  We describe an analytic model for an evolving protoplanetary disk driven by
viscosity and a disk wind. The disk is heated by stellar irradiation and energy
generated by viscosity. The evolution is controlled by 3 parameters: (i) the
inflow velocity towards the central star at a reference distance and
temperature, (ii) the fraction of this inflow caused by the disk wind, and
(iii) the mass loss rate via the wind relative to the inward flux in the disk.
The model gives the disk midplane temperature and surface density as a function
of time and distance from the star. It is intended to provide an efficient way
to calculate conditions in a protoplanetary disk for use in simulations of
planet formation. In the model, disks dominated by viscosity spread radially
while losing mass onto the star. Radial spreading is the main factor reducing
the surface density in the inner disk. The disk mass remains substantial at
late times. Temperatures in the inner region are high at early times due to
strong viscous heating. Disks dominated by a wind undergo much less radial
spreading and weaker viscous heating. These disks have a much lower mass at
late times than purely viscous disks. When mass loss via a wind is significant,
the surface density gradient in the inner disk becomes shallower, and the slope
can become positive in extreme cases.
