In order to judge the vulnerability of the water supply, not only
the average yearlywater precipitation but also minimum accessible
precipitation levels have to be taken into account. This is critical in
regions like South-East Queensland, where annual precipitation can
change considerably and water restrictions may be applied by
regional authorities. In order to facilitate resilience in the light of the
precautionary principle these minimum levels should be used for
the planning process. Therefore, a factor has to be introduced to
consider these effects where the minimum precipitation is related
to the average precipitation. This is especially important in a country
like Australia where on the average 13.1% of available water is
appropriated by human use, which increases in some areas up to
75% due to the large variability in rainfall patterns (Chartres and
Williams, 2006) so that water restrictions are regularly necessary.
The sustainable water yield should account for soil water
requirements and changes and maintaining groundwater stocks as
well as stocks and flows in rivers, wetlands and other surface water
bodies which provide accessible human water supply. If there is
a gap between the precipitation and the water demand in drought
years, water storage is necessary, for example, in dams or in
groundwater bodies. Precipitation in extreme rainfall events can
technically only be stored up to a certain quantity and is often lost
for human use. These limits have to be subtracted from the theoretical
potential.