Effects of free-air CO2 enrichment on energy balance and evapotranspiration of cotton

Abstract

The effects of free-air CO2 enrichment (FACE) at 550 μmol mol−1 on the energy balance and evapotranspiration, ET, of cotton (Gossypium hirsutum L.) were investigated. Latent heat flux, λET was calculated as the residual in an energy balance approach from determinations of net radiation, Rn minus surface soil heat flux, G0, minus sensible heat flux, H. Rn was directly measured. G0 was determined from measurements with soil heat flux plates at 10 mm depth, corrected for temperature changes in the soil above. H was determined from measurements of air temperature with aspirated psychrometers, of foliage temperature with IR thermometers, and of wind speed with cup anemometers.Under ambient CO2 (control) conditions (about 370 μmol mol−1), the λET from the energy balance approach agreed fairly well with values from several other methods, including the Bowen ratio method, lending credence to the technique. However, the results had an uncertainty of the order of 20% associated with the Rn measurements. Therefore, an apparent increase in ET of about 13% in the FACE plots was judged insignificant. The conclusion that any effects of CO2 enrichment to 550 μmol mol−1 on the ET of cotton were too small to be detected was consistent with the results of other investigators who determined ET in the same experiment using stem flow gauges and the soil water balance.