Top panels: ratio of the integrated intensities of the 2₁₁–2₀₂ (752 GHz) and 2₀₂–1₁₁ (988 GHz) water lines versus the bolometric luminosity of the source. Middle panels: ratio of the integrated intensities of the H₂O 3₁₂–3₀₃ (1097 GHz) and 2₀₂–1₁₁ (988 GHz) spectra versus L_bol. Bottom panels: ratio of the integrated intensities of the 3₁₂–3₀₃ (1097 GHz) and 2₁₁–2₀₂ (752 GHz) water emission lines as a function of L_bol. The left-column presents the ratios calculated considering the entire line profile and the right-column shows the ratio of the integrated intensity calculated for the broad velocity component. The value of the ratios with and without correcting by the different beam size factors are indicated by dashed lighter lines and darker dot symbols respectively. Blue lines and symbols correspond to the low-mass Class 0 protostars, black to the low-mass Class I, green to intermediate-mass objects and red lines and symbols to high-mass YSOs. The purple dashed-dotted horizontal lines and the orange dashed horizontal lines indicate the optically thin and optically thick limits, respectively, calculated assuming LTE and an excitation temperature of 300 K. The linear correlation between the dot symbols (ratios not beam corrected) and the logarithm of the luminosity is shown by the dotted black lines in the top panels.