‘Consumptive water use’ does not mean that the water disappears, because
water will remain within the cycle and always return somewhere. Water is a
renewable resource, but that does not mean that its availability is unlimited.
In a certain period, the amount of water that recharges groundwater reserves
and that flows through a river is always limited to a certain amount. Water in
rivers and aquifers can be used for irrigation or industrial or domestic purposes.
But in a certain period one cannot consume more water than is available. The
blue water footprint measures the amount of water available in a certain period
that is consumed (in other words, not immediately returned within the same
catchment). In this way, it provides a measure of the amount of available blue
water consumed by humans. The remainder, the groundwater and surface water
flows not consumed for human purposes, is left to sustain the ecosystems that
depend on the groundwater and surface water flows