Agroforestry systems are an alternative to established agricultural production systems and may help mitigating environmental pollution, biodiversity loss, and climate change. Since there are only a few simpler agroforestry models, the modular model system Expert-N has been extended to include an agroforestry sub-model. This model was applied to a South African citrus agroforestry system focusing on the effect of windbreaks on the crop water use. Additionally, a control simulation without the agroforestry sub-model was carried out. Depending on the distance from the windbreak, the modelled wind speeds were reduced between 11.6% and 38.8% in comparison with the control simulation, while the incoming radiation was reduced between 6% and 45.4% due to the shading caused by the tree row. The application of the agroforestry sub-model resulted in lower crop yields and higher wood growth, while the water consumption was also reduced by about 20% in a simulation with constant irrigation throughout the growing period. However, these effects strongly depend on the distance to the windbreak. A simulation without irrigation provides the total plant water demand at different positions in the field. The results show that up to 40% of crop irrigation water may be saved due to the adjacent tree strip. The study confirms the benefits of agroforestry systems and thus supports their establishment. Additionally, Expert-N may also be applied for temporal and spatial irrigation planning, which is highly relevant in agroforestry systems.