Harriet, it is not just about water conservation it is also about your own particular garden. The garden I grew up with was very productive although under it was pure sand, the top with a couple of hundred years of cultivation being manured yearly had around three four feet of good topsoil although water went through and it became quite dry in hot summers. We had very large water butts at all down comers from the house stables even the pig pens, it was needed as we only had one water tap in the house. My Father taught me to water only the plants and not the whole garden in general as people do with hoses now. A drop of ammonia in the butts for watering plants kept them clean but animal drinking water was kept tightly lidded and sealed, it did go a bit green although any one having drunk water from a ships fresh tanks will know it is drinkable.
Where I live now it is an under strata of brick clay and was dug out for years for brick making. This means the water strata is quite high and water will lay there the local gardens tend to flood. I am on the top of a hill so it is all downhill plus on moving in I dug some deep sink holes with runaways so although with very high rain fall I get the odd pond it is not as bad as some.
Plants do not need water as often as the old house and living in the North East with plenty of water plus the unused facility at Kielder we do not appear to be able to give the stuff away. A year or so back a couple of miles from me they were filling water tankers 24 hours a day and running it into Yorkshire, their reservoir's were dry.
What we are saying is water depends on where you live, as an engineer I can see no reason for not running water pipes down country, filling rivers that could be pumped on South to where it is needed. We once sent coal from this area to London so why not water tankers or is that too simple.