Simulation of surface wetness with a water budget and energy balance approach

Surface wetness plays an important role in environmental studies. In particular, it is a major variable for plant disease prediction. Surface wetness is commonly measured with electronic sensors but simulation with a surface wetness model is an alternative. Recently, the increased use of interpolation procedures and atmospheric models to produce site-specific weather products has created a greater need for reliable surface wetness models. However, surface wetness models have not been widely used operationally because they are often highly complex, do not simulate both dews and rain or do not adapt well to a new spatial scale or crop. Other models estimate surface wetness in units that are cumbersome to observe in the field. In addition, few models have been calibrated to observed surface wetness over a wide range of atmospheric variables and plant leaf properties under controlled environmental conditions. The objective of this study was to develop a surface wetness model that would be appropriate for operational use in site-specific weather products for grapes. For this purpose, we developed the surface wetness energy balance (SWEB) model based on a ‘big leaf’. The SWEB model consists of four sub-modules describing: (i) surface water distribution based on an observed wet fraction; (ii) canopy water budget; (iii) energy balance module based on a combination equation developed by Tanner and Fuchs; (iv) a transfer function based on Bird et al.ʹs generic transfer coefficient that was previously calibrated to surface wetness under controlled conditions. The SWEB model can be adapted to the physical characteristics of a particular crop by adjusting four plant parameters: leaf area index (LAI), maximum fraction of canopy allowed as wet surface area (Wmax), crop height and maximum water storage. The SWEB model is most sensitive to LAI and Wmax. The SWEB model is close to the required criteria for a suitable surface wetness model including simplicity, utility, scalability, easily observable output units and in addition, it has been calibrated under controlled conditions. The SWEB model was validated in a vineyard and in a companion study, compared to a widely used sensor. The overall objective of these studies was to develop a theoretical standard for surface wetness measurement.