Effect of the heterogeneity of the radiation distribution on the crop activity

The crop activity inside a greenhouse is strongly affected by the local microclimatic conditions (radiation, temperature and humidity of air in particular). In order to reduce the consumption of energy and water, the control of these parameters is of prime interest. A better knowledge of the local climate may also help to formalize processes in CFD studies. The objectives of the present study are: (i) to analyse the evolution of the microclimate in the vicinity of the canopy all day long and (ii) to assess the spatial heterogeneity of the radiation distribution, air humidity and temperature above the crop, and crop evapotranspiration. In this prospect, a 100 m2 greenhouse compartment located in Angers (47° 28’ N, 0° 33’ E) was equipped with a set of sensors including temperature and humidity probes as well as solar silicium cells and pyranometers. Crop evapotranspiration was measured at two locations with balances. Under rather clear sky conditions, the horizontal distribution of the climatic parameters was analysed as well as their temporal evolutions which were compared to each other. At a short time scale (2 hours), strong radiation and humidity spatial-heterogeneities are observed without impact on temperature distribution: different evolutions of their characteristics all over the day, mainly due to shadows, are reported, but averaging the values of the climatic parameters over time (from 8 am to 4 pm) erases these heterogeneities. The investigations of the temporal evolution of the climatic parameters evidence the role played by the radiation. Knowing both the spatial distribution and temporal evolution of the crop activity could help to select hypotheses for CFD simulations as well as to identify the plant requirements and to adapt a strategy for heating and irrigation.