When specifying the total blue water footprint by source, one may also like
to explicitly distinguish the consumptive use of harvested rainwater. Rainwater
harvesting is a bit of a particular case, since one may argue whether harvested
rainwater is green or blue water. Mostly, rainwater harvesting refers to the
collection of rain that otherwise would become run-off. Since consumptive use
of harvested rainwater will subtract from run-off, we recommend to consider
such water use as a blue water footprint. Various sorts of rainwater harvesting
techniques exist to provide drinking water, water for livestock or water for
irrigating crops or gardens. As long as one speaks of local collection of run-off
– as in the case of rainwater harvesting from rooftops or other hard surfaces or
in the case of leading the rain to small ponds – one can categorize consumptive
use of this water under the blue water footprint. If, on the contrary, one speaks
of measures to increase the soil water holding capacity or about green rooftops
to retain rainwater, consumptive use of this water for crop production will fall
under the green water footprint