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Chapter 6: Restoring the Hydrological Cycle in the Urban Forest Ecosystem1

Lawrence V. Korhnak2


Forests provide a protective cover for the landscape and cycle much of the precipitation back to the atmosphere. They are essential components of many aquatic ecosystems. When native forests are removed and replaced with impervious surfaces and high maintenance vegetation, much of the water that would have been returned to the atmosphere or percolated into the groundwater, washes off the landscape. The quantity and energy of this runoff erodes landscapes, deteriorates aquatic habitat, and floods human habitat. In addition, the runoff washes away chemicals that have been concentrated on the land to support high maintenance vegetation. Polluted runoff, referred to as non-point source pollution, is our nation's most serious water quality problem. Reestablishing the urban forest can help to protect the landscape and associated aquatic ecosystems. Runoff can be reduced, use of polluting chemicals can be lowered, and aquatic habitat and ecosystem links can be reestablished.

Forest Water Cycle

Forest Water Cycle Overview

On average, two-thirds of precipitation entering U.S. forests is returned to the atmosphere through evaporative processes. Most of the remainder percolates through the porous forest soils to streams or fills underground geological storage space. Forests function as a protective layer and are a key link between the atmosphere and the land in the water cycle (Figure 1).

Figure 1. 

Forests are a key link in the cycle of water between the atmosphere and the land.

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The forest canopy intercepts both the falling rain and its kinetic energy. Some of the intercepted rainfall is evaporated to the atmosphere, while the rest drips to the ground as through-fall or runs down the trunk as stem-flow. Forest soils are generally very porous so little through-fall washes over the soil surface as runoff to water bodies. Instead, most of the through-fall seeps or infiltrates into the soil. The sun's energy evaporates water from inside the leaves in the canopy in a process called transpiration. Transpiration from the foliage creates a moisture deficit that is transmitted as a suction force all the way down to the tree roots. Much of the soil water is sucked up by plant roots to replace the water transpired from the foliage. Depending on the soils, geology, and other factors, some of the remaining soil water will percolate deeper, and some will move laterally into nearby streams.

Interception and Through-fall

Much of the rain falling on a forest landscape will first impact the canopy vegetation (Figure 2). Some will eventually drip to the ground and some will be evaporated from the vegetation back to the atmosphere. This evaporative loss is referred to as interception loss. The percentage of rainfall intercepted and evaporated by the forest canopy in the U.S. ranges from about 12%-48% of rainfall depending on the climate, tree type, and canopy structure. For example, interception losses of 12% were reported for mature hardwoods in the southern Appalachian mountains (Kimmins, 1997), 18% for pine flatwoods in Florida (Riekerk et al. 1995), 40% for ponderosa pine in Arizona, and 43% for a beachforest in New York (Kimmins 1997).

Figure 2. 

Much of the rain falling onto a forest landscape will first impact the canopy vegetation. Some will eventually drip to the ground, but on an annual average 12% to 48% will be evaporated from the vegetation back to the atmosphere.

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The kinetic energy of rainfall can cause significant soil erosion (Figure 3). A one-inch storm will deliver about 2 million foot pounds per acre of kinetic energy. Most of this energy can be adsorbed by the forest canopy and forest litter. Without this shield the rainfall energy will break up soil particles into smaller more easily transportable materials. Most of the splashed soil will move downhill. The fine particles resulting from the rainfall breakup of larger soil aggregates will clog soil drainage and result in more runoff. This can result in sheet flow and sheet erosion. This water energy will concentrate in small depressions called rills, which over time may develop into gullies. Left unchecked, erosion can carve canyons (Figure 4).

Figure 3. 

The kinetic energy of rainfall can cause significant soil erosion. A one inch storm will deliver about 2 million foot pounds per acre of kinetic energy. Much of this energy can be adsorbed by the forest canopy.


Andrew Davidhazy, Rochester Institute of Technology, School of Photographic Art and Sciences

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Figure 4. 

In Georgia at Providence Canyon State Park you can observe the severe erosion that can result from permanently removing the forest canopy from the landscape.

