Calda reservoir: drought shortage, leakage and desalination
The town of Calda has 45,000 residents and a single reservoir with a usable capacity of 2.4 million m³. At the start of the dry season it is 55% full, and the dry season typically lasts 120 days with negligible inflow. Current demand is 14,000 m³ per day, and 22% of treated water is lost through leaks. The water company proposes either a desalination plant costing $30 million and producing 5,000 m³ per day, or a leakage-reduction programme costing $8 million and cutting leakage by half. A new housing development of 1,500 homes is planned, each expected to use 150 m³ per year. Environmental groups oppose desalination because of brine discharge and energy use, and they argue that leakage reduction should come first.
1. Calculate the volume of water used in the 120-day dry season at current demand, the usable storage available, and the shortfall. Then calculate how much water the leakage programme would save over the same period.
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Dry-season demand = 14,000 m³/day × 120 days = 1,680,000 m³. Usable storage at 55% = 0.55 × 2,400,000 m³ = 1,320,000 m³. Shortfall = 1,680,000 − 1,320,000 = 360,000 m³.
Leakage = 22% × 14,000 = 3,080 m³/day. Halving leakage saves 3,080 ÷ 2 = 1,540 m³/day. Over 120 days: 1,540 × 120 = 184,800 m³ saved.
So leakage reduction covers about half the shortfall. After the programme, the remaining shortfall is 360,000 − 184,800 = 175,200 m³, which is still about 12.5 days of current demand. Leakage reduction alone would not close the gap unless demand were also restricted.
2. Evaluate the two options. Which would you recommend, and under what conditions?
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Leakage reduction is much cheaper ($8 million versus $30 million) and reduces waste rather than increasing supply. It saves 1,540 m³/day, but leaves a 175,200 m³ shortfall over the dry season. Desalination produces 5,000 m³/day, or 600,000 m³ over 120 days, which is more than enough to cover the 360,000 m³ gap. It is reliable in drought, but it is expensive, energy-intensive and produces brine, which environmental groups oppose.
I would recommend leakage reduction first, combined with demand management (hosepipe restrictions, tariffs, public information), because it is cheaper and addresses the root inefficiency. If restrictions cannot close the remaining 175,200 m³ gap, a smaller or phased desalination plant should be added, rather than the full 5,000 m³/day plant. The conditions are that leakage savings are measured and verified, that restrictions are enforceable, and that the desalination plant's brine discharge is monitored. The planned 1,500 homes add 225,000 m³/year, or about 616 m³/day, so the housing development makes demand management more urgent.
3. Explain why the planned housing development changes the decision, using a calculation.
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The development adds 1,500 homes × 150 m³/year = 225,000 m³/year. Spread over 365 days, that is about 616 m³/day. Over a 120-day dry season it is about 73,972 m³ of extra demand.
This matters because the current dry-season shortfall is 360,000 m³. The housing adds roughly 74,000 m³ to that gap, so the leakage programme's 184,800 m³ saving becomes even less sufficient. It also means the desalination plant's output would be partly used to supply new homes rather than to restore reliability for existing residents. The decision therefore cannot be based only on current demand: the company must either secure additional supply, enforce water-efficient fittings in the new homes, or phase the development until leakage and demand measures are working.