A big consumer in the fresh water market is agriculture. Whenever articles talk about demand exceeding freshwater supplies, it’s referring to agriculture demand, which usually draw from dams, rivers, or lakes.

City water treatmant plants also usually start with pulling water from a river or water reservoir.

The costs with these consumers suddenly spinning up a saltwater or other advanced purification plant, that could perhaps function without a large freshwater reservoir, is prohibitively expensive. Especially for developing countries, where agriculture could be a large part of the economy