Leaf wetness duration in sunflower (Helianthus annuus): analysis of observations, measurements and simulations.

The presence of free water on a leaf surface caused by dew is the result of the radiative balance determined by the interactions between the canopy and the atmosphere. Leaf wetness duration (LWD) depends on leaf and canopy characteristics (exposure, position on the plant, etc.). To study the LWD in a horizontal canopy, a set of sensors were placed inside a sunflower (Helianthus annuus) experimental field and the records were compared with visual observations. An agrometeorological station was installed outside the crop to collect the variables necessary to feed a LWD model. The simulations were compared both to sensor measurements and to visual observations. The main aims were the verification of the sensor performances in the detection of dew and the analysis of LWD spatial variability inside the crop canopy. The potential of simulation models as an alternative to field instruments was also investigated. Our results showed that in sunflower canopy, organised in different layers, the LWD increases with height in the canopy but the sensors are not always able to reproduce the observed variability. The use of models can be considered as an operational procedure but attention should be paid to their calibration.