Evapotranspiration measurement for tall plant canopies: The sweet sorghum case

A methodological study on a tall canopy in a Mediterranean region was carried out in order to identify the most suitable method for measuring the actual evapotranspiration (ET). ET from a sweet sorghum crop was measured by 4 different methods: (i) energy balance/eddy correlation, (ii) energy balance/Bowen ratio, (iii) energy balance/aerodynamic simplified, and (iv) floating lysimeter (ETmeter). In order to compare a very large range of ET values and to reduce experimental errors due to low gradients of air humidity and temperature, the crop was submitted to two soil drying-wetting cycles. To evaluate the main limitations of each method with respect to crop height, crop ET was monitored during the entire vegetative cycle, from stem elongation (crop height 0.85 m, full canopy) to grain filling (when the crop was 2.5 m high). The comparison between the micrometeorological methods (i, ii, and iii) was made on hourly and daily time steps, while the analysis of ETmeter measurements was made on a daily time step only. On an hourly scale: eddy correlation ET was 106% of Bowen ratio ET and simplified aerodynamic ET was 116%, 125% and 135% of Bowen ratio ET with the first sensors are placed at the top of the canopy and the second sensors at 0.7m, 1.4m and 2.8m from the first sensors, respectively. On a daily scale: eddy correlation ET was 102% of Bowen ratio ET, simplified aerodynamic was 114% of Bowen ratio ET and ETmeter ET was 97% of Bowen ratio ET. In the last case the values are very widely spread and the correlation is really not so good. The results show that the Bowen ratio method and the eddy correlation method are in good agreement on daily scales, however, certain precautions, must be taken concerning the eddy correlation method on an hourly scale. The simplified aerodynamic method failed when crop height was > 1.5 m and the ETmeter failed under windy conditions (wind speed > 2.0 m/s) and limited crop water conditions.