This post is late I know - sorry, I've only just looked at this site. But if you still check in I hope I can help. I live in Australia, and we've just come thru the worst drought in 100 years. I've been connecting up the outlet of my washing machine to a hose that runs outside on the lawn. Just lucky we have a cat door next to the machine. This saves a lot of water and can keep the lawn going without extra watering. Just one tip...you can't put bleach or fabric softener into the actual washing water..I do the bleaching before the wash in a smaller container and that goes down the drain. Our sewage pipes are in a position that we can't pump out the water from showers & baths unfortunately. The only concern I would have is that the raw water doesn't find its way into your lake.

How about using a small solar pump? They are available at hardware stores, and online, are relatively inexpensive, and can be hooked onto a hose. My neighbor had some buried drums in which he collected rainwater. He pumped the water back into his garden using a little solar pump. He didn't get gulley-washing pressure, but he did get a good enough flow to water his vegetables and ornamentals.

Here is a link that might be useful: Desert Gardens