Surface wetness duration under controlled environmental conditions

Surface wetness is an important variable for forecasting plant disease. It is commonly measured with sensors, but these provide an indirect measurement and there is variability between different makes of sensors. Consequently, there is no standard for surface wetness measurement. The objective of this study was to derive and validate a physically based theoretical definition of surface wetness for both drops and films drying under controlled conditions. The validation compared observations of surface wetness with theoretical simulations for a range of factors: atmospheric variables including temperature, humidity, net radiation and wind speed; plant physical properties including surface wettability, leaf width and thickness; and initial water distribution including drop volume and film thickness. The drying of drops and films was studied in a wind tunnel and a laboratory setting. Surface wetness duration was most sensitive to initial drop volume or film depth, relative humidity and surface wettability in the case of water distributed as drops. Surface wetness duration was relatively insensitive to other atmospheric variables. Water distribution whether drop or film, made a four-fold difference in evaporation rate compared to leaf width, which was unimportant for leaves larger than 3 cm. The observations were compared to the theoretical estimations using a surface energy balance model. The surface energy balance model is based on a combination equation and a generic transfer coefficient. The transfer coefficient is dependent upon whether the water is present as drop or film and is assumed to be independent of leaf width or drop dimension. Considerations of drop geometry are ignored, although the initial wet area must be known. The theoretical understanding of drop and film drying under controlled conditions could potentially be useful in a field scale model of surface wetness. Since all the factors that influence surface wetness can be explicitly defined, such a field scale model has potential to be used as a theoretical standard for surface wetness estimation. Additional research is required to test this model under controlled conditions of condensation.