The hydrologic cycle, or the water cycle, is the process by which water is transported from the land surface into the atmosphere and then back down to the land surface.
The sun’s energy heats up water from oceans, lakes, ponds, rivers and causes it to evaporate. In addition, water absorbed by plants pass through them and then transpires through the leaves to the air and changes it into water vapor. The water vapor rises then cools and condenses to form clouds.
Clouds hold the water temporarily as vapor, small liquid droplets, or little ice particles. When cloud temperatures become cold enough, moisture vapor cools and condenses.
The condensed water within clouds forms larger liquid drops or solid moisture particles and is pulled to earth by gravity as precipitation in the form of rain or snow. Some of this rain or snow is absorbed by the ground (infiltration) and then becomes groundwater (0.6 percent). Some water runs off the surface of the ground and becomes nonpoint runoff to streams and rivers, finally returning to the ocean. Most of this water return to the ocean. A very small amount (2 percent) of the precipitation falls in colder areas of the earth as snow and becomes tied up for longer periods of time in glaciers and ice caps.
As important as the cycle itself, are ways we've interfered with it, the modifications humans made over time to the land's natural hydrology. In a natural system, with no human modifications, water "percolates" through many different elements (sand and dirt, rocky and grassy areas) before reaching waterways like streams and rivers. Those elements help filter out pollutants.These changes give pollutants a straight shot into water.
Many rural areas, for example, have been tiled. Tiling is a process of inserting tiles 3-4 feet beneath the surface to drains the landscape and funnels the water directly to a nearby stream. This speeding up of the movement of water off the land surface into a nearby stream essentially short-circuit nature’s intended plumbing system that includes wetlands, vegetation and soil. It’s not just agricultural modifications that short-circuit the system. Urban areas, with acres of concrete, steel and asphalt, do the same thing in a different way. When rain falls on a roof, asphalt parking lot, or a concrete road, it can't absorb. Instead of absorbing, water runs over these solid surfaces, picking up pollutants like sediment and oil. The runoff moves back to waterways, or flows down stormwater drains, which dump directly into streams and rivers.