In this paper, with the aim of considering the fluid flow behavior pattern at the scale of the rock mass, the results of a number of experiments in the Chamshir Dam and Power Plant project have been studied and analyzed. The fluid flow pattern in the rock mass or the relationship between the fluid flow rate and its pressure is usually considered as a linear relationship, while in laboratory fluid flow experiments, this pattern has been divided into three groups: linear Darcy behavior, nonlinear behavior under the effect of high inertia, and nonlinear behavior under the effect of fracture expansion. In this research, by nonlinear fitting of the results of the water pressure experiment, the nonlinear behavior pattern of the fluid flow in the rock mass has been investigated in the form of the two Forchmeier and Izbash relations. This analysis shows that these two relations describe the nonlinear behavior of the fluid flow in the rock mass well. Also, the Forchmeier relation has a better efficiency for describing the fluid flow behavior. Using the existing relations regarding the critical Reynolds number in laboratory tests, a method has been presented to estimate the critical Reynolds number in field tests. This quantification helps to better understand fluid behavior in field experiments such as water pressure tests to estimate the critical Reynolds number and determine the deviation from linear to nonlinear flow.