Relevant model results for the normalized H₂O SLED (a1)−f1)), and for the L_H2O/L_IR ratios (for ΔV = 100 kms^-1) as a function of T_dust and L_IR (assuming a source of R_eff = 100 pc, a2)−f2)). In panels a1)–f1), model results for lines 1 to 8 (Table 1) are shown from left to right. Values for N_H2O/ΔV and τ₁₀₀ are indicated at the top of the figure. The different colors in panels a1)−f1) indicate different T_dust , as labeled in b1), while they indicate different lines in panels a2)−f2) (labeled in a2), see Table 1). Models with collisional excitation ignored (a)−c)), and with collisions included for n_H2 = 3 × 10⁵ cm^-3 and T_gas = 150 K (d)−f)) are shown. The gray lines/symbols in panels d1)−f1) show model results that ignore radiative pumping (i.e., only collisional excitation). Collisional excitation has the overall effect of enhancing the low-lying lines (1 and 2) relative to the others and of increasing the L_H2O/L_IR ratios of all lines (see text). The dashed lines in panels a2)−f2) indicate the average L_H2O/L_IR ratios reported by Y13. When compared with observations, the modeled L_IR values should be considered a fraction of the observed IR luminosities, because single temperature dust models are unable to reproduce the observed SEDs (Sect. 4.3.1); the H₂O submm emission traces warm regions of luminous IR galaxies (see text).