What is scale?
What is the variability in SWD within a canopy?
What is the variability in SWD within a field?
What is the variability in SWD between different farms?

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What is scale?

Scale is the uncertainty associated with variability of a phenomenon over space and time (Russo et al. 1993). Three spatial scales can be distinguished, within canopies, within fields, and between fields/farms within a district.
 

What is the variability in SWD within a canopy?

There may be considerable variation in SWD within a canopy but it may not be related to a particular canopy position. Instead it may be related to the location of very large slow drying drops. Two conflicting factors explain the variation in SWD within canopies. The first is the increase in wind speed and the decrease in relative humidity with height. This favors the longest SWD in the bottom of the canopy. The second is that precipitation and condensation from dew add moisture to the top of the canopy first. Since dew events may often not fully saturate the canopy, the top may have longer SWD. This can explain conflicting reports in the literature as will be seen below.

The variation within a grape canopy is smaller than the influence of sensor precision. In grape, the top may have greater SWD although in most instances there was no significant difference between different parts of the canopy (Magarey et al. unpublished #2, Penrose et al. 1996, Wittisch 1997). The location of the sensor in the canopy was of secondary importance because the sensor has a precision between 1.5 to 5 hours whereas variation due to canopy position was closer to 0 to 3 hours in the few cases where it was important (Magarey et al. unpublished #2). Penrose and Nicol (1996) found that the variation between four sensors in different parts of an apple canopy was over 10 hrs on 44.7% of occasions, but these differences can also be explained by the uncertainty due to precision.

In field crops the portion of the canopy with the greatest SWD may depend upon the type of wetting event. In bean canopies the bottom of the canopy was observed to dry 6-10 hrs after the top, but these results only included drying after artificial wetting (Huber and Itier 1990). However, Weiss et al. 1989 found that in bean canopies the top of the canopy had the greatest SWD as did Wilson et al. 1999 in potato. A 3-5 hour variation in SWD was observed in an onion crop (Sutton et al. 1984).
 

What is the variability in SWD within a field?

Magarey et al. (unpublished #2) found that there was no significant difference in mean SWD between three proximal vines in a vineyard. In an unpublished study Magarey found that 63% of the variability in SWD could be explained by elevation across a 2 km sloping transect on either side of Seneca Lake, NY. Aspect either east or west had no influence. Longer SWD and colder temperatures were observed at greater elevations.
 

What is the variability in SWD between different farms?

Penrose and Nicol (1996) examined the variability in SWD between farms within a district. Seven sites were examined within a 20 km radius and only on approximately 5% of wet hours were all sites wet at the same time. Sites within a 5 km radius had the same total hours of wetness, but a more distant site, 20 km from the central point had significantly more wetness.

 
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