Most of the precipitation falls as surface runoff into the oceans or on the ground, where it is gravity-driven. Some of it enters the rivers in the valleys and then flows through the rivers to the oceans. Surface water and groundwater are collected in lakes and rivers as fresh water. Of course, it does not always reach the rivers due to water percolating underground. Some of the water remains near the surface and rejoins the surface water as groundwater. Some groundwater rejoins the water cycle as freshwater sources through openings in the earth’s surface. Shallow groundwater is absorbed by plant roots and returned to the atmosphere by transpiration through the leaf surface. Some of the water that seeps underground goes deeper and feeds water-saturated underground aquifers, which often store freshwater. With the movement of water, some of it ends up in the oceans, where it completes the water cycle. Water completes its cycle with heat energy provided by the sun (sometimes melting, sometimes freezing). The sun heats the water in the oceans and turns it into water vapor. The vapor rises into the atmosphere where the air is colder. The water vapor condenses into clouds. Volcanoes produce the vapors that make clouds. Wind moves clouds around the earth. Water droplets form in the clouds and fall to the earth as precipitation (rain and snow). In cold climates, this precipitation forms snow, ice and glaciers. The snow melts and flows into rivers, oceans and underground. Some ice evaporates into the air without melting (sublimation). Rain that falls on land moves down from high places as surface runoff to lakes, rivers and oceans as a source of water. Some of the rainwater accumulated on the ground is filtered and stored as groundwater at sufficient depth. Lake and river water can go deep underground. Underground water is in constant motion due to gravity and pressure. Groundwater close to the surface is used by plant roots. Some of the groundwater mixes with lakes and rivers and flows as surface water as spring water. After plants use groundwater, they excrete this water through their leaves in the form of vapor. Some of the groundwater is stored in the groundwater well below the ground. Groundwater accumulated in the oceans ensures the continuation of the water cycle.
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