We have observed 6 clouds along the line-of-sight toward W49A using the Submillimeter Wave Astronomy Satellite ( SWAS ) and several ground-based observatories .
The ortho-H _ { 2 } O 1 _ { 10 } \rightarrow 1 _ { 01 } 
and OH ( 1665 and 1667 MHz ) transitions are observed in absorption , whereas the low-J CO , ^ { 13 } CO , and C ^ { 18 } O lines , as well as the [ C I ] ^ { 3 } P _ { 1 } - ^ { 3 } P _ { 0 } transition , are seen in emission .
The emission lines allow us to determine the gas density ( n \sim 1500 - 3000 cm ^ { -3 } ) and CO column densities ( N ( CO ) \sim 7.9 \times 10 ^ { 15 } -2.8 \times 10 ^ { 17 } cm ^ { -2 } ) using a standard Large Velocity Gradient analysis .

By using both the o-H _ { 2 } ^ { 18 } O and o-H _ { 2 } O absorption lines , we are able to constrain the column-averaged o-H _ { 2 } O abundances in each line-of-sight cloud to within about an order of magnitude .
Assuming the standard N ( H _ { 2 } ) /N ( CO ) ratio of 10 ^ { 4 } , we find N ( o-H _ { 2 } O ) /N ( H _ { 2 } ) = 8.1 \times 10 ^ { -8 } -4 \times 10 ^ { -7 } for three clouds with optically thin water lines .
In three additional clouds , the H _ { 2 } O lines are saturated so we have used observations of the H _ { 2 } ^ { 18 } O ground-state transition to find upper limits to the water abundance of 8.2 \times 10 ^ { -8 } -1.5 \times 10 ^ { -6 } .
We measure the OH abundance from the average of the 1665 and 1667 MHz observations and find N ( OH ) /N ( H _ { 2 } ) = 2.3 \times 10 ^ { -7 } -1.1 \times 10 ^ { -6 } .
The o-H _ { 2 } O and OH abundances are similar to those determined for line-of-sight water absorption features towards W51 and Sgr B2 but are higher than those seen from water emission lines in molecular clouds .
However , the clouds towards W49 have lower ratios of OH relative to H _ { 2 } O column densities than are predicted by simple models which assume that dissociative recombination is the primary formation pathway for OH and H _ { 2 } O .
Building on the work of Neufeld et al .
( 2002 ) , we present photo-chemistry models including additional chemical effects , which can also explain the observed OH and H _ { 2 } O column densities as well as the observed H _ { 2 } O/CO abundance ratios .