This paper describes a novel approach to optimize injection and production well rates in a waterflood using streamline-based (SL) flow simulation. The method provides engineers with an approach that is well beyond the traditional field management workflows offered by standard surveillance and finite difference (FD) simulation. The method is automated and therefore applicable to very large fields with many wells. Streamline based flow simulation is unique in that it allows to quantify the amount of injected and produced fluids between well pairs via well allocation factors (WAF's). WAF's allows calculating the efficiency of injection wells as the ratio of injected water to the oil produced at offset wells. With injection efficiencies known across the field for each injector, water can be reallocated from low-efficiency to high efficiency wells, thereby optimizing production for each barrel of water injected.