I hear what you are saying Laurent1, but take what gianni said and think about ways you can use the naturally given rain water and move it throughout your yard to get the most from it.
I saw a really great video the other day about harvesting rainwater and basically the guy learned how to take a very dry arid site (in Arizona Desert) and use the rainwater to create a huge botanical garden.
He graded the land on his site to move the water in a zig zag motion across the site rather than just having it quickly rush accross the site and not get soaked up by the soil. This way at each zig he would store a little in a burried oil can and use that water to hydrate plants to help draw toxins out of the soil and water (bioremediation) while also using that water to grow food.
Here is the link: http://sustainableroute.com/?p=68
Adam