The volume of water – 325,851 gallons – that would cover an acre of ground to a depth of one foot; roughly a year’s supply for two families.


The process by which air flows through soil. Aeration helps keep soil healthy. In places where the ground has been compacted, aeration is reduced.


A professional trained to provide care and maintenance to trees and other woody plants.


The underground bed or layer of earth, porous stone or gravel that contains or supplies groundwater. See groundwater.



Soil used to fill in around the root area of a newly planted tree or shrub.

bare root

Having no soil around the roots of a tree (for transporting to a planting site).


A wall or mound of dirt that helps hold rainwater within a defined area.

Best Management Practice (BMP)

In a given field, a tool or technique generally recognized as one of the best available. Stormwater BMPs include cisterns, infiltration basins, swales, strategic tree plantings and other technologies.


Bio-remediation uses biological processes to repair pollution damage.
For example, a grass swale can bio-remediate much of the pollution caused by automobile use by holding heavy metals in the soil at harmless concentrations as well as by the action of soil bacteria, which gradually breaks down hydrocarbon waste.


canopy cover

The portion of land area covered by the spread of a tree, including its leaves and branches.

carbon sequestration

The process of removing carbon from the air; one of the many benefits trees provide.

catchment planter

A planting bed that has been specially designed to hold and absorb storm flows from adjacent areas, usually from parking lots.


A planning or creative problem-solving activity in which an interdisciplinary group of participants is assigned a complicated design project and asked to complete it within a very short period of time.


A tank or recess used to capture and store rain water for later use.

citizen forestry

Ordinary people training to take responsibility to grow and care for the forest in their communities.



Holes drilled into the ground-sufficiently deep to allow rainwater to quickly flow back into the ground.



An area that contains living organisms interacting with each other and the non-living environment.


The wearing away of the land by the movement of water, air or ice.


The loss of water from the soil both by evaporation and by transpiration from the plants growing on it.


A plant which has living leaves or needles year round.


filter medium

Any item or substance that is used for filtering impurities.

first-flush rain

The first rain that happens after a long period of dry weather. In between rainstorms, pollution and grime build up on the city’s outdoor surfaces. The first-flush rain washes the accumulated grime and pollution off the streets and into the underground storm drain system, where it eventually makes its way to the ocean.

forest-mimicking technologies

Technologies that re-create natural functions of a forest; some examples: mulched swales, french drains, cisterns and rain barrels.

french drain

A shallow trench lined with a permeable surface collects allows rainwater to seep into the ground.

Functioning Community Forest

An urban community where local residents and businesspeople have joined together to transform their neighborhood into a sustainable ecosystem that functions like a healthy, natural forest.


global warming

The documented, historical increase in Earth’s surface temperature and projected increase predicted for the future. It is closely linked to the increase in greenhouse gas emissions produced by human activity.


Untreated household waste water which has not come into contact with toilet waste; includes: used water from bathtubs, showers, bathroom sinks and water from washing machines and laundry tubs; does not include: waste water from kitchen sinks, dishwashers, toilets or laundry water from soiled diapers

green design/sustainable design

To significantly reduce or eliminate the negative impact of buildings on the environment and on the building occupants, green building design and construction practices address: sustainable site planning, safeguarding water and water efficiency, energy efficiency, conservation of materials and resources, and indoor environmental quality. (U.S. Green Building Council definition)

greenhouse effect

A warming of the Earth’s surface temperature as a result of “greenhouse gases” (carbon dioxide, ozone, water vapor) accumulating in the atmosphere and trapping the sun’s radiation.

green waste

Biodegradable waste – fallen leaves, branches and hedge clippings, etc. – which is typically removed from yards and parks.


Water that saturates the soil at some distance below the surface, held in rocks and soil. See aquifer.

groundwater recharge

The process where by rainwater soaks down into the surface to replenish groundwater supplies.

growing medium

Any substance used for planting. This is almost always soil. Sometimes soil will be substantially amended with additives, fertilizers, and organic material. Substantially amended soil is commonly referred to as growing medium.



