The hydrologic cycle，a major topic in this science，is the complete cycle of phenomena through which water passes，beginning as atmospheric water vapor，passing into liquid and solid form as precipitation，thence along and into the ground surface，and finally again returning to the form of atmospheric water vapor by means of evaporation and transpiration.
水文循環(huán)(hydrologic cycle)，作為該學科中的一個主要課題，指的是水所經(jīng)過的諸現(xiàn)象的整個循環(huán)過程，開始時是作為大氣中的水蒸氣，轉(zhuǎn)而作為雨、雪、露、雹一類的降水量經(jīng)過液體和固體形態(tài)，由此而沿著地層表面分布或進入地層表面，終通過蒸發(fā)和散發(fā)作用再度回復(fù)到大氣水蒸氣的形態(tài)。