Test students' ability to design and size a household rainwater harvesting system by calculating annual harvestable volume from roof area and rainfall (using a runoff coefficient), determining appropriate tank capacity to meet given household demand and reliability targets (including losses and overflow), performing required unit conversions, and estimating annual water savings and percent reduction in mains supply; problems should provide or require monthly/annual rainfall, roof area, demand values, and ask for justification of assumptions and simple calculations.