  A very low ortho-to-para ratio (OPR) of 0.1-0.5 was previously reported in
the Orion Bar photon-dominated region (PDR), based on observations of two
optically thin $\mathrm{H_2^{18}O}$ lines which were analyzed by using a
single-slab large velocity gradient model. The corresponding spin temperature
does not coincide with the kinetic temperature of the molecular gas in this
UV-illuminated region. This was interpreted as an indication of water molecules
being formed on cold icy grains which were subsequently released by UV
photodesorption. A more complete set of water observations in the Orion Bar,
including seven $\mathrm{H_2^{16}O}$ lines and one $\mathrm{H_2^{18}O}$ line,
carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR
code to derive gas-phase water abundance and the OPR, taking into account the
steep density and temperature gradients present in the region. The model line
intensities are in good agreement with the observations assuming that water
molecules formed with an OPR corresponding to thermal equilibrium conditions at
the local kinetic temperature of the gas and when solely considering gas-phase
chemistry and water gas-grain exchanges through adsorption and desorption.
Gas-phase water is predicted to arise from a region deep into the cloud,
corresponding to a visual extinction of $A_{\mathrm{V}} \sim 9$, with a
$\mathrm{H_2^{16}O}$ fractional abundance of $\sim 2\times 10^{-7}$ and column
density of $(1.4 \pm 0.8) \times 10^{15}$ cm$^{-2}$ for a total cloud depth of
$A_{\mathrm{V}}=15$. A line-of-sight average ortho-to-para ratio of $2.8 \pm
0.2$ is derived. The observational data are consistent with a nuclear spin
isomer repartition corresponding to the thermal equilibrium at a temperature of
$36 \pm 2$ K, much higher than the spin temperature previously reported for
this region and close to the gas kinetic temperature in the water-emitting gas.
