hydrologic cycle; water cycle
Rare term for the hydrologic cycle; the more common term is 水循環 (すいじゅんかん).
See also: 水循環
"水天循環" is a rare way to say the water cycle.
The standard term for the hydrologic cycle; 水天循環 is a rare variant.
Compound of 水 (water), 天 (heaven/sky), and 循環 (circulation). The exact derivation is uncertain; it may be a calque or older term for the water cycle.