Since regional water supply systems (RWSS) in Korea are all separated and are tree-shaped networks, the reliability is low and operation is not efficient in an emergency. So connecting RWSS to each other can be a good option to improve the operation efficiency. This study has been initiated to evaluate efficiency looking at a pilot case. Expected shortage has been selected as a surrogate. In this study, we developed pipe failure probability equations which are suitable in Korea by considering material, diameter, year, and length. As a pilot case, the three systems of Jeon-ju (JJ), Bu-an (BA), and Seom-jin (SJ) are selected and we consider two connections: JJ–BA and JJ–SJ. In each connection we develop three alternative plans, and we conclude that connections of JJ–BA and JJ–SJ can improve the operational efficiency as much as 5% and 4%, respectively.