Rainwater Harvesting for the Garden: Roof Catchment, Monthly Yield, Garden Water Demand, Tank Sizing, Overflow Days & Payback

What you'll build

What you will be able to do

Each lab ends with a concrete result that you build yourself and the checker confirms, cell by cell.

• Measure five roof sections in plan, look up a fictional runoff factor for each roof type and turn a year's rainfall into the litres each roof can deliver to a garden tank.
• Turn twelve months of fictional rainfall into gross litres, subtract the water a first-flush diverter throws away on every rain day, and see how much of the year's harvest arrives in the growing season.
• Turn fictional watering needs in millimetres per week into litres for beds, lawn, fruit and pots, scale them to whole months, total the season and find the monthly shortfall the tank cannot cover.
• Run a 1,000-litre tank through twelve days of July: rain in, garden water out, the level capped between empty and full, the litres that overflow and the days the tank runs dry.
• Compare four tank sizes from a season simulation: the share of the garden's demand each covers, the cost per 1,000 litres over the tank's life, the extra water per extra unit of money, and the smallest tank that meets a coverage target.
• Split the roof into the areas each downpipe drains, follow one storm into four tanks – what is stored and what overflows – and fill three tanks linked in series one after the other.
• Work out how many litres each bed needs per watering, reduce the pump's label flow for the hose length with a fictional factor, and turn litres into minutes per watering and pump minutes per week.
• Value the rainwater at a fictional mains water price, subtract running costs, work out the simple payback of two systems, grow the price year by year and chart the cumulative savings over 15 years.