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Purpose <p>Flow-proportional variable-rate fertigation (VRF) systems are used to support site-specific nitrogen (N) management under center-pivot irrigation; however, field-scale evaluations of actual N delivery under spatially variable operating conditions remain limited. This research brief presents a practical, field-based method to quantify as-applied N rates for a flow-proportional VRF system integrated with variable-rate irrigation (VRI).</p> Methods <p>Fertilizer drawdown measurements, pivot flow-totalizer records, and irrigation prescriptions were combined with management-unit geospatial processing to quantify as-applied irrigation depths and N rates during an in-season fertigation event. System-level irrigation and fertilizer volume accounting was performed to ensure mass-balance closure. As-applied N rates were derived assuming uniform fertilizer concentration under flow-proportional injection and were compared against prescribed rates at the management-plot scale.</p> Results <p>Strong spatial agreement was observed between prescribed rates (1–27&#xa0;kg-N/ha) and as-applied N rates (<i>r</i> = 0.993). Root mean square error (RMSE) and mean absolute error (MAE) were 2.52 and 2.41&#xa0;kg-N/ ha, respectively. A consistent positive bias was observed, attributable to irrigation hydraulic variability and small mismatches between the irrigation-flow meter displayed on the pivot control panel and the flow sensor supplying input to the fertigation controller. System-level volume closure was achieved (<i>R</i> ≈ 1.01), supporting the validity of the quantification approach.</p> Conclusion <p>Outcome-based as-applied evaluation provides a practical and reproducible means of verifying VRF performance beyond assumed prescription execution. The proposed workflow enables reliable field-scale quantification of N delivery accuracy and supports improved agronomic record keeping and performance assessment in operational VRF–VRI systems without requiring catch-can measurements or nozzle-scale instrumentation.</p>

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A mass balance method for developing nitrogen as-applied maps in flow-proportional variable-rate fertigation systems

  • 期刊论文

  • 发表日期：

    2026年5月16日

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  • Precious N. Amori,
  • Derek M. Heeren,
  • Guillermo R. Balboa,
  • Tyler Smith,
  • Joe Luck,
  • Yeyin Shi,
  • Daran Rudnick,
  • Alan Boldt

摘要

Purpose

Flow-proportional variable-rate fertigation (VRF) systems are used to support site-specific nitrogen (N) management under center-pivot irrigation; however, field-scale evaluations of actual N delivery under spatially variable operating conditions remain limited. This research brief presents a practical, field-based method to quantify as-applied N rates for a flow-proportional VRF system integrated with variable-rate irrigation (VRI).

Methods

Fertilizer drawdown measurements, pivot flow-totalizer records, and irrigation prescriptions were combined with management-unit geospatial processing to quantify as-applied irrigation depths and N rates during an in-season fertigation event. System-level irrigation and fertilizer volume accounting was performed to ensure mass-balance closure. As-applied N rates were derived assuming uniform fertilizer concentration under flow-proportional injection and were compared against prescribed rates at the management-plot scale.

Results

Strong spatial agreement was observed between prescribed rates (1–27 kg-N/ha) and as-applied N rates (r = 0.993). Root mean square error (RMSE) and mean absolute error (MAE) were 2.52 and 2.41 kg-N/ ha, respectively. A consistent positive bias was observed, attributable to irrigation hydraulic variability and small mismatches between the irrigation-flow meter displayed on the pivot control panel and the flow sensor supplying input to the fertigation controller. System-level volume closure was achieved (R ≈ 1.01), supporting the validity of the quantification approach.

Conclusion

Outcome-based as-applied evaluation provides a practical and reproducible means of verifying VRF performance beyond assumed prescription execution. The proposed workflow enables reliable field-scale quantification of N delivery accuracy and supports improved agronomic record keeping and performance assessment in operational VRF–VRI systems without requiring catch-can measurements or nozzle-scale instrumentation.