Growing Degree Days in Ontario
- A Description
Table of Contents
- Heating Degree
Days
- Cooling Degree
Days
- Growing Degree
Days
- Modified
Growing Degree Days
- Related Links
Heating Degree Days
Heating degree days, or HDD, are used primarily by oil and gas
companies and utilities to estimate the amount of energy (oil, natural
gas, etc.) required for home and commercial heating during the cold
months. To calculate the HDD they first determine the mean temperature
for the day. This is usually done by taking the maximum and minimum
temperatures for the day, adding them together, and dividing by
two. This mean temperature is then subtracted from a base temperature
to give a daily HDD. If the daily HDD calculates to a negative number
it is made equal to zero. The daily HDD is then accumulated over
the heating season. Generally the larger the accumulated HDD is
the more energy we use to heat our homes and businesses.
Cooling Degree Days
Cooling degree days, or CDD, are used primarily by electrical utilities
to estimate the amount of heat that has accumulated and then the
amount of electricity required for home and commercial cooling during
the warm months. CDD are calculated in much the same manner as HDD
are. First the mean temperature for the day is determined. This
is usually done by taking the maximum and minimum temperatures for
the day, adding them together, and dividing by two. The base temperature
is then subtracted from the mean temperature to give a daily CDD.
If the daily CDD calculates to a negative number it is made equal
to zero. The daily CDD is then accumulated over the cooling season.
Generally the larger the accumulated CDD is the more energy we use
to cool our homes and businesses.
Growing Degree Days
Growing degree days, or GDD, are used to estimate the growth and
development of plants and insects during the growing season. They
are calculated in much the same manner as CDD but with different
base temperatures. The basic assumption is that development will
only occur if the air temperature exceeds some minimum developmental
threshold or base temperature. The increase in development is also
assumed to be close to linear in relation to an increase in temperature
or heat energy accumulation. The base temperatures are determined
experimentally and are different for each organism.
To calculate GDD you first determine the mean temperature for the
day. This is usually done by taking the maximum and minimum temperatures
for the day, adding them together, and dividing by two. The base
temperature is then subtracted from the mean temperature to give
a daily GDD. If the daily GDD calculates to a negative number it
is made equal to zero. The daily GDD is then added up (accumulated)
over the growing season. Generally the larger the accumulated GDD
is the greater the plant or insect development.
The GDD equation used by OMAFRA can be written as follows:
dailyGDD = ((Tmax +Tmin)/2) - Tbase
where:
Tmax = the daily maximum air temperature
Tmin = the daily minimum air temperature
Tbase = the GDD base temperature for the organism being monitored
OMAFRA uses the above GDD calculation and accumulation to help
determine the progression of the growing season and to make comparisons
to previous years and long term averages (normals). There are four
factors to consider when comparing GDD accumulations from various
sources or regions.
- Are the base temperatures used the same?
- OMAFRA uses various base temperatures and produces reports
for three basic ones; 3, 5 and 10 degrees Celsius.
- Are the start dates for the accumulations the same?
- OMAFRA uses April 1 as a start date for accumulations.
- Are the equations used to calculate the daily GDD the same?
- Many modifications to the simple GDD calculation have been
developed over the years. Many American States use more complex
ones but still call them simply GDD or Degree Days.
- Are the temperatures used in degrees Celsius or Fahrenheit?
- OMAFRA uses the Canadian degrees Celsius standard for temperatures
while the United States uses the degrees Fahrenheit scale.
Another problem that has caused some confusion over the years is
with the terminology used. Here in Ontario we use the terms Growing
Degree Days (GDD) and Crop Heat Units (CHU), which represent two
different, temperature dependent, development models. The Crop Heat
Units calculation now used in Ontario is identical to the traditional
Corn Heat Units calculation with the name change reflecting its
now broader application to other crops. OMAFRA has developed a CHU
Factsheet with a full description of the CHU calculation and it's
applications. Growing Degree Days are some times referred to just
as Degree Days or Degree Days Averaging Method. Some jurisdictions
also use the term Heat Units interchangeably with Degree Days.
Modified Growing Degree Days
Modified GDD are similar to GDD but with several temperature and/or
calculation adjustments. A common modified GDD calculation used
in the United States to monitor corn development places an upper
limit of 30°C (86°F in the States) on the maximum daily
temperature. Other Modified GDD systems include:
- Baskerville-Emin (BE) Method
- This method uses a sine curve, fitted to the maximum and
minimum temperature profile, to simulate how the daily temperature
would vary. The area under the curve and above the base temperature
is then calculated. This gives a better estimate of the heat
accumulation, but uses a more complex calculation. This method
is commonly used in Michigan, USA.
- Hourly Average Integration Methods
- These methods are usually implemented on automated dataloggers.
They use average hourly temperatures to calculate a hourly
heat accumulation. These are then summed up over the day to
give a daily GDD. Some systems use 15 minute and even 5 minute
average temperatures.
The University of California, Integrated Pest Management Project,
has put together a good description of various modified GDD calculation
methods, particularly those in use across the United States. See
Degree-Days and Phenology Models for more information.
Related Links
