It's hard to tell ... ask some city water departments for estimates.

What do you mean garden to landscape?

Do you mean where are there documents about changing from mesic or oasis style landscaping to Xeric type of landscaping?

If you are talking about someone changing a garden to a landscape, then I don't believe you are going to find any statistics on that.

What exactly are you looking for?

City of Albuquerque gives rebates to folks who switch from grass and/or higher water use yards to xeriscape, so they could give you those stats but it would not include those who don't take advantage of the program, so it would be way under reported. Las Vegas, NV has the strictist home water restrictions in western US (even though the casinos get a pass). Those 2 cities might be a good place to start, depending on what you are looking for.