Maintenance planning for hydropower plants is a crucial problem. In this paper, we evaluate the impact of the Hydropower Production Function (HPF) formulation on the maintenance scheduling. Based on an existing model for Generator Maintenance Scheduling that uses a convex hull approximation for representing the HPF, we developed two additional approximations, one that is piecewise linear approximation and another that uses a polynomial function. Then, we compare these three approximations: first a convex hull approximation, then a piecewise linear approximation and third, a nonlinear approach using a polynomial function fitted on real data. We experiment with two test cases based on real-world hydroelectric systems. The results show that for a one-month planning horizon, depending on the approximation used, maintenance tasks can be shifted by up to 5 days, and the difference in energy production can reach 8,300 MWh.