Rain fall or Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off.
Rainwater uses include water for gardening, water for livestock, water for irrigation, water for domestic use with proper treatment, and indoor heating for houses etc. In many places the water collected is just redirected to a deep pit with percolation. The harvested water can be used as drinking water as well as for storage and other purpose like irrigation.
vCalc's Rain fall harvest folder contains equations that pertain to rainwater harvesting.