Different surfaces may be defined, for example leaf, fruit or canopy. At first it appears counterintuitive to classify SWD as a binary variable. An alternative would be to express the proportion of the hour wet. However, if the interval is partially wet it is not clear what part of the interval was wet or dry. Since small traces of water may persist in the canopy for extended periods it is useful to include a wet surface area threshold, W t , below which the canopy is considered to be dry.
Hours are a suitable unit because weather data is commonly collected on an hourly basis.
The disadvantage of a binary value is that it creates errors that are on the average 0.5 hours, which is not a negligible error. In instances where this is important a sub-hourly binary variable should be considered. Surface Wetting Period - The length of time during which a surface remains wet. This can be a useful means of comparing different sensors or summarizing SWD data. In this volume, SWD is analyzed by individual wetting periods and not daily summaries.