According to the National Weather Service, Pittsburgh receives an average of 36.6 inches of precipitation a year. That’s equivalent to 97,600 gallons of water falling on each acre in a year’s time, or 2.2 gallons of water on each square foot per year. And that doesn’t include the regional moisture that evaporates mid-air before touching the ground.

All of that precipitation goes somewhere. When it reaches the ground and comes in contact with soil and vegetation, rainwater and snowmelt begin an underground journey within soils called infiltration.

It is estimated that in the northeast temperate forest (all of the Allegheny Watershed lies within this region), 70 percent of the water that enters the soil from precipitation is drawn up the stems of plants and into their leaves and is released to the air as water vapor. This process, called transpiration, is the evaporation of internal fluids from a plant's leaves or needles.

Water that is not transpired by plants moves into deeper soil layers. This infiltration into the soil increases with dense vegetative cover, high amounts of organic matter, irregular and uncompacted soil surfaces, numerous root holes, and healthy microbic soil activity. Once water can infiltrate the soil surface, it continues penetrating down into the soil profile and seeping into bedrock fractures. In naturally vegetated systems, there is more infiltration, and generally less surface runoff which occurs when soils become saturated during infrequent and heavy storms and can absorb no more water. In contrast, runoff is high in urban and suburban areas that have a high concentration of impervious surfaces—rooftops, roads, and parking lots.

Water readily percolates through porous soils, such as the sand and gravel deposits found in glaciated sections of the Allegheny Watershed. Sandy soils allow 50 percent of the rainfall to infiltrate the soil. Clay soils are less permeable, allowing 5-15 percent percolation within soil substrates. Nevertheless, clay particles hold onto water longer because they are magnetically charged.

Where water is able to percolate through soils, underground water levels are maintained between rainfalls and snow melts. Most of the water is held in soils and made available to plants. The rest constantly moves underground through bedrock fissures and aquifers. This is why water can be found underground even in a prolonged drought. Subterranean water travels slowly, sometimes over long distances, until it emerges as a spring, is pumped out by a well, or seeps into a headwater stream. It is estimated that 20 percent of freshwater on the planet is stored underground. The forests, headwaters, and rivers of our regional watershed have been consistently sustained by forest soil capacity to absorb, transport, store and slowly re-release water underground.