Purity: Purification of Ground by Rainwater Flow
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 38
Ground that is impure becomes pure if rain falls on it; and if rainwater begins to flow on the ground and while it is still raining it comes into contact with an impure area under a roof [on which rain cannot fall directly], it purifies that area as well.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)