On the impact of leaf-level processes on the water and carbon canopy-turbulent fluxes

Representing the diurnal variability of state meteorological variables, including carbon dioxide, is still an open challenge as shown by the large discrepancies with observations and weather and climate models. These discrepancies translate into different diurnal exchanges of heat, water and carbon dioxide between the canopy and atmosphere. These sub-diurnal differences can propagate to larger temporal and spatial scales.

With a systematic approach, we investigate the diurnal gas exchange of both water and carbon dioxide for an irrigated crop. Our investigation is based on a comprehensive observational dataset that ranges from scales covering from the leaf level to the canopy level  to the atmospheric boundary layer gathered at the LIAISE campaign. This campaign took place over two weeks in summer of 2021 in a region of the Ebro basin located in Catalonia, Spain. We focus specially on one of the “supersites” of the campaign: La Cendrosa, which is an irrigated alfalfa field surrounded by a very dry region.

Our observational approach is bottom-up in which we first analyze the leaf, second the canopy and third the interactions with the atmosphere at a local field scale. Among the observations, we analyzed leaf gas exchange measurements, turbulent surface fluxes and vertical profiles of driving environmental variables such as radiation, wind, temperature, and specific humidity. To support the observational analysis, we use a land-atmospheric interactive model (CLASS model). This model allows the representation at each of the three levels mentioned: (1) leaf, (2) canopy and (3) field.

Our observations show an asymmetry in the diurnal variability of the stomatal conductance, which indicates a larger opening of the stomata during the morning than during the afternoon. To attribute processes to the causality in the stomatal opening, we derive new expressions of the tendency of the stomatal aperture as a function of the mean meteorological drivers including radiation, temperature, atmospheric CO₂ concentration, and water vapor deficit. The asymmetry is only simulated by models once specific characteristics of the crop are considered. It is also observed that dynamics at the leaf level such as a closure of the stomata during the midday can cause a dip in the evapotranspiration and enhance the sensible heat flux. Our results open the debate on the circumstances under which it is important to constrain the leaf gas exchange.