C urrently, increasing the number of PLTMs is one of the important focuses of thegovernment to maximize water resources in Indonesia. Thus, this study focuses ondetermining the process of optimizing the operational performance of PLTM which can bedone by calculating the dependable discharge. In this study, there were several problems,namely the absence of discharge data at the PLTM Tongar location, the lack of observationof river discharge near the PLTM Tongar and incomplete data of rainfall in the Tongarwatershed. To complete the rain data at Sukamenanti station, the Reciprocal method wasused for 2 other stations, and the consistency test for Sukamenanti station was using theRAPS method. To get the parameters of the rain-discharge method, two alternatives werechosen between the Mock method and the NRECA method. The selection of the mostsuitable method for the AWLR discharge used NSE correlation. To get the dependabledischarge, the Flow Duration Curve method is used. Based on the calculation results, thesimulation of the Mock method produces an NSE correlation of 0,55 (qualified), while thesimulation calculation of the NRECA method produces an NSE correlation of -1,01 (doesnot qualified). It was concluded that the most suitable method for AWLR discharge thatcould be used for extended discharge was the Mock method. In addition, the dependabledischarge value obtained from a 90% probability for a generator operating as a base loadobtained from the Flow Duration Curve method is 6.37 m3/second.