Water and energy are the two most essential resources for human civilisation, from day-to-day activities to overall development. These resources are interdependent on each other for their production, which constitutes a nexus between them and is called the water-energy nexus. With increasing development activities, the demand for water and energy is also growing worldwide. Sustainable water and energy resource use has become inevitable for regional energy system planning. This work proposes a graphical method for optimising the water-energy nexus system of any geographical region. The proposed method determines the minimum water and energy generation needed for the demand satisfaction of any geographical region. A mathematical optimisation formulation to verify the graphically obtained results is also presented in this work. The proposed approach can be used as a planning tool for the water-energy nexus system. A case study on Spain's water-energy nexus system is demonstrated using the developed method and determined that only 35,414 × 106 m3/y of water and 273,979.9 GWh/y of energy is needed for demand satisfaction. 45.65 % of nuclear power (Open Loop) and 19.47 % of groundwater can be conserved from Spain's total available energy and water sources.