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This study presents a stochastic security-constrained unit commitment (SCUC) model for the optimisation of coordinated midterm water and natural gas supplies. The stochastic model considers random outages of system components, load forecast errors and water inflow uncertainty, which are modelled as scenarios in the Monte Carlo simulation. Water resources may be used in winter to cover gas unit outages caused by an insufficient gas supply. However, those hydro units then may not be available for peak load shaving in the following summer if the summer happens to be a dry season. Thus, water reservoirs would have to be utilised efficiently throughout the year to provide substantial cost reductions while maintaining the power system reliability. The proposed model also considers the impact of midterm security-constrained scheduling of water and gas on the power system reliability. Accordingly, hourly SCUC is incorporated in the reliability calculation. The proposed stochastic problem is formulated as a two-stage optimisation in which the first stage optimises the water and gas usages in the first month and the second stage considers the schedule via multiple scenarios in the following 11 months. Numerical simulations indicate the effectiveness of the proposed stochastic approach for the optimal scheduling of midterm water and gas usages.