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One way researchers measure interception losses is to measure rainfall inputs into the forest (either above the canopy or in a nearby open area), and at the same time measure through-fall with collection devices (for example troughs and funnels) under the canopy (Figure 5). Interception losses are the difference between these two measurements. Interception is related to canopy leaf area, which can be measured with leaf fall traps.

Figure 5. 

Through-fall is measured with troughs and funnels placed under the canopy. The measurements are often correlated with canopy leaf area, which is estimated in this figure with leaf fall traps.

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Transpiration is the evaporation of water from within living plant tissue. Solar energy creates a water potential gradient by evaporating water through leaf openings called stomata (Figure 6). This gradient is transmitted to the roots where soil water is absorbed and transported to the foliage via the conductive network of xylem. Transpiration in the continental US ranges from about 30%-60% of precipitation and is a function of climate, vegetation type, and stand structure (leaf area). A Florida pine forest transpires almost a million gallons per acre in a year (Riekerk et al.1995).

Figure 6. 

Energy from the sun evaporates water from inside living plant tissue through openings called stomata. The guard cells can open and close the opening and provide some regulation of the process.


The Center for Microscopy and Micro Analysis

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The "Transpiration Pump" also helps to draw nutrients from the soil into the tree. Trees have been described as "solar powered chemical machines that mine the soil for minerals" (Figure 7). In addition to sucking up water, trees also draw in their required nutrients. For vigorously growing forests, trees will uptake about 100 kg/ha/yr of Nitrogen and 15 kg/ha/yr of Phosphorus (Kimmins 1997).

Figure 7. 

With the aid of the "transpiration pump" trees can remove significant amounts of nutrients from the soil.

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Transpiration is difficult to measure, but two methods are the sap flow gage and the leaf chamber. Sap flow is measured by applying a known heat source around the trunk of the tree and measuring the heat energy that is removed by the sap flowing up the trunk to replace transpired water (Figure 8).

The leaf chamber is a small transparent chamber that encloses the leaf and measures the moisture that enters and exits the chamber (Figure 9). The positive difference is transpired moisture. One major difficulty of both these methods is scaling up the measurements from individual trees and leaves to the forest.

Figure 8. 

A sap flow gage measures sap flowing up the tree trunk on its way to be transpired from the leaves.

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Figure 9. 

The leaf chamber measures water transpired from foliage enclosed in the chamber. Scaling these measurements up to the forest level is a challenge.

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The sun's energy will evaporate water from many of the components of the forest ecosystem. Often researchers will combine all the evaporative losses into one measurement, called Evapotranspiration (ET). Evapotranspiration includes transpiration, interception evaporation, soil evaporation, and water body surface evaporation (Figure 10). In temperate forest regions about 70% of the precipitation is returned to the atmosphere through evapotranspiration (Hewlett 1982).

Figure 10. 

Evaporation is a term used for the sum of all the evaporative water losses in a forest.

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Infiltration is the movement of water from the soil surface into the soil (percolation is the movement of infiltrated water through the soil). Generally, there is a lot of space between the soil particles in forest soils and this allows water to easily seep into the soil (Figure 11).

Figure 11. 

Water moves into the soil through both the small spaces between soil particles and the larger spaces between blocks of soil.

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For coarse to medium textured forest soils, the infiltration capacity is high and ranges from about 15 to 75 mm/hr (Brooks et al. 1991). Vegetation, both in the canopy and on the forest floor, protect the soil from compaction by rain energy. Forest floor vegetation, both alive and dead, prevents rain splash erosion from clogging soil pores with colloidal material (Figure 12). In addition, forest floor vegetation increases infiltration capacity by retarding surface flow, thus giving water more time to sink in. Raking the forest floor clean of vegetation, as is done in many urban parks, will reduce the ability of the forest to soak in rainfall and thus increase storm water runoff. Roots and old root channels also make the soil more pervious.

Figure 12. 

The live and dead vegetation on the forest floor serve important functions in the infiltration process.


Ken Clark

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Surface runoff in the forest landscape occurs when the rainfall (or through-fall) intensity exceeds the infiltration capacity of the soil and surface storage is full. Forest soils generally have infiltration capacities that exceed most rainfall events. So how does storm flow occur in the forest? Precipitation falling on the stream channel and saturated areas near the stream are the source of most early storm flow. As rain continues to fall, the saturated source area expands due to direct precipitation and infiltration, and from water infiltrating elsewhere and moving down slope. This expanding saturated variable source area contributes most of the storm flow to forest streams (Figure 13).

Figure 13. 

An expanding saturated source area contributes most of the storm flow to forest streams.

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One method scientists use to answer questions regarding the hydrological impacts of forest management is with paired watershed experiments. In this method the water outputs of similar drainage basins are measured with hydrological structures like flumes and weirs (Figure 14). Data are collected from the watersheds for several years before treatment in order to establish statistical relationships. Then the treatment is applied to one of the watersheds and the post treatment data is analyzed to determine if the statistical relationship changed in a significant way.

Figure 14. 

A weir is one type of structure used for measuring forest stream flow. It is an important tool for answering questions about the effects of land management on the hydrological cycle.


Hans Riekerk

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Seepage and Groundwater

Much of the water infiltrating into the soil supplies evapotranspiration demands. The remainder will seep down (percolate) until it hits a permeability barrier, for example, clay or rock, and then will move down laterally. Lateral seepage provides flow to streams in dry weather (base flow). In more permeable soils, seepage may move deeper down into porous geological formations, called aquifers. Depending on the geology, the groundwater may remain stored in the aquifer for less than a week or for over 10,000 years. In regions with dissolved limestone geology (karst) groundwater will often move down gradient in undergrounds rivers. When these underground rivers intersect surface openings they form springs. When they intersect openings in the ocean floor they form blue holes. Occasionally the pressure of the spring flow will force the water above the ground surface to form fountain-like artesian springs. Most of the earth's water is in the oceans, but over 99% of the liquid water associated with the land is groundwater. Groundwater is an essential resource for drinking water (Figure 15). In many areas of the country forest land is being bought to protect ground water supplies from pollution associated with other land uses. High quality groundwater is also important for growing the food we eat (Figure 16).

Figure 15. 

In much of the U.S. groundwater supplies critically needed drinking water. This photo shows groundwater returning to the surface as a spring and some of its surrounding forested catchment area. Springs keep many rivers flowing during periods of dry weather.

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Figure 16. 

Good quality groundwater is also important for irrigating and growing the food we need to eat.

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Impacts of Urbanization on the Water Cycle


Forests provide a protective cover for the landscape and cycle much of the precipitation back to the atmosphere. They are also essential components of many aquatic ecosystems. When native forests are removed and replaced with impervious surfaces and high maintenance vegetation, water that would have been returned to the atmosphere or percolated into the groundwater, washes off the landscape (Figure 17).

Figure 17. 

The urban landscape distorts and shortens the hydrological cycle.

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The percent of runoff increases almost in direct proportion to the impervious area. In addition, impervious surfaces prevent storage of water in the soil and urban activities often fill in natural water storage areas like flood plains and wetlands. The result is that increased amounts of water are delivered to water bodies in a shorter period of time. More water moving faster causes floods and erosion that damage both life and habitat (Figure 18).

Figure 18. 

The replacement of forest with urban impervious surface will degrade stream health. Source: Schueler 1992.

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Water washing over the urban landscape transports nutrients and other chemicals into aquatic ecosystems. This type of pollution is termed "non-point source," and it is our nations most serious water quality problem. Nutrients can stimulate algae production to the point where the ecosystem is no longer inhabitable by native organisms. Other pollutants have toxic effects on aquatic organisms and contaminate drinking water.

Forests are an integral component of many aquatic ecosystems. They provide water temperature moderation, support food webs, provide in-stream habitat, and stabilize stream banks. Breaking the forest ecosystem-aquatic ecosystem link will diminish the biological value of aquatic ecosystems.

Water Quantity Problems

Altering the Landscape Will Alter the Hydrology

Disturbing a forested landscape with agricultural and urban activities will alter the response of the landscape to precipitation events. Forests retain and evaporate most of the incoming precipitation (Figure 19). The hydrograph (graph of discharge over time) for the forest watershed reflects this lower and more gradual release of water (Figure 20).

Figure 19. 

In the forest water cycle, most of the precipitation is returned to the atmosphere and infiltrates into the soil. Flow to streams is slowed and moderated by the forest's complex structure.

