each RG has always enough space to accommodate a tank for water storage;
each RG have a nearby building or structure to be used as a catching surface for rain water;
since data on building footprint are not available, every RG is associated with a catchment surface (i.e. a roof) of
a predetermined size equal to 100 m2;
a standard value of efficiency (60%) is attributed to the catchment area for accounting of leaks, wind, rainfall
rates. In fact, during a slow gentle rain, with no leaks in the system, collection efficiency can reach about 95%,
while, during a very fast, heavy rain, the efficiency would be closer to 60-75 % because gutters overflow and gutter
covers are overrun with water.
Under the mentioned assumption, the total amount of water that can be harvested can be computed by the
following equation: