in the experimental region) and different irrigation strategies (i.e. full irrigation treatment [FIT], limited irrigation of 75% FIT and 50% FIT and rainfed treatment). Kco and Kcr values were developed for each soybean growth stage. The seasonal ETc ranged from 460 mm for rainfed to 489 mm for FIT in 2014 under the same seeding rate of 247,000 seeds ha-1. In 2015, ETc ranged from 308 mm for rainfed conditions under the highest seeding rate (432,250 seeds ha-1) to 395 mm for FIT conditions under 308,750 seeds ha-1. Greater differences in weekly ETc were observed across irrigation levels than seeding rates, indicating that water availability has a greater impact on ETc than seeding rates. Kc values were strongly correlated with days after emergence (DAE) and growing degree days (GDD). The seasonal maximum Kco varied among seeding rates and was observed between GDDs of 924 and 1200oC, corresponding to the R5 (beginning seed) and R6 (full seed) growth stages

Actual and Reference Evaporative Losses and Surface Coefficients of a Maize Field during Nongrowing (Dormant) Periods

Effective water resources planning, allocation, management, and use in agroecosystems require accurate quantification of actual evapotranspiration (ETc) during growing and nongrowing (dormant) periods. Prediction of ETc for a variety of vegetation surfaces during the growing season has been researched extensively, but relatively little information exists on evaporative losses during nongrowing periods for different surfaces. The objectives of this research were to evaluate ETc in relation to available energy, precipitation, and grass and alfalfa-reference ET (ETo and ETr) for a maize (Zea mays L.) field and to analyze the dynamics of surface coefficients (Kc) during the nongrowing period (October 15–April 30). The evaporative losses were measured using a Bowen ratio energy balance system (BREBS) on an hourly basis and averaged over 24 h for three consecutive nongrowing periods: 2004–2005 (Season I), 2005–2006 (Season II), and 2006–2007 (Season III). BREBS-measured ETc was approximately 50% of available energy (Rn – G; Rn is net radiation and G is soil heat flux density) during normal and wet seasons (Seasons I and III) and 41% of available energy during a dry season (Season II). Cumulative ETc ranged from 133 mm in Season II to 167 mm in Season III and exceeded precipitation by 21% during the dry season. The ratio of ETc to precipitation was 0.85 in Season I, 1.21 in Season II, and 0.41 in Season III. ETc was approximately 50% of ETo and 36% of ETr in both Seasons I and III, whereas in Season II, ETc was 32% of ETo and 23% of ETr. Overall, measured ETc during the dormant season was generally most strongly correlated with radiation terms, particularly Rn, albedo, incoming shortwave radiation, and outgoing longwave radiation. Average surface coefficients over the three seasons were 0.44 and 0.33 for grass and alfalfa-reference surfaces, respectively. Using geometric mean Kc values to calculate ETc using a Kc ETref approach over the entire nongrowing season yielded adequate predictions with overall root mean square deviations of 0.64 and 0.67 mm day–1 for ETo and ETr, respectively. Estimates of ETc using a dual crop coefficient approach were good on a seasonal basis, but performed less well on a daily basis. Regression equations that were developed (accounting for serial autocorrelation in the ETc and ETref time series) yielded good estimates of ETc. Considering nongrowing period evaporative losses in water budget calculations would enable water regulatory agencies to better account for water use in hydrologic balance calculations over the entire year rather than only for the growing season and to better assess the progression and availability of water resources for the next growing season

Surface Energy Balance, Evapotranspiration, And Surface Coefficients During Non-Growing Season In A Maize-Soybean Cropping System

Surface energy balance components, including actual evapotranspiration (ET), were measured in a reducedtill maize-soybean field in south central Nebraska during three consecutive non-growing seasons (2006/2007, 2007/2008, and 2008/2009). The relative fractions of the energy balance components were compared across the non-growing seasons, and surface coefficients (Kc) were determined as a ratio of measured ET to estimated alfalfa (ETr) and grass (ETo) reference ET (ETref). The non-growing season following a maize crop had 25% to 35% more field surface covered with crop residue as compared to the non-growing seasons following soybean crops. Net radiation (Rn) was the dominant surface energy balance component, and its partitioning as latent heat (LE), sensible heat (H), and soil heat (G) fluxes depended on field surface and atmospheric conditions. No significant differences in magnitude, trend, and distribution of the surface energy balance components were observed between the seasons with maize or soybean surface residue cover. The cumulative ET was 196, 221, and 226 mm during the three consecutive non-growing seasons. Compared to ETref, the cumulative total measured ET was 61%, 63%, and 59% of cumulative total ETo and 43%, 46%, and 41% of cumulative total ETr during the three consecutive seasons. The type of residue on the field surface had no significant effect on the magnitude of ET. Thus, ET was primarily driven by atmospheric conditions rather than surface characteristics. The coefficient of determination (R2) for the daily ET vs. ETr data during the three consecutive non-growing seasons was only 0.23, 0.42, and 0.42, and R2 for ET vs. ETo was 0.29, 0.46, and 0.45, respectively. Daily and monthly average Kc values varied substantially from day to day and from month to month, and exhibited interannual variability as well. Thus, no single Kc value can be used as a good representation of the surface coefficient for accurate prediction of ET for part or all of the non-growing season. A good relationship was observed between monthly total measured ET vs. monthly total ETref. The R2 values for monthly total ET vs. monthly total ETref data ranged from 0.71 to 0.89 for both ETr and ETo. Using pooled data for monthly total ET vs. monthly total ETref, R2 was 0.78 for ETr and 0.80 for ETo. The slopes (S) of the best-fit line with intercept for the monthly total ET vs. monthly total ETref data were consistent for all three non-growing seasons, with S = 0.45 ±0.05 for ETr and S = 0.62 ±0.08 for ETo. The parity in R2 and S across the three non-growing seasons suggests that the same regression equation can be used to approximate non-growing season ET for field surfaces with both maize and soybean crop residue covers. Considering the extreme difficulties in measuring ET during winter in cold and windy climates with frozen and/or snow-covered conditions, the approach using a linear relationship between monthly total ET vs. monthly total ETref appears to be a good alternative to using a surface coefficient to approximate non-growing season monthly total ET. The conclusions of this research are based on the typical dormant season conditions observed at the research location and may not be generally transferable to other locations with different climatic and surface conditions

Maize response to irrigation and nitrogen under center pivot, subsurface drip and furrow irrigation: Water productivity, basal evapotranspiration and yield response factors

Information and data about quantification and comparison of crop water productivity indices for various irrigation levels and methods and nitrogen (N) application timings “simultaneously” under the same conditions do not exist. Unprecedented and extensive field experiments were conducted for maize (Zea mays L.) in 2016 and 2017 under center pivot (CP), subsurface drip irrigation (SDI) and furrow irrigation (FI) methods with full irrigation treatment (FIT), 80% FIT, 60% FIT and rainfed treatment (RFT) with three N application timings. N treatments were: (i) traditional (TN), (ii) non-traditional-1 (NT-1) and (iii) non-traditional-2 (NT-2). Irrigation yield production functions (IYPF); evapotranspiration-yield production functions (ETYPF), basal evapotranspiration (ETb), crop water productivity (CWP), irrigation water use efficiency (IWUE); evapotranspiration water use efficiency (ETWUE) and yield response factors (Ky) were quantified for each treatment and irrigation method. SDI method required the least seasonal irrigation amount in achieving maximum yield, followed by CP (\u3e~30 mm more than SDI) and FI (\u3e~55 mm more than SDI). Average crop water requirement for achieving maximum grain yield varied among the N treatments within and between the irrigation methods. Irrigation amounts for achieving maximum yields were about 160, 175 and 175 mm in TN, NT-1 and NT-2 nitrogen treatments, respectively, in the CP method; 130, 150 and 150 mm in TN, NT-1 and NT-2 nitrogen treatments, respectively, in the SDI method; and 184 mm in TN management in the FI method. The highest grain yield production per 25.4 mm of applied irrigation followed the order of CP-TN (2.07 Mg ha–1) \u3e SDI-NT-2 (1.91 Mg ha–1) \u3e FI-TN (1.22 Mg ha–1). Across all treatments for the given irrigation method, the highest averaged CWP of 3.00 kg m–3 (slope = 0.067 kg m–3) was observed in the SDI method (p \u3c 0.05) followed by 2.84 kg m–3 (slope = 0.052 kg m–3) in the CP method (p \u3c 0.05) and 2.51 kg m–3 (slope = 0.046 kg m–3) in the FI method. The lowest ETb was observed in FI-TN (169 mm), followed by CP-NT-2 (172 mm) and SDI-TN (255 mm). For two consecutive years, N treatments did not have significant (p \u3e 0.05) influence on IWUE in the CP or SDI methods. The highest IWUE, CWP and ETWUE were always obtained with limited irrigation treatments (60% FIT and/or 80% FIT) whereas the lowest with FIT. Maize under limited irrigation management had Ky \u3c 1 with CP, SDI and FI along with lower Ky values than the respective TN treatment in CP and SDI, suggesting that the yield reduction is impacted to a lesser degree from the magnitude of water stress. The overall conclusion disclosed that utilizing the combination of limited irrigation (80% FIT) with NT-1 fertigation under SDI and CP, while 80% FIT under FI can be viable management practices for achieving high grain yield and CWP in conditions similar to those presented in this research