  To create self-controlled radiation photonics systems, it is necessary to
have complete information about the nonlinear properties of the materials used.
In this paper, the vibrational mechanism of the giant low-inertia cubic
nonlinearity of water in the terahertz frequency range is experimentally
proven. Its dominance which manifests itself when the temperature of the liquid
changes is demonstrated. The measured nonlinear refractive index in the THz
frequency range for water jet at temperatures from 14$^{\circ}$C to
21$^{\circ}$C demonstrates a correlation with the theoretical approach, varies
from 4 to 10$\cdot$10$^{-10}$ cm$^2$/W and is characterized by a inertial time
constant of less than 1 ps.
