GDAY (Generic Decomposition And Yield) is a simple ecosystem model that simulates carbon, nitrogen, and water dynamics at the stand scale (Comins and McMurtrie, 1993; Medlyn et al. 2000; Corbeels et al. 2005a,b). The model can be run at either a daily time step, or a 30-minute time step. When the model is run at the sub-daily timescale, photosynthesis is calculated using a two-leaf (sunlit/shade) approximation (de Pury and Farquhar, 1997; Wang and Leuning, 1998), otherwise photosynthesis is calculated following Sands (1995;1996). The sub-daily approach (photosynthesis & leaf energy balance) mirrors MAESPA, without the complexity of the radiation treatment. In the standard model the water balance is represented simply, with two (fixed) soil water "buckets", which represent a top soil (e.g. 5 cm) and a larger root-zone. If you are using the sub-daily version, there is now the option to use a SPA-style representation of hydraulics. GDAY-SPA resolves multiple soil layers, soil and leaf water potential and limits gas exchange following the Emax approach (see MAESPA for more details).