One niggling point is that the Romans used gravity to transport their water. We would have to use pumps and pipe. Another is that they were moving water tens of miles, we're talking thousands of miles.
That said, if we made water nearly as important in the grand scheme of things as the Romans did - if we designated a similar portion of our GNP to water as they did, I bet you could water your roses all day long and nobody would care. Let's see...
$1 trillion (a significant fraction of what we spent in Iraq and Afghanistan) would buy a lot of water pumps and pipe.
The Alyeska pipeline,
http://www.alyeska-pipe.com/pi... is 48 inches in diameter and 800 miles long. It cost $8 billion to build. It would probably about $32 billion in today's dollars, but some of the engineering and construction techniques required to build oil pipeline across tundra might not apply to water pipe. Maybe we'd save 30%. So maybe you could get four pipelines per $100 billion, or forty per $trillion. The Alyeska pipeline has delivered 15 billion barrels of oil in 30 years - thats 500,000 bbl/year of crude which is much harder to transport than water.
Figure you could push five times as much water through or 2.5 million bbl water/year = 130 million gallons of water per year for each pipeline. Forty pipelines would deliver something like seven billion gallons of water per year. That's equal to about 22,000 acre-feet. A square mile is about 640 acres. And the average rainfall in CO flatlands is about 12 inches. So you could transport in a year about as much water as you would catch in thirty square miles in a year (assuming no evaporative losses, etc.) Of course, if you used much larger diameter pipe constructed of cheaper materials, the cost might be materially less.
Looked at another way, it has cost about $.50 per bbl or $.10 per gallon to transport crude oil 800 miles across Alaska (excluding operating costs). Transporting water using a pipeline would probably be about a fifth the price or about $.02 per gallon. That is a great deal for high-value things like drinking or bathing. But it's still expensive for watering crops and lawns.
I've been wanting to do that calculation for some time.
