Rainwater harvesting begins with the roof. Every square meter of roof that receives rainfall and drains to a gutter and downspout is a catchment surface. The amount of water that can be collected from a given roof in a given year is determined by a simple formula: Annual collection (liters) = Annual rainfall (mm) × Catchment area (m²) × Collection efficiency (typically 0.75–0.85, to account for evaporation, first flush losses, and roof absorption). For example, a building in Taos, New Mexico, with an annual rainfall of approximately 330 mm and a roof catchment area of 150 m², can expect to collect approximately 37,000 to 42,000 liters per year (330 × 150 × 0.80 = 39,600 liters). A household of two to three people using the conservative water budgets appropriate to a regenerative building (40 to 50 liters per person per day) will use approximately 30,000 to 55,000 liters per year….