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The evolution of low- and intermediate-mass stars on the asymptotic giantbranch (AGB) is mainly controlled by the rate at which these stars lose mass ina stellar wind. Understanding the driving mechanism and strength of the stellarwinds of AGB stars and the processes enriching their surfaces with products ofnucleosynthesis are paramount to constraining AGB evolution and predicting thechemical evolution of galaxies. In a previous paper we have constrained thestructure of the outflowing envelope of W Hya using spectral lines of the$^{12}$CO molecule. Here we broaden this study by modelling an extensive set ofH$_{2}$O and $^{28}$SiO lines observed by the three instruments on boardHerschel using a state-of-the-art molecular excitation and radiative transfercode. The oxygen isotopic ratios and the $^{28}$SiO abundance profile can beconnected to the initial stellar mass and to crucial aspects of dust formationat the base of the stellar wind, respectively. The modelling of H$_{2}$O and$^{28}$SiO confirms the properties of the envelope model of W Hya derived from$^{12}$CO lines. We find an H$_2$O ortho-to-para ratio of2.5\,$^{+2.5}_{-1.0}$, consistent with what is expected for an AGB wind. TheO$^{16}$/O$^{17}$ ratio indicates that W Hya has an initial mass of about 1.5M$_\odot$. Although the ortho- and para-H$_{2}$O lines observed by HIFI appearto trace gas of slightly different physical properties, a turbulence velocityof $0.7\pm0.1$ km s$^{-1}$ fits the HIFI lines of both spin isomers and thoseof $^{28}$SiO well. The ortho- and para-H$_2^{16}$O and $^{28}$SiO abundancesrelative to H$_{2}$ are $(6^{+3}_{-2}) \times 10^{-4}$, $(3^{+2}_{-1}) \times10^{-4}$, and $(3.3\pm 0.8)\times 10^{-5}$, respectively. Assuming a solarsilicon-to-carbon ratio, the $^{28}$SiO line emission model is consistent withabout one-third of the silicon atoms being locked up in dust particles.

The wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya