Calculations
Growing Degree Days (GDD)
Growing degree days are used to estimate the maturity of
crops during a growing season. We calculate GDD10 values (GDD50
for Fahrenheit), which means that the average daily temperature
is accumulated only if it is above 10°C (50°F), and below 30°C (86°F). More
specifically, we use the standard GDD formula:
GDD10 = max(0, Tavg−10)
GDD50 = max(0, Tavg−50)
where Tavg is a day's mean temperature. Each day's GDD value
is then added to the current total for the period, and this
summed result is displayed on the web page.
Richardson Chill Units (RCU)
Plants need a certain amount of cold weather during the winter
in order to mature properly later on. Straight chill units simply
count the number of hours below 7°C, but we use the Richardson
Chill Units to provide a more accurate model for orchards and
vineyards. First we calculate the average temperature for each
hour, and then use the table below to accumulate the RCU:
Temperature (°C)  Temperature (°F)  RCU (per hour) 
T < 1.5  T < 34.7  +0.0 
1.5 ≤ T < 2.5  34.7 ≤ T < 36.5  +0.5 
2.5 ≤ T < 9.2  36.5 ≤ T < 48.6  +1.0 
9.2 ≤ T < 12.5  48.6 ≤ T < 54.5  +0.5 
12.5 ≤ T < 16.0  54.5 ≤ T < 60.8  +0.0 
16.0 ≤ T < 18.0  60.8 ≤ T < 64.4  −0.5 
T ≥ 18.0  T ≥ 64.4  −1.0 
Bacchus Botrytis
Botrytis Cinerea is a disease that affects soft fruits, mainly wine grapes,
usually affecting vines that experience constant wet or humid conditions,
or over irrigation. It can lead to the loss of produce.
To detect Botrytis risk we monitor leaf wetness using the Decagon leaf wetness sensor.
For best results the sensor is recommended to be placed NEAR the crop but
not in it to avoid it being sprayed. It is also recommended that the sensor
be mounted at a 10 degree angle so it is almost horizontal. This will give the
best data results.
By measuring the duration that leaves have been wet, and utilising a
temperature sensor we can apply these to the Botrytis disease risk model and provide
an output in the form of an infection index value. When this index reaches 1.0 there
is a high probability of infection and crops should be sprayed or irrigation reduced
or stopped.
Data from the leaf wetness sensor is fed into a calculation based on research from the
University of California  Agriculture & Natural Resoursces. Disease model 1 was used  as
referenced in this article.
The equation specifically is:
I= 84.37  7.238T + 0.1856T2
Where I is risk index and T is the mean air temperature during a wet hour. The I values were summed
for the duration of each wet period. After four hours of "dry" the risk index is reset.
Dew Point
The Dew Point is the temperature at which a pocket of air
(in this case around the temperature/humidity sensor) would form dew.
To calculate the Dew Point temperature, the unit requires a temperature and
humidity sensor to be attached.
The Dew Point temperature is directly related to the temperature of the air in a
given humidity. If the humidity is 100% then the Dew Point temperature will be equal
to the temperature of the air. As the humidity decreases, so does the Dew Point temperature.
The Dew Point is helpful for predicting if and when dew will form on the vines.
It is also important to note that if the Dew Point falls below freezing (0°c)
then it is known as the frost point. This is a good indication that any dew
forming will instead be in the form of a frost.
Wet Bulb
Wet bulb is a calculation that refers to the lowest temperature on the vine that
will be experienced by the evaporation of water alone.
To calculate the Wet Bulb temperature, the unit needs to have a humidity and temperature
sensor attached.
Using the air temperature and the dew point (calculated with humidity and air temperature),
the Wet Bulb temperature is able to be calculated.
The application of the Wet Bulb temperature is primarily in regards to frost protection.
It is used to give an indication of when to start/stop misters or overvine sprinklers to
release latent heat and increase air temperature.
Evapotranspiration
Evapotranspiration is a term that describes the amount of water that
travels to the air from sources of evaporation, such as water bodies, soil,
and vine/tree canopies, and water lost through plant transpiration.
To calculate Evapotranspiration we require the weather station to have a
temperature and humidity sensor, wind speed and direction sensor,
and a solar radiation sensor.
Data from the sensors are fed into a calculation based
on an article (see the article here)
by Jay M .Ham, Professor, Department of Agronomy, Kansas State University.
The calculated value is recorded in mm/hr as well as total Evapotranspiration,
in mm, experienced over the time period being viewed.
By knowing how much water has been lost due to Evapotranspiration, a grower
is able to confidently know how much irrigation will be needed to replace the
water lost.
Soil Moisture Measurement using the Topp Equation
In June of 1980, G. Clarke Topp and his team members, J.L Davis and P. Annan, published a
watershed paper which included what would come to be known as the Topp equation. The equation
is so commonly used that some people don’t even know they’re using it. The Topp Equation makes
it possible for us to measure an electrical property of the soil (the dielectric permittivity)
and correlate that electrical property with the water content in the soil. The beauty of the
equation is that it is pretty accurate in most typical soils, saving us the job of doing custom calibrations.
The equation specifically is:
T (m3/m3) = 4.3 X 106 * e3  5.5 X 104 * e2 + 2.92 X 102 * e 5.3 X 102
Topp and his team showed that the volumetric water content of soil can be determined from the apparent
dielectric constant of the soils, independent of soil type, soil temperature, and soluble salt content.
True Time Domain probes like the Acclima measure dielectric permittivity and report that value
as well as calculating and reporting the Volumetric Water Content (VWC) using the Topp equation.
Reference  Topp, G.C., J.L. David, and A.P. Annan 1980. Electromagnetic, Determination of Soil Water Content:
Measurement in Coaxial Transmission Lines. Water Resources Research 16:3. p. 574582.
Powdery Mildew
Powdery mildew is a fungal disease that affects many different plants. The damage is caused to both
the leaves and the fruit, and if left unchecked, will result in the death of the plant.
The calculation we use is based on the model developed by Doug Gubler from the University of Calfornia.
It is an extremely complex and detailed calculation based upon the temperature we read from the desired
sensor.
The resulting risk allows you to gauge the optimal time for disease development and thus the most
appropriate time to use powdery mildew applications to control the disease.
