![Hard rain on a roof.jpg](https://upload.wikimedia.org/wikipedia/commons/thumb/6/65/Hard_rain_on_a_roof.jpg/300px-Hard_rain_on_a_roof.jpg)
The following are calculators for Rainwater Cathment (aka Rainwater Harvesting):
- http://rainwaterharvesting.tamu.edu/calculators/
- https://cantaroazul.shinyapps.io/calculadora/ (in Spanish and quite comprehensive)
- http://www.pequals.com/rain (map function currently not working)