Learning from the physics around crop transpiration

Crop transpiration is essentially a process based in physics but has a strong botanical component as well. The one crop transpires notably more than the other, which makes that growers pay a lot of attention to crop transpiration. Providing there is sufficient water and a healthy root system, transpiration is primarily determined by (sun)light and in second place by the humidity of the greenhouse air. With venting, screening and misting, growers can manipulate the transpiration but due to the strength of solar radiation, especially in summer, the options to manipulate are small. Still, understanding the process helps to interpret observations and to set out the climate control strategy. During darkness the greenhouse air humidity is the factor that influences transpiration most, but also the long wave radiative heat loss to the cover and/or sky plays a role. These factors can be manipulated with screens, heating and ventilation. Proper climate control equipment and settings can lead to more favorable crop growth. In order to provide an educational tool to quantify these processes and to show the possibilities of manipulating the growing conditions, Wageningen UR has developed an online application that shows the effect of the major components on the transpiration rate of a crop. Also, the temperature distribution in three (lumped) canopy layers is shown. The public accessible application KASSIM shows a static snapshot of greenhouse conditions in a user-defined combination of outside conditions and greenhouse controller settings. This snap-shot image is actually the final equilibrium condition as calculated with the dynamic simulation model Kaspro, fed with external conditions and control settings as constants. This means that in order to get realistically computed steady-state conditions, this dynamic simulation model has to be accurate and complete. Therefore, in this project the equations that play a role in the computation of crop transpiration were refined and parameterized such that the dynamic calculation of crop transpiration and the crop temperature was close to measurements on four crops during several weeks in summer. This paper presents the theory and results obtained.