If quality or quantity go below the minimum thresholds, we
enter a “zone of unsustainability”, which can quickly lead to
complete system collapse (Godfrey and Todd, 2001). This is an
important issue to consider when planning water supply and use,
as different water demands require different minimum water
quality standards. By ensuring that high quality captured water is
directed to uses that require high quality water and that lower
quality water is appropriately dedicated for uses that do not need
high quality, supply is effectively enhanced and water can be allocated
in a more sustainable way. Liu et al. (2010) consider the
sustainable use ofwater at three different levels e the small cycle at
an individual firm level; the medium cycle at an inter-firm level;
and the large cycle at a regional level. The regional level cycle is one
where certain wastewater streams may be directed, after treatment,
into a number of re-use streams (Liu et al., 2010). This could
be considered a partial very high level view of what the WSF
attempts to achieve e as a fully developed, refined tool or process
to target water supplies from all sources (not simply re-use or
recycling) to an appropriate use based on source quality and
intended use. The WSF shows that Liu et al. (2010) large cycle
concept has been improved and defined in order to be a valuable
tool in sustainable water planning that can be adapted to any
number of different situations.