hydatoscopy

From hydato- +‎ -scopy. Hydat- is the stem of Ancient Greek ὕδωρ (húdōr, “water”), whence also hydro-.

hydatoscopy (uncountable)

1. (rare) Divination by interpreting rainwater. Omens are drawn from the various formations and sizes of rain puddles, and bowls of rainwater are gazed upon until clairvoyant visions appear on the surface of the water, or in the mind's eye. This is considered in some cases to be more important than hydromancy because rain has fallen from the heavens, making it sacred.

• hydromancy - with water
• pegomancy - with spring-water