The Rain
Ledgerof Marrow Bay

Marrow Bay receives 1,180 millimetres of rain a year — more than three quarters of what it drinks. Until 2017 essentially all of it ran off tarmac, through nine storm culverts and back into the estuary within four hours of falling. The utility then bought the same water back from an upriver reservoir 62 kilometres inland, lifted it 210 metres, and lost a fifth of it to leaks on the way home.

The Ledger began as an accounting exercise, not an environmental one. Someone in the treasury asked what the city was paying per litre to move water past its own rooftops.

The first year of the Ledger produced a single unglamorous chart. It is still the most quoted page in the dossier, because it shows that the largest category of urban water is not consumption — it is loss.

Rather than pilot sequentially, the city ran all three interventions from the same budget year, accepting messier attribution in exchange for speed. Adoption is measured as share of eligible parcels enrolled by the end of 2025.

Each square below is one of the city's 100 census blocks. Teal blocks now retain more than half their rainfall on site; amber blocks are between a quarter and a half; grey blocks have not moved measurably since baseline.

The grey band is not random. Thirty of the thirty-three unchanged blocks sit on the reclaimed flats, where the water table is within a metre of the surface and infiltration is physically unavailable. The Ledger's honest conclusion is that a third of Marrow Bay cannot be fixed with soil.

Capital spend is public record; the avoided-cost column is the authority's own model, which assumes no growth in imported-water tariffs. It is therefore conservative by construction.

Imported water as a share of total municipal supply. The 2020 flat spot is a construction pause; the 2024 uptick is a drought year in which cisterns emptied faster than they refilled.