Left: gas thermal pressure nT vs. I(557) / I(1670) as determined by a series of LVG models. Each panel summarizes the result from more than 1000 LVG models of different density and temperature (see text), together with a constant value of N(H₂O) / ΔV indicated in the upper right corner (units are cm^-2 km^-1 s). The blue-shaded region marks the interval of ratios between 2 and 4 that contains 60% of the outflow sample. Right: thermal pressure nT vs. N(H₂O) / ΔV for the outflows in our sample. Each point represents an outflow (open squares correspond to L1157-B1 and L1148-R4), and the mean values and error bars have been determined from the LVG results shown in Fig. 12. The dashed lines labeled with R (= I(557) / I(1670)) have been derived from the models in the left panels. Again, the region between R values of 2 and 4 is shaded in blue.