Water Cycle does not have an Antonym as such... Antonym usually refers to an opposite meaning... Of which water cycle really has none... However if you are asking for an antonym for cycle there are a few. Such as stillness, immobile, innert, unmoving, frozen and stiff.
The water cycle is important for sustaining life because water is an essential component of all living organisms. Organism require water to produce energy. The water cycle provides a continous source or usable water to organisms.