Slide 20: Derived Variables
Derived variables are calculated from the primary variables by a variety of different functions, and these variables, whilst not as directly accurate, may also provide some insight into the distribution of a species. Growing degree days are the number of days in which the temperature is above a minimum level required for plant growth. Potential evapotranspiration is calculated using one of several formulae calculated by Thornthwaite, Priestly-Taylor, Linacre or several others. From this can be derived the water balance of the soil, which is crudely defined as mean annual rainfall minus the PET. Hence the number of favourable soil moisture days for plant growth can be modelled using one of several models, and finally the Palmer drought stress is calculated using the PDSI programme. Each of these variables represents a further step in abstraction, through which potential errors might be compounded, so it is useful to bear in mind that derived variables are necessarily less accurate than primary environmental variables.