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Why a measurement standard is needed?
The need for a measurement standard for surface wetness has long been
recognized (Anonymous 1990). The consequences of the absence of a standard
are numerous. First, SWD data can not be easily transferred from one group
to another. Second, SWD climatic databases can not be constructed for large
regions. Third, SWD data must be interpreted according to their local observation
protocol. Finally, the calibration of sensors becomes problematic.
What is the current method of calibrating sensors?
At present the recognized method of calibrating sensors is to compare
visual observations of wetting and drying in the crop canopy with the output
of the sensor. Potraz et al. (1994) gives an example of this using
unpainted sensors above a tomato crop. They found that a resistance threshold
of 6999 kOhm corresponded well to the onset and dry-off of dew. It may
not be possible to calibrate sensors accurately within the laboratory.
The authors calibrated the dryoff of sensors in the lab and derived a resistance
value of 900 kOhm. Under field conditions this value was found to underestimate
dew duration by an average of 1.5 hrs, whereas the field derived threshold
of 6999 KOhms underestimated dew duration by an average of 0.3 hrs.
What standards have been proposed?
The most common suggestion for standardization is the adoption of a
specific sensor design as the standard. Getz (1991) recommended a particular
sensor and protocol for measuring SWD. The protocol specified the placement
of sensors inside an evergreen bush. Other workers have also established
standard practices for the use of sensors (Fisher et al. 1992).
As a consequence, there is no universal standard for wetness measurement.
An alternative approach: a theoretical standard
Another approach is to define SWD in physical terms, as a water budget for a plant surface. Water is added to a budget by precipitation or condensation from dew and lost by evaporation. A surface energy balance model can describe these physical processes as a balance of energies. For example during a dew event, radiant energy is lost from the plant surface and is converted into latent and sensible heat. The latent heat flux effects evaporation and condensation, while sensible heat flux affects the heat losses and gains at a leaf surface. The partition of radiation into latent and sensible heat depends upon the atmospheric conditions and the plant and soil physical properties. Such a physically based understanding of SWD offers the potential to define a theoretical standard for SWD.
What are the advantages of a theoretical standard?
A theoretical standard offers many advantages. First, one only needs plant properties and atmospheric variables to determine SWD. Second, the theoretical approach provides a physical basis for deriving SWD. Third, and most importantly for measurement, SWD sensors can be compared to a standard to determine their precision and accuracy, and their calibration. Fourth, the standard can be easily distributed and used by the scientific community. Fifth, simulated data can be used as input. The simulated data may be historical, current or forecast weather data.
Proposed theoretical standards
The SWEB simulation model has been proposed as a theoretical standard.
The following material will be provided here as an example of how a model
may serve as a theoretical standard.
The following chapter from Roger Magarey's thesis can be downloaded
as a word 97 document. This informatioin will be available once the papers
have been published.
Abstract, Table of contents
An introduction to a theoretical standard
The development and calibration of the theoretical standard
Evaluation of the theoretical standard in grapes with on-site weather
data
Evaluation of the theoretical standard with off-site weather data
Conclusions
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the SWEB model as a Excel spreadsheet.
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