Growing
Degree Days
Growing Degree Days are a measurement of the growth and development
of plants and insects during the growing season. Development does not occur at
this time unless the temperature is above a minimum threshold value (base
temperature). The base temperature varies for different organisms. It is
determined through research and experimentation. The actual temperature
experienced by an organism is influenced by several factors. These factors
affect growth and development. For instance, depending on the weather, an
organism's temperature may be a few degrees more or less than that recorded. An
organism may spend its time in the shade or under direct sunlight. The fertility
and nutrient content of the soil directly affect the growth rate of insects and
plants. The presence of weeds and precipitation may indirectly influence
development. Due to these factors and some other scientific considerations, a
base temperature of 50 degrees Fahrenheit is considered acceptable for all
plants and insects.
There are three
main methods of calculating the GDD:
a) Simple Method:
This method compares the daily mean
temperature (Tmean)
to the base (Tbase)
or threshold temperature. The daily mean
temperature is the average of the daily maximum and minimum temperatures. If
this average is greater than the threshold temperature the GDD accumulated for that day is
the threshold temperature subtracted from the daily mean temperature. If the
daily mean temperature is less than the base temperature, then the GDD for that
day is zero. Adding the GDD of all the days previously considered and the GDD of
that day calculated the GDD of a particular day.
b)
Modified Growing Degree Days:
This method sets an upper and lower threshold for temperature. Typically, 86F
and 50F are used. If the daily maximum temperature is greater than 86F, then it
is reset to 86F. If the daily minimum temperature is less than 50F, then it is
reset to 50F. These modified temperature are averaged and compared to the base
temperature in a similar manner to the simple method. This method is used to
mainly to monitor the development of corn, the assumption being that development
does not take place beyond 86F.
c) Sine Wave Method:
This method is considered the most accurate way to calculate the GDD. First, a
graph is made. The rate of development is plotted on the y-axis (1/time) and the
temperature is located on the x-axis. The curve produced by this graph resembles
a sine wave. The base temperature is considered to be 50F.
The GDD is then calculated thusly:
AvgDailyTemp = (MaxDailyTemp + MinDailyTemp)/2
k = 50 (the base temp.)
a = (MaxDailyTemp -
MinDailyTemp)/2
theta = ((k - AvgDailyTemp)/a)
If MaxDailyTemp < k
GDD = GDD + 0
Else If MinDailyTemp > k
GDD = GDD + AvgDailyTemp – k
Else
GDD = GDD + (1/3.141593) * [ (AvgDailyTemp – k) * ( (
3.141593/2 ) – arcsine( theta ) ) + ( a * Cos( arcsine( theta ) ) ) ]
Phenology
Basically, phenology is the study
of cyclic events of nature in responses to seasonal and climatic changes to the
environment. The cycles studied are usually those of plants and animals.
Phenologists are interested in recording and comparing the dates of various
natural events such as bird migration; plant budding, flowering or fruiting;
insect activities and stages of birth and death
(http://phenology.osu.edu).
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