Economist here. Imo the market rate would be the price at which supply and demand were equal. In this case, the government should choose the supply such that

Economist here. Imo the market rate would be the price at which supply and demand were equal. In this case, the government should choose the supply such that it's ecologically sustainable. So the price is the price at which that sustainable level of water usage is demanded by the market. That would not count as monopoly pricing, which would instead set the price to maximize revenue