Portions of a property covered by buildings, pavement and other hard and impervious materials.

heat gain

Heat can slowly build up in an object over time. This is called heat gain. In a building, heat gain is most often the consequence of many hours of sunshine striking and warming the exterior walls and roof.

heat island effect

The increase in ambient temperature caused by a prevalence of heat-retaining buildings and paved surfaces. According to the U.S. Environmental Protection Agency, on hot summer days urban air temperatures can be up to 10ºF hotter than the surrounding countryside.



The absorption of surface water by the soil. Also called percolation.

infiltration field

An area particularly well suited and/or altered for directing storm water back into the soil.

impermeable pavement

Pavement that blocks water from soaking into the ground beneath it. Almost all asphalt and concrete pavement is impermeable.


LEED (Leadership in Energy and Environmental Design)

A rating system established by the U.S. Green Building Council to encourage and accelerate the global adoption of sustainable green building and development practices through the creation and implementation of universally understood and accepted tools and performance criteria. TreePeople’s Conference Center earned a platinum LEED rating (the highest).


An instrument used to measure water that percolates through soil.



A ground covering, especially of organic materials, that holds water, slows evaporation and enriches the soil. Raking up dropped leaves and branches takes away natural mulch and causes the soil to dry out, harden and become less healthy for the trees and plants in it.


one hundred-year storm

A probability-based measure of storm magnitude. On average, a 100-year storm can be expected to occur every 100 years. Similarly, a 50-year storm is expected to occur every 50 years, on average.



The absorption of surface water by the soil. Also called infiltration.

percolation rate

The rate at which water can filter into the soil. Some soil types, such as sand, have a very high percolation rate; other soils types, such as clay, have a very slow percolation rate.

permeable pavement

Permeable pavement is honey-combed with voids, or air-pockets. These voids allow water to migrate down through the pavement into the soil below.

potable water

Water that is fit to drink.


rain barrel

A barrel used to store rainwater harvested from rooftops and drains; the water can be saved for irrigating in dry weather.


Bringing back the healthy, natural functions of an ecosystem.


The reestablishment of a forest by planting or by natural regrowth.

retention grading

A method of sloping your land to create lowered areas that can retain and absorb rainwater.

riparian retention and treatment area

A retention or recharge area where plants native to rivers or lakes are installed to consume and clean the water therein.


Stormwater flowing across the surface of the earth. In urban environments, runoff becomes contaminated with pollutants as it flows across impermeable surfaces such as streets, roofs and parking lots.


service learning

A method of teaching and learning that combines curriculum-based instruction with meaningful service to the community.

shade trees

Trees large enough to shade a two-story building. In some climates, shade trees lose their leaves in the winter. Some evergreen trees are suitable shade trees, but they may shade the house or street during the winter when people would prefer to have the light and warmth of the sun.

sheet flow

Sheet flow is stormwater that flows in even sheets across a flat surface, such as a parking lot.

soaker hose

Soaker hoses are water conserving means of watering shrub beds especially. These hoses contain small perforations that allow water to flow gradually and continually onto the soil. They work particularly well with cisterns as they operate well with the low water pressures typically delivered by cisterns

storm drain

An opening along a road through which runoff flows and then enters an underground system leading to the ocean.


Rainwater that hits the surface of the earth. Stormwater can evaporate, percolate into the ground or flow across the surface to the nearest storm drain inlet, stream, or wetland area. If stormwater does not evaporate or percolate into the ground, it becomes runoff.


The soil layer below the "topsoil" layer.


Below the surface of the ground.


The ability for humans to maintain a successful long-term existence by taking measures to ensure that our natural resources are not depleted or irreparably damaged.


A natural or sculpted channel that slows runoff. Usually vegetated or covered with mulch, it can filter pollutants and increase aquifer recharge.


T.R.E.E.S. Project

Transagency Resources for Environmental and Economic Sustainability Project.


urban forest

The forest (trees, plants and soil) that exists in an urban area.

urban forestry

Growing and caring for the urban forest to maximize its benefits to humans, while protecting natural resources.



An organism, such as a mosquito or tick, that acts as a carrier of disease-causing microorganisms.



The area of land drained by a particular body of water. Also called a drainage basin. Watersheds help supply us with water by feeding underground aquifers or channeling water into rivers. Gravity moves water through a watershed from higher to lower areas.

watershed management

An integrated approach to protecting and restoring of a watershed, based upon a careful understanding of the relationships between the trees, people, land and water in it.