An empirical model for describing daily courses of net photosynthesis in Hammada scoparia is being developed. The model is based on the functional relationships, by which various environmental factors affect the photosynthetic activity and which can be measured by experiment in the field. In a sequence of steady-states daily courses of net photosynthesis are predicted during a growing season considering the variability of the physiological states and the capacity for regulative adaptations. The rate of net photosynthesis at a certain date is calculated from the maximal rate of CO2 uptake being expected at that season and from the effects of light, temperature, and air humidity which are scaled from 0 to 1. All factors are connected multiplicatively. The light function accounts for the seasonal changes in the light curve, the temperature function is based on the seasonal shift of the temperature optimum, and the humidity function considers the increasing sensitivity of the stomatal humidity response at increasing water stress. The model is built to be a submodel of a general ecosystem model, where various other submodels (i.e. water stress model, phenology model) are supplied. The present model is tested by predicting daily courses at extreme climatic conditions during the year and by comparing the predicted values of gas exchange with values being measured in an independent experimental procedure. The result shows that the model is able to simulate the natural behaviour of Hammada scoparia during the growing and dry season of a desert habitat. The problems of incorporating the influence of water stress, the interaction of the various factors, and the phenological aspect of the photosynthetic activity is being discussed.