If you can terraform a bit of desert: 1. There's still not going to be a lot of water, you'd think that many other less water intensive crops would be a good choice, even if you are fully efficient and never lose any water it's a lot of water to be tied up in a field. 2. There's technology around or being designed to fully automate rice production. If you have the technology to terraform a desert you'd be able to automate so much you'd only need a couple of people to run it probably. 3. If you did need bulk labour I can't see how making and training clones could be cheaper than either automation or shipping in ready made peasants with the skills you need.