Rainwater Harvesting: Sizing & Modeling Guide
TL;DR: Calculate effective rainwater harvesting tank size using simple formulas and local data to maximize water capture and minimize overflow.
- Calculate basic yield with catchment, rainfall, and runoff coefficient.
- Use 0.8 runoff coefficient for rooftops; adjust for local factors.
- Size storage volume based on catchment, design storm, and depth.
- Pre-storage filters are crucial for maintaining water quality.
- Verify calculations with local site and climate data.
- Conservatively select coefficients for reliable system performance.
Why it matters: Accurate rainwater harvesting system sizing is critical for ensuring a reliable water supply, reducing municipal water demand, and effectively managing stormwater runoff for sustainable landscapes.
Do this next: Measure your roof or catchment area and research your local average annual rainfall to begin sizing a system.
Recommended for: Practitioners and homeowners aiming to design efficient and reliable rainwater harvesting systems for urban and rural settings.
This technical wiki provides sizing and modeling methods for rainwater harvesting, including simple approximations and advanced variables. Basic yield: 5% of average annual = 0.05 × catchment area (m²) × annual runoff coefficient × rainfall (mm) × filter efficiency (0.9). Rooftop runoff coefficient around 0.8 from Ontario studies, factoring evaporation, snow loss, overflows. Storage volume: VS = Ac × Cvol,E × Rd, where Ac is catchment (m²), Cvol,E design storm coefficient, Rd depth. Practical for low-impact development: size tanks for 5% annual yield rule-of-thumb, validate with site data. Emphasizes pre-storage filters at 90% efficiency. Case insights from three Ontario sites confirm 0.8 Cvol,A empirically. Actionable steps: measure footprint, apply local rainfall stats, select coefficients conservatively. Supports modeling for reliability in variable climates, aiding drought resilience via overflow-minimized designs. Depth suits practitioners integrating with green infrastructure, offering formulas for custom LID swales or cisterns.