Where the water goes

The rain

Every supplied litre begins as weather over the Greycap hills. The water company’s model divides 640 square kilometres into small cells, estimates how much rain lands in each, then subtracts what returns to the air or remains in soil and plants. What is left is called effective rainfall. It reaches streams quickly on bare, saturated slopes and slowly through peat and fractured rock.

The twelve-year record below contains wet winters, ordinary springs and two deliberately severe dry summers. The average annual rainfall is just over 900 millimetres, but an annual average hides the timing that runs the system. Winter rain arrives when demand is low and the ground is willing to pass it on. Summer rain can fall on dry soil, replenish that soil first and contribute little to the reservoir.

In this model, May to August 2018 brought 91 millimetres. The same months in 2022 brought 106 millimetres. Neither year is treated as a prediction. They are stress periods placed in a synthetic record to show how a plausible system responds when inflow and demand move in opposite directions.

Water is taken from four upland streams through screened intakes. A legal minimum flow remains in each stream; the model never sends all available water towards the city. That rule reduces supply in dry weather, precisely when extra supply would be useful. It is there because a river is habitat before it is infrastructure.

The reservoir

Vale Reservoir is less a bucket than a bank account. Rain credits it unevenly; evaporation, river releases and the treatment works make withdrawals every day. At full supply level it stores 74,000 megalitres. Not all of that is available. The bottom 9 per cent sits below the usable draw-off or is reserved to protect water quality.

The level line responds after the rainfall bars. In a wet month, catchment soils must first become wet enough to generate runoff. In a dry month, the reservoir keeps falling even if a shower passes over the city. That delay is why a week of rain does not necessarily end a shortage.

During the 2018 dry summer, the modelled store falls from 78 per cent in April to 52 per cent by September. It enters that season with room to absorb the loss. In 2022, a dry spring precedes the summer and the store reaches 38 per cent. The company then lowers pressure overnight, pauses non-essential main flushing and asks large users to reduce demand. Household restrictions are not triggered in this scenario, though the margin narrows.

Operators also choose which depth to draw from. Warm surface water can carry algae; deeper water may contain more dissolved iron and manganese. A movable intake lets the works select the layer that requires the least intervention. The choice saves chemicals and energy, but it cannot remove the need for treatment.

Storage turns variable rain into a steady supply. Its useful number is not simply today’s level, but how long that level can support demand under a range of possible weather.

Treatment

Raw water enters the works carrying fine clay, natural colour from peat, microorganisms and traces of compounds washed from roads and fields. Treatment removes a sequence of risks; it does not make an abstract substance called purity.

First, a coagulant gives tiny suspended particles enough attraction to gather into flakes. Slow paddles help those flakes grow. They settle in broad tanks and are removed as sludge. The clearer water passes through beds of sand and granular carbon, which catch remaining particles and adsorb some dissolved organic compounds. Ultraviolet light and a small chlorine dose provide barriers against microorganisms. Operators measure turbidity, pH and disinfectant continuously, with laboratory checks as a separate line of evidence.

The synthetic works can treat 94 megalitres a day. In 2025 it processed an average 66 megalitres. Pumps, mixers, ultraviolet lamps and compressed-air systems used 438 kilowatt-hours for each megalitre produced. That ratio rises when filters are dirty or raw water is difficult to treat. The lowest-energy litre is therefore not always the one moved downhill; water quality changes the work required.

Three litres in every hundred are consumed within treatment. They wash filters, carry removed solids and clean process equipment. Most washwater is settled and returned near the head of the works, but recycling it indefinitely would concentrate contaminants. The model counts only the net use after recovery.

The mains

Clean water leaves the works under pressure and enters 3,860 kilometres of pipe. Some trunk mains are wide enough to walk through; the final service pipe to a home may be narrower than a wrist. Age alone does not decide whether a pipe leaks. Material, soil movement, pressure, workmanship and the number of joints all matter.

