Hydrology

Water is a limited resource that is essential to all life.  Under natural conditions, rain falls on vegetation, drips and filters into the soil which recharges ground water supplies and slowly flows into creeks and rivers.  In contrast, human-altered landscapes are often designed to treat water as a waste product, with the goal of moving it offsite as quickly as possible.  Conventional drainage systems typically deliver larger volumes of water to streams in shorter amounts of time, leading to increased downstream flooding, erosion, water quality degradation and fewer opportunities to enjoy the aesthetic and recreational benefits of streams and lakes. The effects of these human modifications on landscapes can affect regional and even global ecology.   

Did you know?

According to the U.S. Environmental Protection Agency (U.S. EPA), the demand for water is up 209 percent since 1950 and is rising.1  The U.S. EPA also found that approximately 30 percent of the water used daily by a family of four in the U.S. is devoted to outdoor uses such as watering lawns and gardens.2

Half of irrigation water can be wasted as a result of evaporation, wind and over-watering, but weather-based irrigation systems can reduce irrigation water use by 20 percent or 24 billion gallons per year.3

The Natural Resource Defense Council found that New York State experienced 1,280 beach closure or advisory days in 2006, many due to sewer systems being overwhelmed by stormwater runoff.4

Current Practices

Disturbing or removing soils and vegetation can severely diminish or even eliminate the interception, evapotranspiration and infiltration capacity of the landscape.  Disturbed soils and vegetation are commonly covered with impervious surfaces, on which up to 95 percent of rainfall becomes runoff.5  When impervious surfaces cover more than 10 percent of a watershed, it degrades the physical quality of streams and stormwater.6,7  Even lawns and landscape beds, which are generally viewed as more permeable than pavements and roofs, experience soil compaction during construction and installation that can result in reduced infiltration.8,9  For example, a residential lawn can be up to 40 percent impervious cover.10  Disturbance of soils and vegetation can result in dramatic increases in the rate, volume, duration and frequency of runoff, increased pollutant loadings of stormwater and reduced groundwater quantity and quality.

Conventional drainage and stormwater management systems are traditionally designed to move water offsite quickly, causing flooding, erosion, scoured stream banks, increased pollution and sediment and fewer opportunities to enjoy the benefits of streams and lakes.11  Stream channel degradation and increased sedimentation can impact aquatic habitat. 

Contaminated stormwater runoff from developed land is the leading cause of water quality problems12 and accounts for 70 percent of urban water pollution.13  Runoff from developed areas can contain oil, grease, excessive nutrients, pathogens such as. E. coli and hepatitis A, persistent bioaccumulative toxic (PBT) chemicals and heavy metals.  For example, land use changes throughout the Mississippi River watershed have caused significant increases in surface runoff and nutrient delivery to the northern Gulf of Mexico.  This leads to changes in dissolved oxygen levels that have caused fish and shrimp catches to drop to zero.14

Inefficient water use, at a time when the demand for water is rising (up 209 percent in the US since 1950),15 reduces the supply of drinking water and the flow in streams and lakes.  Most sites are designed to use potable water only once before discharging untreated or partially treated water.  In addition, high-quality municipal drinking water is often used for purposes that may not require potable water, such as lawn and garden irrigation.  Because compaction affects the infiltration rates of lawns, the water used to irrigate lawns commonly does not seep into the water table and ends up as runoff or evaporation.  Thirty-six states anticipate local, regional or state-wide water shortages in the next five years.16

As we recognize the value of water, we must also recognize the value of natural systems to store, clean and distribute available fresh water.  We have technology to integrate systems that mimic nature’s capacity to store, filter and clean water.

Examples of Sustainable Hydrology Practices

Balance your water budget
A water balance calculates inputs and outputs of water on a site.  To achieve water balance, the inputs — precipitation, surface flow and piped-in water supply — equal outputs — evapotranspiration, runoff and water that infiltrates into soil.  A site should identify goals for the post-development water balance based on the historic condition and local or regional issues of concern. 

Incorporate water infiltration into the site design
Install a rain garden or small vegetated catchment areas which filter rainwater and increase groundwater recharge by capturing excess water.  

Reuse water
Utilize rainwater, gray water and wastewater for on-site non-potable water needs, such as landscape irrigation, cleaning outdoor surfaces and water features.  

Clean and slow the flow of water to protect and enhance down stream water bodies.
Water treatment methods for rainwater runoff include dry wells, vegetated swales instead of curb and gutter systems, vegetated filter strips and infiltration facilities. 

[1] U.S. Environmental Protection Agency. Why Water Efficiency? WaterSense: Efficiency Made Easy [webpage] 2007 [cited 30 Aug 2007]; Available from: www.epa.gov/WaterSense/water/why.htm.
[2] Ibid.
[3] U.S. Environmental Protection Agency. Outdoor water use in the United States. (EPA Pub 832-F-06-005) Department of the Interior, Editor. 2007
[4] Natural Resources Defense Council. Pollution prompts beach closings to double along New York and New Jersey coasts.  2007  [cited 7 Aug 2007; Press release]. Available from: http://www.nrdc.org/media/2007/070807b.asp.
[5] Anacostia Restoration Team. Watershed Restoration Handbook, Department of Environmental Programs: Metropolitan Washington Council of Governments, Editor. 1991.
[6] Klein, R.D., Urbanization and Stream Quality Impairment. Water Resources Bulletin, 1979. 15(4): p. 948-963.
[7] Schueler, T.R., Site Planning for Urban Stream Protection, Department of Environmental Programs: Metropolitan Washington Council of Governments, Editor. 1996.
[8] Kelling, K.A. and A.E. Peterson, Urban lawn infiltration rates and fertillizer runoff losses under simulated rainfall. Soil Science Society of America Proceedings, 1975. 39(2): p. 348-352.
[9] Pitt, R., S. Chen, and S. Clark. Infiltration Through Compacted Urban Soils and Effects on Biofiltration Design. in Low Impact Development Roundtable Conference Proceedings. 2001. Baltimore, MD.
[10] Anacostia Restoration Team, Watershed Restoration Handbook. 1991
[11] Low-Impact Development: An Integrated Design Approach. Programs and Planning Division Department of Environmental Resources, Editor. Prince George's County, Maryland, 1999. 
[12] U.S. Environmental Protection Agency, Nonpoint Source Pollution:  The Nation's Largest Water Quality Problem, Department of the Interior, Editor. 1996.
[13] Loizeaux-Bennet, S., Stormwater and Nonpoint-Source Runoff:  A Primer on Stormwater Management. Erosion Control, 1999. 6(7): p. 56-69.
[14] National Science and Technology Council, Integrated Assessment of Hypoxia in the Northern Gulf of Mexico, Committee on Environment and Natural Resources, Editor. 2000. 
[15] U.S. Environmental Protection Agency. Why Water Efficiency?
[16] Ibid