Water is the phase of deepest stillness in the Wu Xing, the point furthest from Fire on the cycle and the one from which growth begins again. In the generative, or Sheng, order Metal feeds Water, condensing into it, and Water in turn feeds Wood, as rain feeds a root. In the controlling, or Ke, order Water controls Fire, extinguishing it, while Water is itself controlled by Earth, which dams and absorbs it. The cycle closes here only to open again, Water's own child, Wood, beginning the sequence anew.
In Chinese medicine Water governs the Kidney and its paired Bladder, and by extension the ears, the bones, and the body's deepest reserves, both of fluid and of constitutional vitality; an imbalance is read through the fear the phase is said to carry and through the exhaustion of stores it cannot quickly replace. Water's season is winter, its direction north, and its colour black, correspondences that place it at the year's most withdrawn point, when growth is stored rather than spent.
Because Water is held to be the root reserve behind every other phase, five-phase diagnosis treats a depleted Kidney as a limit on the whole system's capacity to recover, not merely a fault local to Water itself.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown