Studies by Garcia (2003) and Geerts et al. (2006) on the effects of water stress in different phenological stages of quinoa under well controlled conditions were used to set up field experiments in the Bolivian Altiplano. During the growing seasons 2005-2006 and 2006-2007, the water productivity of quinoa under rainfed cultivation, deficit irrigation and full irrigation was assessed. The experiments demonstrate that the building up of the harvest index after flowering followed a model with an initial lag phase, a linear phase and a final plateau. Unless water stress occurred during the early grain filling stage, all treatments resulted in the same Harvest Index. Canopy growth followed a logistic curve which was temporary inhibited during periods of water stress. Biomass accumulation in comparison to accumulated transpiration, referenced by the reference evapotranspiration, showed a conservative linear behavior with an average slope of 12 g/m². Results of both the controlled and field experiments were used to calibrate and validate the robust FAO-AquaCrop model for quinoa. Comparisons between simulated and observed parameters of the water balance, biomass production and final yield were used for the calibration. The water stress factor for leaf expansion was calibrated, and a type of dormancy before suffering senescence stress was added. Quinoa proved to be a highly indeterminate crop regarding the relation between canopy growth and timing of flowering. Adequate calibration (R² = 0.8 and Efficiency = 0.6) and validation (R² = 0.6 and Efficiency = 0.4) were obtained. Future items that need to be addressed to improve the performance of FAO-AquaCrop for quinoa are the adjustments of the length of the growth cycle as a result of water stress and the possible adjustments of water stress factors due to hardening caused by droughts in the vegetative stages. The calibration and validation of the FAO-AquaCrop model will permit to carry out scenario analyses in the future such as the effect of deficit irrigation under various environmental conditions, and of the El Niño Southern Oscillation on quinoa production in the Bolivian Altiplano.