Calculating GP
The PACE Turf temperature-based growth potential, or GP, is a utility function that takes the input of average air temperature, and gives an output of GP on a scale of 0 to 1, or 0 to 100%. This isolates the potential effect of temperature on turfgrass growth, and conveniently expresses that potential on a scale we can all understand.
The growth potential equations have some constants and one variable. The variable is the actual air temperature. The constants are the optimum air temperature and the variance. But there’s a little more to it, and I’ve seen an error made multiple times that I want to highlight here, in the hope that the error can be avoided in the future.
Let me explain. One can use Fahrenheit temperatures to calculate GP, and that necessarily requires Fahrenheit temperatures as optimum air temperature, and that temperature scale has a specific variance in the equation. The optimum air temperature and the variance differ for the cool-season grass GP equation and for the warm-season grass GP equation.
The error I’ve seen, for example here, is changing the actual air temperature to the Celsius scale, changing the optimum air temperature to the Celsius scale, but leaving the variance term as it is for Fahrenheit as well.
The result of such a calculation is shown in this chart,
As the air temperature approaches the optimal temperatures for net photosynthesis, the GP by both equations converges on 100%. But because the variance term has not been adjust to the Celsius scale, when the temperatures move away from the optimum, the GP does not change the way it should. The standard GP equation for warm-season grass produces a GP close to 0% when the average air temperature is 50 °F (10 °C). Even though grass will stop growing at that temperature, the erroneous equation produces a GP of more than 20% at that temperature.
For more, see the related posts below, along with these links: