Water security is essential for the countries' sustainable development and economic growth. Currently, the uncertainties related to anthropogenic and non-anthropogenic factors in the water security threats bring more complexity to water resources policymakers. In addition, studies emphasise the huge number of financial resources required in traditional hard infrastructure to reduce water security threats. Blending spatially and temporally nature-based solutions (green investment) with hard infrastructure (grey investment) can decrease the total cost of investments. This research proposes an economic optimisation modelling to identify an ideal pathway of green and grey investments in space and time to non-increase water security threats. The results demonstrate that blending green and grey investments in 20 years can reduce almost 20% of the total cost of capital spending when only grey investment is considered. This tool can be useful for policymakers because it pinpoints the area where the investment in grey infrastructure could be postponed with appropriate green investment in place. Finally, the outcomes can support the water resources decision makers to optimise the water-related investments and accelerate the achievements of the SDG 6 targets.