Context : Existing SWAS observations and future HIFI/Herschel data require a clear sense of the information content of water emission and absorption lines .

Aims : Investigate wether the ground-state transition of ortho- \mathrm { H _ { 2 } O } ( \mathrm { 1 _ { 10 } } \rightarrow \mathrm { 1 _ { 01 } } ) at 557GHz can be used to measure the column density throughout an interstellar cloud .

Methods : We make use of a multi-zone escape probability code suitable for the treatment of molecular line emission .

Results : For low abundances , i.e. , X ( \mathrm { H _ { 2 } O } ) \lesssim 10 ^ { -9 } , the intensity of the \mathrm { 1 _ { 10 } } \rightarrow \mathrm { 1 _ { 01 } } transition scales with the total column of \mathrm { H _ { 2 } } .
However , this relationship breaks down with increasing abundance , i.e. , optical depth , due to line trapping and , – for \mathrm { T _ { dust } } \gtrsim 25K , X ( \mathrm { H _ { 2 } O } ) \lesssim 10 ^ { -8 } and n \sim 10 ^ { 4 } \mathrm { cm ^ { -3 } } , – absorption of the dust continuum .

Conclusions : An observed decline in intensity per column , expected if \mathrm { H _ { 2 } O } is a surface tracer , does not necessarily mean that the water is absent in the gas phase at large columns , but can be caused by line trapping and subsequent collisional de-excitation .
To determine the amount of water vapour in the interstellar medium , multiple line measurements of optically thin transitions are needed to disentangle radiative transfer and local excitation effects .