The objective of this work was to evaluate the use of plant height as a calibration variable for improving indirect measurements of the leaf area index (LAI). Three experiments were conducted with different crops - corn (Zea mays), soybean (Glycine max), and sugarcane (Saccharum officinarum) -, to compare the performance of the LAI measured indirectly (LAIind) and corrected by the calibration variable with the LAI measured directly (LAIref). Without the proposed correction, the LAIind tended to be overestimated by 20%, on average, compared with the LAIref, for the three crops. After crop height was used to adjust the LAIind, a strong positive relationship was observed between the LAIref and the corrected LAIind (R2 = 0.96); overestimation was reduced to 4% and the root-mean-square error decreased to 0.35 m2 m-2. The variable canopy height is promising for the correction of the LAI of the soybean, corn, and sugarcane crops.

SUMMARY: Due to the lack of basic information on water required by maize (Zea mays L.) in Brazil, the large amount of water applied usually exceeds crop requirements, wasting water and energy. In this study, we measured crop evapotranspiration (ETc) as evaporative heat flux from a centre pivot-irrigated maize plantation in Southern Brazil during winter and summer seasons, using the Bowen ratio method to evaluate how the degree of canopy-atmosphere coupling affects crop water needs and irrigation management. Irrigation requirements were determined by comparing ETc with reference evapotranspiration (ETo), derived from the Penman–Monteith equation and expressed as the ETc/ETo (Kc) ratio. In this study, the average Kc values obtained were 1.31 and 0.90 for the winter and summer, respectively. Using aerodynamic and canopy resistance measurements, the decoupling factor (Ω) was computed. Ω values tending to zero (0.09 and 0.20 for winter and summer, respectively) showed that strong coupling of maize plants to the atmosphere and sensitivity to high air temperatures, vapour pressure deficits and wind speed caused variations in Kc in relation to ETo ranges. During the experimental period, the Kc value ranged from 0.92 when the ETo exceeded 4 mm d⁻¹ to 1.64 when the ETo was less than 2 mm d⁻¹.