For most of low-head pump stations, pumps simultaneously undertake important tasks such as flood control, irrigation and drainage where more attention should be paid on involved hydraulic stability. In order to establish a suitable numerical prediction model, two kinds of water surface treatment namely volume of fluids (VOF) and rigid-lid hypothesis (RLH) methods, for upstream and downstream reservoirs, are presented and corresponding results are compared. System transient characteristics during power-off process are predicted, where the obtained maximum runaway speed by VOF method is found to be closer to the one from experimental results. The dynamic parameters of VOF method change slower than those of RLH method and the static pressure peak value is smaller. The usage of VOF method to water dynamics at the reservoir's free surface makes the water flow in reservoirs relatively smooth, whereas large-scale vortices appear in reservoirs for the RLH method. Moreover, the turbulent kinetic energy is found to be larger if the reservoir's free surface is not taken into consideration. This article established a novel and accurate prediction model, which otherwise would provide the foundation of further studies in terms of transient characteristics prediction within pumping stations taking into account the air-water interactions. (C) 2020 Elsevier Ltd. All rights reserved.