The dominant relaxation mechanism for pure water magnetization at low and intermediate magnetic fields is the dipolar relaxation mechanism.mechanism described in the earlier note by Daniil.
However, in Biomolecular NMR studies of water solutions, (which are usually carried out by using high magnetic fields and high-Q probes), other relaxation mechanisms may dominate, e.g., radiation damping. See the earlier discussion on radiation damping in this forum.
For some relevant data (but no equation) see the following paper: Journal of magnetic resonance. Series B 112 (2), 200. Note: the spectra described in this publication were for a solution of water containing a protein (not for pure water).
It is advisable to identify all the significant relaxation mechanisms before fitting the data to available equations.
