Inaccuracy in earth fill dam’s analysis is one of the most important reasons of breaking this huge structures. assuming soil saturated only and disregarding time in rapid drawdown analysis cause errors in results of analyses that this article discuses about them. In this article Gulak earth fill dam has been modeled in three situations named saturated only, saturated-unsaturated with linear and nonlinear functions and the results show that modeling dam in saturated only situation not only effects on seepage results, but also causes    about 25 percent error on safety factor of dam. In the fallowing this result achieved that by using nonlinear functions for permeability  and vol. water content and choosing required parameters for van genuchten equation from laboratory result accuracy of analyses increases about 13 present than using a linear function. Also In rapid drawdown analysis to avoid shocking to the model, by using a transient analyze, drawdown of the reservoir was modeled as a process depending on time and results show  that disregarding the time in rapid drawdown of reservoir analysis cusses about 4.8 percent error that this error increases to 13 percent by reducing drawdown rate.