The Lake Promo Tile was an unlocked stretch goal in the Petrichor base game Kickstarter.
Any drop that lands on the Lake tile is immediately returned to its owner’s personal supply. If an existing cloud is moved onto the Lake via a Wind card action, the player performing the Wind action must immediately add 3 of their own drops into the cloud - signifying the water the breeze evaporates from the Lake’s surface. If a cloud is moved off from the Lake tile via a Wind card action, it can be moved any number of times (even changing directions). The cloud must stop moving if merged with another cloud, and cannot re-enter the Lake during the same Wind card action it left it.
For more information, please refer to the BGG listing.