From my Data Requirements for Reservoir Site Suitability post, I mentioned the following data layers as possible contributors:

Hydrology
Soils
Vegetation
Elevation
Roads
Parcels
Structures
Airports

Additional data layers that could be used are:

Rain Levels
Subdivisions
Zoning

The first big test is availability.  If I can’t get the data layer then it doesn’t do me any good to add it to my study.  Layers that I can [...]