Soft water, unlike hard water and seawater, contains very little dissolved salt. It makes up 3% of the hydrosphere. Almost three quarters is polar ice, a little less than a quarter is underground water and a minute part is superficial water. In the end, only about 0.01% of soft water on Earth is directly usable. Water circulates continuously between the different reservoirs (seas and oceans, continental superficial and underground waters, atmosphere and biosphere). Thus, even if stocks are not limitless, some are always being renewed. If we give it enough time, water is a completely renewable resource.