It was designed for use in larger-scale ecosystem, climate and hydrological models concerned with fluxes of CO2, water, and heat. Given a set of environmental and biological parameters, PGEN calculates instantaneous rates of net photosynthesis and transpiration, and associated conductances to CO2 and water. It is intended to predict species-specific behaviour with minimal need for empirical parameterisation. The biochemical model of photosynthesis is based on the Farquhar approach. This biochemical model is embedded in a model of the leafs energy balance, which is based on the work of Monteith and Jones.
Stomatal conductance is calculated using an optimisation concept.
In this concept there is an assumed tradeoff between CO2 entering and water leaving the leaf, resulting in a single stomatal conductance for each set of environmental conditions, that maximises a function including the costs and benefits. The source code is available from A. Friend upon request. References Friend, A.D. 1995. Kramer, K. 1995. Modelling comparison to evaluate the importance of phenology for the effects of climate change on growth of temperate-zone deciduous trees.