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Figure 20. 

Water from the forest is released in lower amounts and more slowly compared to other land uses. Source: Beaulac and Reckhow 1982.

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In agricultural landscapes, heavy machines and livestock compact the soil. Compacting squeezes the soil particles closer together and reduces the soil pore space. With less pore space, rainfall will not soak into (infiltrate) the soil as well. A landscape with a reduced infiltration capacity will produce more runoff (Figure 21). The hydrograph will have a higher peak and because more water travels the faster surface route, the peak flow rate will occur earlier.

Figure 21. 

In the agricultural landscape, soil compaction results in less infiltration and increased runoff.



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In the urban landscape even more runoff will be produced faster because the soil is often highly compacted or covered with impervious surfaces (Figure 22). Impervious area distorts the hydrological cycle. Infiltration, storage, and transpiration are reduced and runoff increases in proportion to the percent impervious area (Figure 23). Urban impervious surfaces are designed to move water quickly off site. More runoff and less delay of runoff results in higher peak-flows and flooding. Figures 24, 25, and 26 show generalized changes in the water cycle resulting from different levels of impervious area in urban landscapes (EPA 1993a).

Figure 22. 

In the urban landscape, impervious surfaces produce more runoff in a shorter period of time.

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Figure 23. 

When forests are replaced with impervious surfaces, transpiration and infiltration are reduced and runoff increases in proportion to the percent impervious area. Source: Novotny and Olem 1994.

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Figure 24. 

In low density residential areas with 10 to 20% impervious area, evapotranspiration and groundwater account for most of the water loss.

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Figure 25. 

As the percent impervious area increases in higher density residential area outputs to evapotranspiration and groundwater are reduced and surface water runoff increases.

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Figure 26. 

Surface water predominates the water cycle in commercial and industrial areas.

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The Importance of Storage

In the forest water cycle, precipitation is captured and stored by the forest vegetation, forest litter, and soils. If preconditions are dry and the amount of rainfall is moderate, much of this water will be temporally stored and returned to the atmosphere through evaporative processes. Under wetter conditions there is less storage, and more rainfall may become stream flow. However, the complex structure of the forest landscape creates a tenuous path that delays the water's release from the land. This delay will result in more gradual stream inputs and a gentler rise in stream flow (Figure 27).

Figure 27. 

Storage of precipitation, in the forest canopy, litter, soil, and wetlands, is important for reducing flood hazards.

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In urban systems, the storage capacity of vegetation is reduced, soil compaction reduces soil storage space and impervious surfaces prevent rainfall from entering much of the soil altogether. Often flood plains, wetlands and other depressional storage sites are filled in, further reducing storage (Figure 28). As a result, more water reaches the stream in a shorter period of time.

Figure 28. 

In urban areas flood plains and wetlands are often filled in reducing hydrological storage. In addition, these areas near the water are often prime real-estate. These factors combine to set up conditions for destructive flooding events.

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Flooding and Aquatic Habitat Degradation

Flooding and erosion resulting from altered landscapes are serious concerns for human life and property. They also impact aquatic organisms and degrade their habitat. Impervious surfaces often form an effective conveyance system for rapid transport of runoff into urban water bodies such as streams. The quantity of stream flow is equal to the cross sectional area of the stream channel multiplied by the average stream velocity. To convey the additional runoff produced from disturbed landscapes, the cross sectional area of the stream and/or the stream velocity must increase. Streams increase their cross sectional area by rising up their banks, and many have natural flood plains for conveying runoff from extreme precipitation events. In the urban landscape, the flood plain may be filled in and built in, and flooding will occur (Figure 29).

Figure 29. 

Reduced storage, high runoff rates, and concentrated peak flows will often result in flooding in urban landscapes.

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The energy of water increases exponentially as its velocity increases. High energy urban stormwater runoff scours stream bottoms, and erodes and undercuts their banks (Figure 30). Stream side vegetation and aquatic habitat are washed away and conditions are set for destructive landslides.

Figure 30. 

High energy urban stormwater runoff scours stream bottoms, erodes and undercuts their banks. This degrades aquatic habitat and creates dangerous landslide conditions.

