In assessing the blue water footprint of a process it may be relevant (depending
on the scope of the study) to distinguish between different sorts of blue water
sources. The most relevant division is between surface water, flowing (renewable)
groundwater and fossil groundwater. One can make the distinction by speaking
respectively of the blue surface water footprint, the blue renewable groundwater
footprint and the blue fossil groundwater footprint (or the light-blue, dark-blue
and black water footprint if one really likes the use of the colours). In practice, it
is often very difficult to make the distinction because of insufficient data, which
is the reason the distinction is often not made. It is possible, however, if data
allow, to specify the blue water footprint by source (see examples in Aldaya and
Llamas, 2008; Aldaya and Hoekstra, 2010; Mekonnen and Hoekstra, 2010b)