Systems and methods for generating inerting gas on vehicles are described. The systems include a proton exchange membrane (PEM) inerting system, a pure water replenishment system configured to provide pure water to the PEM inerting system, wherein the pure water replenishment system is in fluid communication with the PEM inerting system to replenish water lost during operation of the PEM inerting system, and a control system configured to control operation of the pure water replenishment system to automatically replenish pure water to the PEM inerting system.