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Water Quality Problems

Non-Point Source Pollution

Increased runoff is not the only concern when the forested landscape is altered. Generally, forest ecosystems require little if any extraneous inputs of chemicals and disturbance is infrequent. On the other hand, to sustain agricultural and urban activities, nutrients, pesticides, herbicides, and energy producing chemicals are concentrated on the landscape. Urban impervious surfaces are associated with intensive land uses that generate pollution. They function as an efficient conveyance system for transporting pollutants directly to aquatic ecosystems, bypassing the pollutant removal functions of the soil (Figure 31).

Figure 31. 

Roads often function as an efficient system for transporting pollutants to aquatic ecosystems.

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Soil disturbance is frequent in agricultural and urban watersheds. Construction in urban watersheds removes the protective vegetative cover and erosion can produce 10 to 100 times more sediment than natural areas (up to 50,000 ton/km2/yr) (Novotny and Olem 1994) (Figure 32).

Figure 32. 

Pollution washed from altered landscapes is referred to as non-point source pollution. This aerial photo shows a sediment plume in a lake washed from upstream construction in an urban watershed.


Hans Riekerk

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Stormwater generated from urbanized landscapes will wash pollutants into aquatic ecosystems, often causing severe dysfunction (Figure 33). This type of diffuse pollution is called non-point source pollution. In contrast, point source pollution originates from focused sources such as the effluent from waste water treatment plants (Figure 34).

Figure 33. 

Stormwater runoff will wash many pollutants off urban impervious surfaces into aquatic ecosystems.

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Figure 34. 

Point source pollution often originates from waste water treatment plants and factories whose discharges are emitted at discrete, identifiable locations such as pipes and ditches.

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Much progress has been made in cleaning up point source pollution, but treating non-point source pollution problems are generally more difficult and costly. Non-point source pollution is responsible for the majority of the impaired use of our nations waters. Of the total pollution load to our nations waters, non-point sources contribute 90% of nitrogen, 90% of the fecal coliform bacteria, 70% of the oxygen demand, 70% of the oil, 70% of the zinc, 66% of the phosphorus, 57 % of the lead, and 50% of the chromium (Thompson et al. 1989).

Measurement of Non-Point Source Pollution

Different land uses have been measured to export different amounts of substances (Figure 35). Activities that increase runoff (such as soil compaction and paving), and activities that expose pollutants to washing off the land (such as over fertilization), will contribute to higher export rates. The exports are usually measured in kilograms leaving the land area (per hectare) for a year. These values are determined by measuring the quantity and quality of water leaving a known area of drainage basin.

Figure 35. 

The forest landscape exports much less pollutants than more intensive land uses.

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Typically, the first step in measuring the amount of water leaving a land area is to develop a stream height-discharge relationship (rating equation) for a stable section of the stream channel. On smaller streams the stream cross section is often modified into a more hydraulically uniform shape by a flume (Figure 36) or weir (Figure 37).

Figure 36. 

Flumes are flow modification structures designed to accurately measure the amount of water passing through them. They are self cleaning and can work with relatively low head loss, but they are very expensive.

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Figure 37. 

Weirs also measure flow, and they are less expensive than flumes. However, they dam up the water behind them which can cause many problems.

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Discharge is the product of cross sectional area of the stream channel multiplied by the average stream velocity. Depth measurements are taken along the cross section to calculate the area, and velocity measurements are taken at different depths at different locations to determine the average velocity. This process is repeated for a wide range of flow conditions, and the data are used to construct an equation that will estimate stream flow from stream height. These equations have been determined under lab conditions for weirs and flumes, but real world conditions will modify their flow characteristics, so on-site calibration is good practice.

Flow proportional sampling is required for an accurate determination of the amount of substance (for example, nitrogen or phosphorus) passing through the measurement station. This is accomplished by a microcomputer that reads the stream stage, calculates a flow from the rating equation, and activates an automated sampler to take a water sample when the specified volume of water has passed through the measurement section.

Here is a simple hypothetical export calculation. From a topographic map and an inspection of the watershed, the contributing area to a stream gaging station was determined to be 10 ha. The total water passing through the measurement channel for a year was 10,000 m3. The average total nitrogen concentration of the volume weighted samples was 5,000 mg/m3. The mass of nitrogen is calculated by multiplying the flow volume by the concentration. For this example:

10,000 m3 x 5,000 mg/m3 = 50,000,000 mg or 50 kg of nitrogen. Thus the land export was 50 kg/10 ha/yr or 5 kg/ha/yr.

