What RAINFAL does
In water planning, firm yield is the amount of water a system can supply with 100% reliability even under the worst drought on record. RAINFAL runs a daily water-balance across decades of historical rainfall at a given location, letting you test how different roof sizes, tank capacities, and household demand affect whether a system would have met that demand every single day of the record.
The model originated as a Meadows Center study of household-scale rainwater harvesting across all of Texas's regional water planning areas, challenging the long-held assumption that rainwater is too unreliable to count as a real water source.
What the research found
It works statewide
With an appropriately sized roof and tank, a system can meet 100% of a household's needs even in Texas's driest regions.
Design beats geography
Reliability depends more on roof area, storage capacity, and water use than on location or local drought history.
Drought isn't a dealbreaker
Larger catchment and storage carry systems through dry stretches; wetter areas need far less to reach full reliability.
No strain on supplies
Even universal rooftop harvesting would reduce runoff and recharge by less than 1% — negligible next to urban development.
Project leads
Robert Mace, Ph.D., P.G.
Executive Director & Chief Water Policy Officer
Professor of Practice in Geography at Texas State and project lead. Faculty profile ↗
Ricardo Briones
Former Research Assistant, The Meadows Center
Developed the RAINFAL model and authored the founding master's thesis, The Firm Yield of Rainwater Harvesting in Texas (2023).
Reports & downloads
Sponsors & funders
Funding for the study was provided by The Meadows Foundation and the National Oceanic and Atmospheric Administration.
The RAINFAL web interface (rainfal.app) is funded, developed, and maintained by The Rainwater Exchange.