Modelling of actual evapotranspiration in open top chambers (OTC) at daily and seasonal scale: Multi-annual validation on soybean in contrasted conditions of water stress and air ozone concentration

The present study describes an analytical model, based on the Penman-Monteith equation, for estimating the daily evapotranspiration (ET) inside open top chambers (OTCs). It includes two functions, making it possible to separately take into account the effects of water stress and of high concentrations of air ozone. The input variables of the model are: (i) the diurnal values of global radiation and temperature, usually measured routinely in a standard agro-meteorological station; (ii) the daily values of the AOT40 index accumulated (accumulated ozone over a threshold of 40ppb during daylight hours, when global radiation exceeds 50Wm⁻²); and (iii) the periodic measurements of the predawn leaf water potential. These latter two variables were determined inside the OTC. The ability of the model to take into account contrasting conditions of ozone air concentration and water stress was evaluated over three successive years, for 468 days, in 10 crop growth cycles, excluding the days employed to calibrate the model. Tests were carried out in several chambers for each year and take into account the intra- and inter-year variability of ET measured inside the OTCs. On the daily scale, the slope of the linear regression between the ET measured by the soil water balance and that calculated by the model under different water conditions are 0.94 and 1.05 for the filtered and unfiltered (or enriched) OTCs. The correlation coefficients r ² observed are 0.71 and 0.64 for the two situations. On the seasonal scale, the mean difference between measured and calculated ET is equal to +3% and +10% for the filtered and unfiltered OTCs respectively. The ability of the model to estimate the daily and seasonal ET inside the OTCs was quite good in all of the experimental situations. Thus, the model hypothesis and the calibration considered seem to be realistic and robust. The modifications of the daily ET as a result of the use of OTCs in a Mediterranean region, as compared to a crop cultivated in an open field, were also evaluated. Finally, the possibility of developing a version of the model using only automatically collected variables inside the OTCs was also discussed.