Hydrologic Cycle
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The hydrologic cycle is truly an amazing system and is enormously important to the Earth. Water moves throughout all four of Earths systems and helps to move materials, as well. Amazingly, water exists in each system. In the lithosphere it plays a significant role in crystalline structures. In the biosphere it is a substantial constituent of all life. In the atmosphere it becomes a vapor and moves through a process which brings it back to the Earth as precipitation. They hydrologic cycle is what truly sets Earth apart from other planets.
Water moves about the Earth through a variety of pathways and processes. Evaporation is the process in which water is changed from a liquid to a vapor. This vapor moves from reservoirs like lakes, streams, and oceans into the atmosphere, where it condenses, meaning that it changes back into a liquid or a solid, which then cause precipitation in forms like rain, snow, and hail. Water can also evaporate into the atmosphere by transpiration. This process involves plants releasing water that they have absorbed directly into the atmosphere as vapor. Once the water has fallen to Earth through precipitation, it can become surface runoff, which is how the water runs and drains over the land or into stream channels. Infiltration is the process that allows water to seep back into the ground through cracks and other small openings.
Interestingly, the hydrologic, or water, cycle is a closed cycle of open systems. It is considered closed because the amount of water in the cycle never changes. The amount of water on the planet presently is the same amount that it always has been and always will be. It is open because water can be transported to all the other Earth systems. The amount of water in each reservoir, including lakes, groundwater, and oceans, can gain or lose water at any time, but that water will become part of another reservoir.
The hydrologic cycle, unlike the rock cycle, is something that scientists can actively observe and is a remarkable system. It is a precious commodity in many areas of the world, which is not surprising considering that the fresh water available for human and animal consumption is extremely limited, as compared to the saline water, primarily in the Earths oceans which equals 97.5 percent of all the water on Earth.
Reference
Murck, B. W., Skinner, B. J., & Mackenzie, D. (2008). Visualizing Geology (2nd ed.). Toronto, Canada: Wiley.