  Habitable planets are often defined as terrestrial worlds capable of
maintaining surface liquid water. As a result, atmospheric water vapor can be a
critical indicator of habitability. Thus, habitability-themed exoplanet
investigations emphasize detection of water vapor signatures for their targets.
A variety of concept missions for exoplanet direct imaging in reflected light
have seen recent study, including the HabEx and LUVOIR concepts. Here, it is
important to understand how direct imaging in reflected light --- coupled with
moderate-resolution spectroscopy --- could be used to detect various water
vapor amounts in Earth-like exoplanetary atmospheres. To investigate water
vapor detection for terrestrial exoplanets, we generated reflectance spectra
over a grid of water vapor column masses and used an instrument model to
explore requisite integration times for spectral feature detection at either
visible or near-infrared wavelengths. Lower-resolution near-infrared
spectroscopy is generally optimal for detecting water vapor in the atmospheres
of Earth-like exoplanets when using direct imaging in reflected light. This
holds true for dry or cold terrestrial planets, whose atmospheres would contain
relatively little water vapor. Atmospheres richer in water vapor, such as
planets undergoing a moist or runaway greenhouse, could have water vapor
efficiently detected at visible wavelengths. Understanding details like an
exoplanet's size, temperature, and location relative to the habitable zone can
aid in determining appropriate wavelength ranges for atmospheric
characterization. Overall, water vapor detection for Earth-like exoplanets is
quite feasible for future direct imaging missions.
