Temperature controls the developmental rate of many organisms. Plants and invertebrate animals, including insects and nematodes, require a certain amount of heat to develop from one point in their life cycles to another. This measure of accumulated heat is known as physiological time or thermal time. Theoretically, physiological time or thermal time can provide a common reference for the development of organisms. The amount of heat required to complete a given organism's development does not vary—the combination of temperature (between thresholds) and time will always be the same. Physiological time or thermal time are often expressed and approximated in units called degree-days (°D).
Upper and lower developmental thresholds have been determined for some organisms through carefully controlled laboratory and field experiments. The lower developmental threshold for an organism is the temperature below which development stops. The lower threshold is determined by the organism's physiology. It is independent of the method used to calculate degree-days. For example, the lower developmental threshold is 15°C and the upper developmental threshold is 35°C for potential dengue carrier mosquito Aedes aegypti. Thresholds vary with different organisms. The upper developmental threshold is the temperature above which the rate of growth or development begins to decrease or stop as determined by the cutoff method. The physiological interpretation of the upper threshold is dependent on the cutoff method.
Degree-day calculations are based on the area under the diurnal temperature curve and between the thresholds. For a site with maximum and minimum temperatures data available, the different methods for calculating degree-days are single sine, double sine, single triangle, and double triangle. From the simplest to the most mathematically complex, these are: single triangle, double triangle, single sine, and double sine. All of these are linear methods because the rate of development is presumed to be a straight line directly related to temperature.
SimCLIM 4.0 for Desktop provides two degree day models. The first is a spatial model which calculates degree day from monthly mean temperature data using a method described in Carter et al. (1991). The second is a site specific model which calculates degree day from daily time series of maximum and minimum temperatures. The method has been described in detail by Baskerville & Emin (1969), (Andrewartha & Birch (1973), (Allen (1976), and (Wilson & Barnett (1983).
The cutoff method refers to the manner in which the degree-day calculation area will be modified in relation to the upper threshold.
There are three cutoff methods - horizontal, vertical, and intermediate—to be used in conjunction with the sine and triangle calculation methods:
- Horizontal cutoff method assumes that development continues at a constant rate at temperatures in excess of the upper threshold. Mathematically, the area above the upper threshold is subtracted from the area above the lower threshold.
- Intermediate cutoff method assumes that development slows, but does not stop, at temperatures above the upper threshold. Mathematically, the area above the upper threshold is subtracted twice from the area above the lower threshold.
- Vertical cutoff method assumes that no development occurs when a temperature is above the upper threshold
The single triangle method draws a straight line between a day's minimum temperature and maximum temperature, assumes the next day's minimum temperature is the same, and draws another line to that point, forming two sides of a triangle. This method assumes the temperature curve is symmetrical around the maximum temperature. Degree-days are estimated by calculating the area within the triangle and between the thresholds.
The double triangle method draws a straight line between a daily minimum and maximum temperature, and another line vertically through the maximum temperature, forming two sides of a triangle. Degree-days are estimated by calculating the area within the triangle and between the thresholds. The second 12-hour period uses the same configuration with the minimum temperature of the following day. Degree-days for the day are the sum of the degree-days for the two half-days.
The single sine method uses a day's minimum and maximum temperatures to produce a sine curve over a 24-hour period, and then estimates degree-days for that day by calculating the area above the threshold and below the curve. This method assumes the temperature curve is symmetrical around the maximum temperature.
The double sine method fits a sine curve from the minimum temperature of the day to the maximum temperature of the day and then fits a separate sine curve from the maximum temperature of the day to the minimum temperature of the next day. Degree-days for the day are the sum of the degree-days for the two half-days.
Select Run an Impact Model from the Tools dropdown menu.
Select Heat Accumulation.
You can then choose between Area wide and Site specific scale.
Select your study area from the top right. If it is spatial like a state or province choose Area wide for your analysis.
Enter the base temperature and click on .
The SimCLIM 4.0 for Desktop scenario generator dialogue box will be displayed. Leave it in its default form to conduct and analysis of baseline conditions or add a year and scenarios to explore the impact of climate change.
Select the months or months of interest.
Click . A image will be produced with the criteria in the header. You can click on at the bottom right of the image to get complete information on the output.
You can run a different scenario for the same area and then choose the synch images tool from the toolbar to ease your analysis of change.
The site specific version of the degree day model is slightly different. Select site specific and click.
Depending on the study area you have chosen in the upper right of the main dialogue box a map will be generated with site data indicted.
Click on the site of interest in the map or click on the site drop down dialogue box.
Click and then click on the column in table to generate a graphic display of the results.
Left Click in the model output window for a graphic display:
You can also click on model output to toggle between output and weather data.