We address the problem of minimizing the energy cost generated by pumping operations for supplying an elevated water tanks from a low source reservoir of water distribution systems (WDS). Pumping operations can be activated by using simple ON-OFF trigger levels without advanced control system. A static optimal solution of simple trigger levels may not be robust when being applied to real world situation. In this paper, we propose an integer programming model using the idea of Additional Time Slots of [Quintiliani et. al., Water Resour Manage, 2019] to make the optimal solution more robust. In addition, computational results on real-world data with single and multiple pumps will be presented.