My Aquifer > Groundwater Basics > What is the hydrologic cycle?
The world's water is in constant motion, flowing downhill by gravity, pumped into the atmosphere by evaporation fueled by Sun's heat, and returned as rain and snow. Except for the oceans, most of it moves underground. The hydrologic cycle is a simple way to represent this motion (Figure 1). Water at the surface (wetlands, lakes, rivers, oceans) evaporates into the atmosphere, leaving impurities behind. Moisture moves around the globe with the weather patterns, and rain and snow condense from it. Some of this precipitation runs back to surface water bodies and some percolates into the ground to become soil moisture. Micro-organisms are constantly purifying this water as it moves in streams, wetlands, and soil. Plants transpire some of this back to the atmosphere, and some continues to percolate down to the water table where it becomes ground water. Ground water flows slowly through the earth, and eventually returns to surface water bodies or to the oceans to start the cycle all over. [Click for additional info]