  The depth-dependent abundance of both gas-phase and solid-state water within
dense, quiescent, molecular clouds is important to both the cloud chemistry and
gas cooling. Where water is in the gas phase, it's free to participate in the
network of ion-neutral reactions that lead to a host of oxygen-bearing
molecules, and its many ortho and para energy levels make it an effective
coolant for gas temperatures greater than 20K. Where water is abundant as ice
on grain surfaces, and unavailable to cool the gas, significant amounts of
oxygen are removed from the gas phase, suppressing the gas-phase chemical
reactions that lead to a number of oxygen-bearing species, including O2. Models
of FUV-illuminated clouds predict that the gas-phase water abundance peaks in
the range Av ~3 and 8mag of the cloud surface, depending on the gas density and
FUV field strength. Deeper within such clouds, water is predicted to exist
mainly as ice on grain surfaces. More broadly, these models are used to analyze
a variety of other regions, including outflow cavities associated with young
stellar objects and the surface layers of protoplanetary disks. In this paper,
we report the results of observational tests of FUV-illuminated cloud models
toward the Orion Molecular Ridge and Cepheus B using data obtained from the
Herschel Space Observatory and the Five College Radio Astronomy Observatory.
Toward Orion, 2220 spatial positions were observed along the face-on Orion
Ridge in the H2O 110-101 557GHz and NH3 J,K=1,0-0,0 572GHz lines. Toward
Cepheus B, two strip scans were made in the same lines across the edge-on
ionization front. These new observations demonstrate that gas-phase water
exists primarily within a few magnitudes of dense cloud surfaces, strengthening
the conclusions of an earlier study based on a much smaller data set, and
indirectly supports the prediction that water ice is quite abundant in dense
clouds.
