The following is a short primer on stormwater. A glossary of terms is below.

Stormwater Runoff and storm sewer discharges are the second most significant cause of water pollution in the nation's estuaries. Stormwater runoff is increased by human activities through construction, paving, soil compaction or changes in the vegetation growing on the land. As runoff increases, flooding often follows and can cause streambank erosion and degradation to stream channels and aquatic habitats. Runoff moving across the ground after a rainfall carries pollutants, excess nutrients, and soil to nearby streams, rivers, lakes and estuaries. These substances degrade water quality and can have serious impacts on drinking water and aquatic life.

Increased stormwater runoff is also related to reduced groundwater infiltration. Some portion of annual rainfall needs to infiltrate into the ground to replenish water that humans have drawn out of the ground. Without this recharge, groundwater supplies will continue to decrease, creating potential shortages for individual drinking water wells and other public supplies that rely on pubilc wells.

Current stormwater regulations in Pennsylvania are aimed at reducing the impacts of stormwater runoff, increasing recharge to public and private water supplies, improving stream corridor habitats and restoring the overall environmental integrity of the watershed.

Stormwater is part of the natural hydrologic cycle of precipitation, infiltration, evaporation and groundwater discharge. In undisturbed landscapes, the natural environment maintains an equilibrium by accepting and absorbing rainfall. Healthy vegetation and soil with its organic matter, porosity, and microorganisms use and absorb rainwater in their living processes. Excess water infiltrates into groundwater to discharge slowly and steadily into streams and wetlands, supporting essential aquatic ecosystems during dry weather.

Development radically alters the natural hydrologic patterns generating excessive amounts of surface runoff through vegetation removal, soil compaction, impervious paving surfaces and structures. Impervious pavement collects and concentrates runoff. The pollutants that accumulate on these surfaces are collected in the runoff that is directed into streams and estuaries. Uncontrolled urban runoff also contributes to hydrologic and habitat modification, two important sources of river impairment identified by the EPA.

Stormwater Glossary
aquifer--a geologic formation(s) that is water bearing. A geological formation or structure that stores and/or transmits water, such as to wells and springs. Use of the term is usually restricted to those water-bearing formations capable of yielding water in sufficient quantity to constitute a usable supply for people's uses.

base flow--sustained flow of a stream in the absence of direct runoff. It includes natural and human-induced streamflows. Natural base flow is sustained largely by ground-water discharges.

condensation--the process of water vapor in the air turning into liquid water. Water drops on the outside of a cold glass of water are condensed water. Condensation is the opposite process of evaporation.

discharge--the volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second. 

watershed or drainage basin--land area where precipitation runs off into streams, rivers, lakes, and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large drainage basins, like the area that drains into the Mississippi River contain thousands of smaller drainage basins. Also called a "watershed."

evaporation--the process of liquid water becoming water vapor, including vaporization from water surfaces, land surfaces, and snow fields, but not from leaf surfaces. See transpiration.

evapotranspiration--the sum of evaporation and transpiration.

flood--An overflow of water onto lands that are used or usable by man and not normally covered by water. Floods have two essential characteristics: The inundation of land is temporary; and the land is adjacent to and inundated by overflow from a river, stream, lake, or ocean.

flood, 100-year--A 100-year flood does not refer to a flood that occurs once every 100 years, but to a flood level with a 1 percent chance of being equaled or exceeded in any given year.

flood plain--a strip of relatively flat and normally dry land alongside a stream, river, or lake that is covered by water during a flood.

ground water--(1) water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper surface of the saturate zone is called the water table. (2) Water stored underground in rock crevices and in the pores of geologic materials that make up the Earth's crust.

ground-water recharge--inflow of water to a ground-water reservoir from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Also, the volume of water added by this process.

hydrologic cycle--the cyclic transfer of water vapor from the Earth's surface via evapotranspiration into the atmosphere, from the atmosphere via precipitation back to earth, and through runoff into streams, rivers, and lakes, and ultimately into the oceans.

infiltration--flow of water from the land surface into the subsurface.

million gallons per day (Mgd)--a rate of flow of water equal to 133,680.56 cubic feet per day, or 1.5472 cubic feet per second, or 3.0689 acre-feet per day. A flow of one million gallons per day for one year equals 1,120 acre-feet (365 million gallons).

non-point source (NPS) pollution--pollution discharged over a wide land area, not from one specific location. These are forms of diffuse pollution caused by sediment, nutrients, organic and toxic substances originating from land-use activities, which are carried to lakes and streams by surface runoff. Non-point source pollution is contamination that occurs when rainwater, snowmelt, or irrigation washes off plowed fields, city streets, or suburban backyards. As this runoff moves across the land surface, it picks up soil particles and pollutants, such as nutrients and pesticides.

percolation--(1) The movement of water through the openings in rock or soil. (2) the entrance of a portion of the streamflow into the channel materials to contribute to ground water replenishment.

permeability--the ability of a material to allow the passage of a liquid, such as water through rocks. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas unpermeable material, such as clay, don't allow water to flow freely.

point-source pollution--water pollution coming from a single point, such as a sewage-outflow pipe.

precipitation--rain, snow, hail, sleet, dew, and frost.

river--A natural stream of water of considerable volume, larger than a brook or creek.

runoff--(1) That part of the precipitation, snow melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains or sewers. Runoff may be classified according to speed of appearance after rainfall or melting snow as direct runoff or base runoff, and according to source as surface runoff, storm interflow, or ground-water runoff. (2) The total discharge described in (1), above, during a specified period of time. (3) Also defined as the depth to which a drainage area would be covered if all of the runoff for a given period of time were uniformly distributed over it.

sediment--usually applied to material in suspension in water or recently deposited from suspension. In the plural the word is applied to all kinds of deposits from the waters of streams, lakes, or seas.

storm sewer--a sewer that carries only surface runoff, street wash, and snow melt from the land. In a separate sewer system, storm sewers are completely separate from those that carry domestic and commercial wastewater (sanitary sewers).

stream--a general term for a body of flowing water; natural water course containing water at least part of the year. In hydrology, it is generally applied to the water flowing in a natural channel as distinct from a canal.

streamflow--the water discharge that occurs in a natural channel. A more general term than runoff, streamflow may be applied to discharge whether or not it is affected by diversion or regulation.

subsidence--a dropping of the land surface as a result of ground water being

surface water--water that is on the Earth's surface, such as in a stream, river, lake, or reservoir.

transpiration--process by which water that is absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plant surface, such as leaf pores. See evapotranspiration.

water cycle--the circuit of water movement from the oceans to the atmosphere and to the Earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transportation.

water table--the top of the water surface in the saturated part of an aquifer.

watershed--the land area that drains water to a particular stream, river, or lake. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large watersheds, like the Mississippi River basin contain thousands of smaller watersheds.