We report an investigation of the temperature-dependent far-infrared spectrum of liquid water. We have employed a new experimental technique based on ultrashort electromagnetic pulses (THz pulses). This technique allows for fast and reliable data of both index of refraction and absorption coefficient for highly absorbing liquids. The temperature dependence reveals an enthalpy of activation corresponding to 2.5 kcal/mol in agreement with recent Raman experiments, but lower than the enthalpy observed in dielectric relaxation experiments. This demonstrates that part of the orientational relaxation in liquid water takes place without breaking of hydrogen bonds with bonding energy of 5 kcal/mol, as suggested in recent theoretical model.