A process-based model for semi-arid grassland ecosystems was developed. It is driven by standard daily meteorological data and simulates with a daily time step the seasonal course of root, aboveground green, and dead biomass. Water infiltration and redistribution in the soil, transpiration and evaporation are simulated in a coupled water budget submodel. The main plant processes are photosynthesis, allocation of assimilates between aboveground and belowground compartments, shoots and roots respiration and senescence, and litter fall. Structural parameters of the canopy such as fractional cover and LAI are also simulated. This model was validated in southwest Arizona on a semi-arid grassland site. In spite of simplifications inherent to the process-based modelling approach, this model is useful for elucidating interactions between the shortgrass ecosystem and environmental variables, for interpreting H2O exchange measurements, and for predicting the temporal variation of above- and belowground biomass and the ecosystem carbon budget.