A model predicting biomass production and water use in tea plantations is described. Biomass production is calculated from the amount of light intercepted and a radiation conversion efficiency. Available assimilate (current production plus reserves) is distributed according to the ‘potential’ demand of each of the shoots, stems and roots. Any excess is stored in the stem and root reserve pool. In the case of the demand for assimilate exceeding the supply, allocation is according to the relative demands of each component. Simulations with the model suggest that assimilate supply is unlikely to limit shoot growth under most conditions. Soil water movement and crop water uptake are simulated using the water balance sub-model used by the CERES crop growth models. The model calculates a water stress factor (Φ) as the ratio of actual to potential crop transpiration, and uses this factor to adjust various crop processes such as dry matter production and shoot development and extension rates. Predicted yields match observed yields well across a range of watering treatments and for different years at sites in Tanzania and Zimbabwe, providing some degree of confidence in the model's ability to predict tea yields for environments in East Africa.