Frehner et al. (2026) Revisiting energetic limits on crop coefficients in heterogeneous canopies

Identification

• Journal: Agricultural and Forest Meteorology
• Year: 2026
• Date: 2026-08-13
• Authors: Ethan Frehner, Thomas N. Buckley, Brian N. Bailey
• DOI: 10.1016/j.agrformet.2026.111411

Research Groups

• Department of Plant Sciences, University of California, Davis, USA

Short Summary

The paper challenges the widely accepted theoretical upper limit of the crop coefficient ($K_c$) of 1.2–1.3, arguing that this limit ignores the effects of advection and canopy heterogeneity on evapotranspiration (ET).

Objective

• To evaluate whether the presumed energetic limits of $K_c$ are physically absolute or if they are based on simplifying assumptions that fail to account for 3D canopy energy balance dynamics.

Study Configuration

• Spatial Scale: Canopy scale (with specific reference to heterogeneous environments such as California orchards).
• Temporal Scale: Not specified (theoretical/conceptual analysis).

Methodology and Data

• Models used: Penman-Monteith (PM) framework; Crop coefficient approach ($ETc = Kc \times ET_0$).
• Data sources: Theoretical analysis and review of existing literature regarding standard grass reference surfaces ($ET0$) and alfalfa reference surfaces ($ETr$).

Main Results

• Putative $K_c$ maxima (1.2–1.3) are based on assumptions of lateral homogeneity and limited horizontal energy exchange.
• Factors such as aridity, canopy heterogeneity, aerodynamic coupling, and advection (horizontal energy transfer) can enhance transpiration beyond these presumed limits.
• Observed ET values exceeding current $K_c$ limits should not be automatically rejected as physically impossible, as they may reflect actual energetic demands in complex canopies.
• In heterogeneous canopies, observed ET at or below the putative maxima may be limited by water availability or physiology rather than energy balance.

Contributions

• Critiques the practice of treating $K_c$ limits as axiomatic indicators of energetic constraints.
• Highlights the necessity of accounting for 3D canopy energy balance and advection when estimating water demand in non-homogeneous agricultural systems.

Funding

• Not specified in the provided text.

Citation

@article{Frehner2026Revisiting,
  author = {Frehner, Ethan and Buckley, Thomas N. and Bailey, Brian N.},
  title = {Revisiting energetic limits on crop coefficients in heterogeneous canopies},
  journal = {Agricultural and Forest Meteorology},
  year = {2026},
  doi = {10.1016/j.agrformet.2026.111411},
  url = {https://doi.org/10.1016/j.agrformet.2026.111411}
}