http://www.sciencedaily.com/releases/2009/08/090804071400.htm
All natural cycles coexist with each other in the world. In other words, they are "coupled". One cannot exist with out another. For example, the nitrogen cycle cannot exist without the carbon cycle. No cycle, even cycles of trace elements such as iron, can exist in isolation. It is important to study said cycles as coupled. The reason for that is it makes more practical sense to have an understanding of the cycles in the state they occur naturally, as "coupled". It may be great to know them in isolation, but that lends us no help. Each cycle behaves differently when coupled, and something that may affect a different cycle may indirectly affect another cycle with which it is coupled with. So things that we may not expect to affect the water cycle, may actually affect the water cycle, because it'll be coupled with the carbon cycle for example, and something may affect the carbon cycle, thus indirectly affecting the water cycle.
