Abstract

Although growers have considerable control over crop production, a major concern is the anticipated increase in evapotranspiration (ET) due to global warming. ET rates, however, are also affected by radiation, humidity, wind speed, crop morphology, and crop physiology in addition to temperature. Crop ET (ET_c) is commonly estimated as the product of reference ET (ET₀) and a crop coefficient (K_c), and the main factors affecting K_c values are net radiation, aerodynamic resistance, and canopy resistance differences between the reference and crop surfaces. The standardized ET₀ equation has fixed values for the canopy resistance (r_c), and different values are likely for other crops. The r_c values might also adjust with increasing CO₂ and higher temperature. Aerodynamic resistance (r_a) depends on atmospheric stability, wind speed, and surface roughness. The relative aerodynamic contributions of sensible heat to ET₀ and ET_c could change if the canopy development or the wind speed climatology are modified by global warming. In this paper, we will discuss how the ET₀ and K_c values vary with microclimate and how K_c values and ET₀ rates might react to global warming.

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