A watershed in which the observed discharge data is not available or incomplete becomes a problem in thewater utilization projects. Observation discharge data obtained from Pos Duga Air Sungai Temef is less reliable because the measurement of water level data for the last few years has been carried out by direct observation without using a discharge measuring device. Obtain river discharge data, it can be done by simulating rainfall data into discharge data, one of them is NRECA Model. In NRECA Model, there are several parameters, namely index soil moisture capacity (nominal), Percent Sub Surface (PSUB), Ground Water Flow (GWF), and crop coefficient (kc). Determination of the parameters NRECA Model was obtained by trial and error based on the value limit so that the coefficient correlation (r) between observation discharge and simulation discharge was close to 1. The values used for the index soil moisture capacity (nominal), percent sub surface (PSUB), ground water flow (GWF), and crop coefficient (kc) are 0,20; 0,85; 0,50; and 0,60.

Temef Dam is located in Oenino Village Oenino District and Konbaki Village Polen District, Timor Tengah Selatan Regency which has a capacity of 45.79 millions m3. This research aimed to determine the amount of potential evapotranspiration using Penman Modification Method, the amount of dependable discharge with three methods that are NRECA, F.J. Mock, and Tank Methods, from those three methods will be selected a method that have closest characteristic of Temef Watershed, and filling timein Temef Reservoir. The maximum half monthly potential evapotranspiration in Temef Watershed using Penman Modification Method is 142.72 mm and the minimum is 71,25 mm. The average of annual discharge using NRECA Method is 249.89 m3/sec, F.J. Mock Method is 258.66 m3/sec, and Tank Method is 92.15 m3/sec. Based from three methods that have been used, Tank Method is the closest method to the characteristics of Temef Watershed, which RMSE value from Tank Method is 2.99. Filling time of Temef Reservoir to reach the planned reservoir volume in normal water year condition is three months and fifteen days and in low water year condition is six months.