Why is Australia mostly desert?

[Talroth]

I'm kind of wondering what kind of effect you could have on the world if you were to take areas that create rain shadows, and dam off the wet side part of the way down the water shed, and build tunnels going through the mountain to the far side.

Allow just enough water to flow through irrigation pipes to everywhere as needed on each side.

[Ascension]

Underground water tables serve a similar function and is why we get oases.