What is the Hydrologic Cycle
There is a finite supply of water on the Earth, but it is continually recycled naturally. Water may occur as a solid, liquid, or gas, and can be found in a wide variety of locations. This circulation that purifies and redistributes water is called the hydrologic cycle.

Water can enter the atmosphere by evaporation, transpiration, or sublimation. Evaporation occurs as liquid water becomes a gas; the water can come from anything on the surface, such as plants, soil, rivers, lakes, and oceans. Transpiration is the process by which water is released from green plants into the atmosphere. In many cases it is very difficult to distinguish evaporation from transpiration; evapotranspiration refers to the combination of the two. Sublimation is the process of a solid turning directly into a gas; snow and ice changing into vapor is only a minor part of water entering the atmosphere.

Water changes from a gas into a liquid by condensation and returns to the Earth primarily in the form of precipitation (snow and rain). Many different things can happen to precipitation ? in some cases it does not even reach the ground. It can evaporate before hitting anything, or can be intercepted by vegetation. When water reaches the ground, it can infiltrate into the ground, be stored on the surface, or travel on the surface until it can infiltrate or be stored.

Groundwater travels through rock and sediment by percolation. It moves by gravity and pressure until the water table intersects the ground surface. Water can then be discharged at springs or any other body of surface water. Once returned to the surface, this water can be used by plants, stored on the surface, or evaporated.

Importance of the Hydrologic Cycle for Groundwater
Infiltration supplies aquifers with a continual source of water to replace that pumped from wells and discharged naturally (such as at springs). During infiltration water can pick up acids in the soil that can subsequently expand the pore space in aquifers, sometimes creating caves. Large areas of impervious cover, such as parking lots, do not allow infiltration. High volumes of rain over a short period of time also reduce the amount of water infiltrating; slow rainfall best recharges groundwater.

Infiltration and percolation are usually slow processes requiring water to move through a tight maze of sediment which acts as a filter. Together with biological activity, water is cleaned as it moves through the ground. This natural scrubbing is one of the reasons groundwater is so commonly used for drinking.