The government should legalize the harvesting of rainwater. I don't see anything wrong with rainwater. Just as we are exposed to the sun and get vitamin D through it, so should the harvesting of rainwater by a renter not be a problem. The client will have to agree on this matter so that no conflict can arise from it.
In our current social make up, as a society we need to be harvesting and using as many different types of resources as possible. If it is possible to harvest rainwater and use it in an effective way, it could help price and availability of drinking water. It could also help move water to areas where it is scarce.