  The water molecule occurs in two nuclear-spin isomers that differ by the
value of the total nuclear spin of the hydrogen atoms, i.e., $I=0$ for
para-H$_2$O and $I=1$ for ortho-H$_2$O. Spectroscopic transitions between
rovibrational states of ortho and para water are extremely weak due to the tiny
hyperfine nuclear-spin-rotation interaction of only $\sim30$ kHz and so far
were not observed. We report the first comprehensive theoretical investigation
of the hyperfine effects and ortho-para transitions in H$_2$$^{16}$O due to
nuclear-spin-rotation and spin-spin interactions. We also present the details
of our newly developed general variational approach to the simulation of
hyperfine effects in polyatomic molecules. Our results for water suggest that
the strongest ortho-para transitions with room-temperature intensities on the
order of $10^{-31}$ cm/molecule are about an order of magnitude larger than
previously predicted values and should be detectable in the mid-infrared
$\nu_2$ and near-infrared $2\nu_1+\nu_2$ and $\nu_1+\nu_2+\nu_3$ bands by
current spectroscopy experiments.
