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The hydrologic cycle controls water available for use. Rainfall, evaporation, runoff, percolation, and transpiration combine to influence the water available at a specific location at a particular point in time. The speed at which water moves among stages in the hydrologic cycle and the amount of time it spends in storage at any stage affects water availability to users. The conjunctive use issue refers to the portion of the hydrologic cycle where groundwater and surface water interface and influence each other.
Users who couldn't depend on an irregular flow of water over time developed systems that modify the variable nature of the hydrologic cycle to change the time water is available for use. This occurs through reservoir storage, land management, groundwater storage, weather modification, and desalination.