Harbour Vale’s uncomfortable number is 21 per cent. For every 100 litres entering distribution, 21 do not reach a billed customer. The balance includes water escaping from mains and service pipes, along with small amounts used for firefighting and network maintenance or missed by meters. The model calls the whole quantity leakage because the other uses are minor, but a real audit would separate them.

Most leaks never break the road surface. Water follows permeable gravel around a pipe, enters a drain or disperses through soil. Engineers listen for vibration, compare flows during the quietest hours of the night, and divide the network into metered zones. A zone using more water at 3am than its expected household minimum becomes a place to search.

The city loses 21 litres in every 100 after treatment. Those litres have already been abstracted, pumped, filtered and disinfected.

The district comparison is not a league table of staff performance. Old Quay has brittle cast-iron mains, dense traffic and many difficult-to-isolate service connections. South Reach was laid later in welded polyethylene. Replacing the worst sections reduces bursts, but replacing every old pipe at once would consume money and carbon that could prevent more losses elsewhere. The operating question is which repair avoids the most future leakage per pound and per road closure.

The tap

A delivered litre crosses the property boundary and becomes household use. In the model, showers and baths take 34 per cent; toilet flushing 24 per cent; kitchen taps and drinking 13 per cent; clothes washing 12 per cent; dishwashing 5 per cent; and cleaning, gardens and other uses the remaining 12 per cent. These shares vary more between homes than a city average can show.

Per-person use falls from 154 litres a day in 2014 to 136 in 2025. Efficient toilets and washing machines account for part of the change. The metering programme, started in 2019, also matters. A meter does not save water mechanically. It gives a household a quantity, makes some leaks visible and, where charges vary with use, creates a reason to act. The model applies a gradual reduction rather than an instant step.

Use rises briefly in 2020, when more people spend weekdays at home. That shift is included because household readings can move without the city as a whole using more: offices, cafés and schools use less at the same time. Our series describes homes, not total urban demand.

The return

After a shower, flush or wash, the litre changes category. It is now wastewater: mostly water, carrying organic matter, nutrients, grit, fibres and household chemicals. Harbour Vale’s sewers capture 92 per cent of household wastewater. The rest is attributed to properties with private systems, misconnections and losses that the model cannot locate.

The older centre has combined sewers that accept rainwater and wastewater in the same pipe. Newer districts keep them separate. In ordinary weather the combined flow reaches the Water Recovery Works. During intense rain, storage tunnels fill first; if both tunnels and treatment capacity are exhausted, screened diluted flow can discharge to the estuary. Our annual model represents the volume of those events but not their hour-by-hour ecological effect. That is a material limitation.

At the works, screens remove wipes and larger objects. Grit settles. In aerated tanks, microbial communities consume dissolved organic material and convert ammonia. A later stage removes nitrogen before the water is disinfected. Sludge is thickened and digested without oxygen, producing biogas that supplies part of the site’s heat and electricity.

The works returns an average 48 megalitres a day to the estuary. The synthetic model removes 81 per cent of incoming nitrogen. Removal is not the same as absence: a treated litre still carries nutrients, salts and traces of substances the process was not designed to capture. Estuary monitoring therefore compares conditions upstream and downstream, across tides and seasons, rather than treating the outfall as a single pass-or-fail point.

The returning litre may mix with seawater, enter sediment, evaporate or move out beyond the harbour. Some of it will fall on the Greycap hills again. That is the physical cycle. The civic system inside it is narrower and more deliberate: abstract, treat, distribute, use, collect, treat again, return.

A pipe network can move water out of sight. It cannot move it out of the same catchment and estuary that supply the city.

Seen end to end, the choices connect. Repairing a leak reduces abstraction and treatment energy. Lower household use leaves more storage for a dry summer and sends less volume through the sewer. Better rain separation gives wastewater treatment more time. None removes uncertainty about future weather or the exact condition of buried assets. They make the system less dependent on being lucky.

Read how the synthetic model was made and download its data.