From the perspective of the receiving water body, for example an urban lake, the land export is referred to as a load. The loading rates of the nutrients nitrogen and phosphorus into water bodies are one of the crucial factors that determine their biological and physical conditions. Proposed changes in land use in a lake's watershed can be used to predict the change in nutrient loads and the probable biological and physical impacts to the lake. Export and load information are used to guide watershed restoration efforts.


Nutrient loading of aquatic ecosystems causes eutrophication or nutrient enrichment. Symptoms of eutrophication may include decreased water clarity, algal blooms, nuisance growth of macrophytes, unpleasant taste and order, dissolved oxygen depletion, fish kills, and altered species diversity and richness (Figure 38) (National Academy of Sciences 1969).

Figure 38. 

Nutrients washed from high maintenance urban landscaping may stimulate algae growth and distort system ecology. In severe cases the resulting environmental changes will make the ecosystem uninhabitable to native species.

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Nutrients in urban storm water runoff are the leading source of impairment of our nation's estuaries (EPA 1996). Developmental stresses pose a serious threat to the health of these productive and complex ecosystems (Figure 39). By the year 2010 almost half of the U.S. population will live near coastal waters, and the population of many coastal cities is predicted to triple in the next 15 years (EPA 1996). Nutrients imported into estuarine watersheds to sustain high maintenance landscapes are washing into the estuaries and disrupting ecological relationships. For example, nitrogen from fertilizers can stimulate dense growth of algae that will shade out sea grass. Sea grass is critical spawning and nursery habitat for much of our seafood (Figure 40).

Figure 39. 

Increasing development in our coastal areas will result in more storm water runoff making an already serious problem worse.

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Figure 40. 

Fertilizers in storm water runoff can destroy critical habitat for many of the species that provide us delicious seafood.


Philip by Greenspun, M.I.T.

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Nutrients are essential for the existence of both terrestrial and aquatic ecosystems but the level of nutrients will play a major role in determining the character of the ecosystem. When urban storm water washes excess nutrients into an aquatic ecosystem, the nature of the ecosystem will change. This human influenced process of nutrient enrichment of aquatic ecosystems is called cultural eutrophication. In severe cases the resulting environmental changes may make the ecosystem uninhabitable to native species.

Most often the root of the problem is excessive inputs of the critical plant nutrients, nitrogen and phosphorus. When one or both of these nutrients limit plant growth, additional inputs will stimulate aquatic weed and algae growth. The aquatic plant community often provides the primary source of organic carbon energy and forms the foundation of the ecosystem. Changes in this critical component of the ecosystem will have system wide impacts.

Often the impacts are undesirable. Algal blooms will decrease water clarity. This lowers the recreational and aesthetic value of the water body. If the water body is an important drinking water supply, algal blooms may impart a bad taste and odor to the water and clog treatment systems. In addition, dense algal blooms will shade out submerged aquatic plants. These aquatic plants are important breeding and nursery grounds for many sport and food fish. Conditions in highly nutrient rich water bodies favor filter and bottom feeding fish. These will multiply to the detriment of many other species and reduce the species diversity of the ecosystem. Aquatic ecosystems, especially shallow ones and those with low flushing rates, tend to keep and recycle the nutrients they obtain. Therefore, it is difficult and expensive to restore many impacted water bodies.


Urban storm water can reduce dissolved oxygen levels in aquatic ecosystems by reducing the dissolved oxygen holding capacity, by stimulating algae respiration with nutrients, and by stimulating microbial respiration with organic carbon sources (Figure 41).

Figure 41. 

Urban storm water can reduce dissolved oxygen levels in aquatic ecosystems by reducing the dissolved oxygen holding capacity, by stimulating algae respiration with nutrients, and by stimulating microbial respiration with organic carbon sources.

