Water.

Assuming that rich means lots and lots of money and not just comfortably off, I'll be creating water-saving solutions.

The whole centralised storage (dams) and distribution of water astounds me; there are countries that have tied up more than 90% of their water supplies already, and it all goes into this inefficient system. Whole rivers are channelled into central points, purified, and pumped long-distance at great cost.

This centralised system wasn't designed to cope with the changes in weather either (bigger, but less frequent storms, meltwater drying up, severe drought interspersed with flooding etc), so better ways to manage water must be found.

I would love to fund thousands of small local research projects where local solutions to the water-based problems are found. Not fund the systems, but rather fund the research, educate, give the expertise; find ways that people can make money out of taking care of their own water.

If it doesn't fund itself it will never work in the long run anyway.