Yield variability is one of the major problems in growing faba beans. In this thesis, the effect of water supply pattern on yield variability of the crop is studied with experiments in the field and under controlled conditions, and with a simulation model. In a series of field experiments, water shortage during flowering, followed by plenty of water after flowering resulted in 30-200% higher pod retention at early formed nodes, a 7% higher Harvest Index, equal average seed yields (6 t ha -1, 100% d.m.), but a 57% larger seed yield range, compared with plenty of water during and after flowering. In some experiments, mild water shortage during flowering resulted in final seed yields which were significantly higher (0.3-0.6 t ha -1) than with plenty of water both during and after flowering. Water shortage after flowering resulted in yield limitations of more than 3 t ha -1and a 200% larger seed yield range. Crop physiological measurements showed that faba beans have insufficient osmotic adjustment and/or adaptation of cell wall elasticity. Thus, the turgor of young stems and leaves, the expansive growth and the vegetative sink strength decrease already with mild water shortage. The stomatal conductance and photosynthesis decrease only at more severe water shortage. It is argued how this may explain the positive effect of mild water shortage on the dry matter partitioning to reproductive organs. The effects are compared with those found in cotton. In a simulation model the physiological knowledge is implemented and calibrated. The model outcomes correspond with the measured average seed yields and account for up to 80% of the measured yield variation of data sets of several locations in Western Europe. Applying plenty of water after flowering increases the average seed yields with 17%-42% and reduces the standard deviation of the seed yields, a measure for the variability, with 43%-73%. Plenty of water during and after flowering has almost no additional effects. It is shown that the positive effect of mild water shortage during flowering on seed yield has only limited value as a target for crop management. But, plenty of water after flowering is crucial for high and stable seed yields. Model explorations show that a doubling of the rooted depth reduces the seed yield variability with about 30%, but a doubling of the water extraction capacity of the crop does not reduce the yield variability at all. It is concluded that variations in water availability after flowering (i.e. during the grain filling period) are a major factor in yield variability of faba beans in Western Europe. However, when water shortage is eliminated as the limiting factor, yield reducing factors, especially diseases, may be more important than was expected before. It is shown how feasibility studies with the model can support management and breeding research by evaluating 'ideotypes' and conditions for optimum productivity in present and future climate. As an example, some effects of climate change on average yield and yield variability of rain-fed and irrigated crops are assessed.