One of the most essential inputs of water balance models is the part in which potential evapotranspiration (PET) is predicted. Especially in the conceptual-based lumped rainfall–runoff models, the steady runoff simulations can be made with acceptable PET predictions. The presented study is about exploring alternative PET equations that can be adapted to a parametric lumped model termed as the dynamic water balance model (dynwbm). Although the use of the Penman–Monteith equation often appears in the literature, a performance assessment was conducted on the dynwbm by using 21 PET equations. The implementation was performed on different river branches in the Gediz Basin, Turkey. The satisfactory PET equations have been selected by means of statistical techniques. As a result of the evaluation, it was observed that one of the radiation-based equations, McGuinness–Bordne, provided the most consistent performance. Alternatively, the presence of parsimonious equations requiring less meteorological variables has been questioned, thus locally calibrated temperature-based PET equations reflecting the PET estimations of McGuinness–Bordne have been proposed so as to be practically utilized in water balance modeling experiments for the basin.