Hydrologic Cycle: Surface
When precipitation reaches Earth's surface, it does one of two things:
- Flows overland, OR
- Soaks into the soil
Water that is moving on or through land accounts for about 8% of the water in the hydrologic cycle at any given time! (Water has a much longer residence in deep-ocean circulation, groundwater, and glacial ice - it can remain in those parts of the water cycle between 3000-10,000 years!)
Infiltration is the process where precipitation soaks into and through the soil surface.
When water is unable to infiltrate the ground surface, it becomes overland flow: water that flows down hill (downslope). Overland flow is also known as surface runoff.
Overland flow can occur because the ground surface is impermeable - paved over with asphalt or concrete, for example. However, overland flow can also occur if the soil is infiltrated to its full capacity and has become completely saturated - for instance, storms where the rate of precipitation happens faster than the water can cycle through the subsurface, as the next page describes.
Excess water may stay in place on Earth's surface as a puddle or pond. However, if there is a downslope, the water will flow until it forms channels, becoming a streamflow or surface water flow.