  The habitable fraction of a planet's surface is important for the
detectability of surface biosignatures. The extent and distribution of
habitable areas is influenced by external parameters that control the planet's
climate, atmospheric circulation, and hydrological cycle. We explore these
issues using the ROCKE-3D General Circulation Model, focusing on terrestrial
water fluxes and thus the potential for the existence of complex life on land.
Habitability is examined as a function of insolation and planet rotation for an
Earth-like world with zero obliquity and eccentricity orbiting the Sun. We
assess fractional habitability using an aridity index that measures the net
supply of water to the land. Earth-like planets become ``superhabitable'' (a
larger habitable surface area than Earth) as insolation and day-length increase
because their climates become more equable, reminiscent of past warm periods on
Earth when complex life was abundant and widespread. The most slowly rotating,
most highly irradiated planets, though, occupy a hydrological regime unlike any
on Earth, with extremely warm, humid conditions at high latitudes but little
rain and subsurface water storage. Clouds increasingly obscure the surface as
insolation increases, but visibility improves for modest increases in rotation
period. Thus, moderately slowly rotating rocky planets with insolation near or
somewhat greater than modern Earth's appear to be promising targets for surface
characterization by a future direct imaging mission.
