In this study, the position of the truncation boundary, which is an important issue when modeling the RESERVOIR in the finite element formulation and determining the hydrodynamic pressure on the dam, is investigated. Water in the RESERVOIR is assumed to be compressible, and the Sommerfeld boundary is used to model the far field. The Euler-Lagrange method is used to analyze the RESERVOIR-dam interaction. The serendipity element is used to model the RESERVOIR and dam. The foundation is considered to be rigid. Most researchers assume that the truncation boundary is located three times the total height of the dam away from the dam body, and they use the Sommerfeld boundary at this location. For this purpose, the RESERVOIR was disconnected at different intervals. The Pine Flat Dam is commonly analyzed with a Taft earthquake. The results of the analyses for different positions of the truncation boundary in RESERVOIR models show that the position of the Sommerfeld boundary at five times the height of the dam away fromthe dam body is a proper place for the truncation boundary. Moreover, comparing the results of the present study based on the compressibility of water and those of previous research based on the incompressibility assumption indicates that the maximum hydrodynamic pressure is approximately 153.8% for a Taft earthquake. Therefore, the assumption of water compressibility plays an important role when evaluating the Pine Flat Dam.