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The oxygen holding capacity of water is a function of the water temperature. Specifically, colder water can contain more oxygen than warmer water. For example, water at 3°C can contain 13 mg/l of dissolved oxygen while water at 35°C will only hold 7 mg/l of dissolved oxygen. In an urban system, water from heated buildings, hot streets and roofs can raise the temperature of water bodies. Removal of trees that shade urban streams will also raise water temperatures. To compound the problem, elevated water temperatures will often increase the metabolic rate of cold blooded aquatic organisms, thus increasing their need for oxygen.

Nutrients, especially phosphorus and nitrogen, can stimulate increases in algae populations. When there is adequate sunlight and inorganic carbon, algae will produce large amounts of oxygen during photosynthesis. In fact, oxygen levels may actually climb above saturated levels in a system with high densities of algae during bright sunlight. However, at night or during extended cloudy periods, the algae will remove large amounts of oxygen from the water for their metabolic needs. Under extreme conditions, the algae can deplete the dissolved oxygen supply and fish kills will occur (Figure 42). This is most common under conditions where diffusion of oxygen from the atmosphere into the water is impaired, such as when the water is covered with ice or when the water column is prevented from mixing due to thermal stratification.

Figure 42. 

Under certain conditions, high levels of algae can deplete oxygen in water resulting in fish kills.

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Algae and fish are not the only competitors for dissolved oxygen in aquatic ecosystems. Aquatic bacteria will feed on organic materials washed into water bodies. They convert oxygen into carbon dioxide in a biochemical process similar to our metabolism of food. When large amounts of organic materials are washed into a water body, bacterial growth and metabolism can be stimulated to the point that their consumption of oxygen will exceed system inputs. For many bacteria, when the oxygen is used up they can make use of alternate oxidants such as nitrate, and oxidized forms of manganese, iron, and sulfur. Unfortunately, many higher level aquatic organisms are dependent on dissolved oxygen, and when it is depleted they will die. Also, certain chemicals, for example ammonium, will combine with dissolved oxygen and make it unavailable. The oxygen depleting properties of pollution are often measured as Biochemical Oxygen Demand or BOD. BOD is determined by measuring the oxygen loss of a water sample in a sealed bottle kept in the dark for five days.

Aquatic Habitat Alteration

Even if urbanization had no impact on water quality and quantity, there are often other severe impacts on aquatic life. In many urban areas the physical structure of aquatic habitats are modified for municipal functions to the detriment of biological functions. Trees removed from stream banks expose the stream to less moderated temperature conditions (higher in the summer, colder in the winter) (Figure 43).

Figure 43. 

In a forested stream, trees moderate water temperatures, support food webs, provide stream habitat and stabilize the banks.

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Removing trees also removes an important source of fuel for detrital food webs. During urbanization, stream channels are straightened, large woody debris removed, and even the bottom substrate may be covered with pavement (Figure 44). These types of modifications remove critical stream habitat and sterilize the aquatic ecosystem's ability to support aquatic life. In extreme cases, urban streams are "blacked out" by enclosing them in pipes and covering them up.

Figure 44. 

In many urban streams the forest has been removed and the aquatic ecosystems that they supported can not exist.


Judy Okay

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Restoring the urban forest can help to restore the hydrological cycle and improve the functioning of aquatic ecosystems. Significantly increasing tree canopy coverage will reduce stormwater runoff and peak flow, and increase the water storage capacity. Urban forests are particularly critical near creeks, streams, and rivers, where they act as riparian forest buffers (Figure 45).

Figure 45. 

Urban forests are particularity critical near creeks, streams, and rivers, where they act as riparian forest buffers. Forested riparian areas stabilize banks, uptake nutrients, and provide shade, habitat, and food for aquatic ecosystems.

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Forested riparian areas stabilize banks, uptake nutrients, and provide shade, habitat, and food for aquatic ecosystems. The magnitude of chemicals used to support high maintenance urban landscapes is overwhelming our efforts to treat polluted runoff. Programs that encourage landscaping with native forest trees can help because these trees will often require less inputs of chemicals and water. Urbanization alters and fragments aquatic ecosystems, sometimes so severely that they cease to function. More environmentally orientated planning can prevent the problem, and reforestation is often the key element in restoring the system.

Increasing Tree Coverage

Increasing or preserving tree coverage in an urban watershed can have water quantity and quality benefits. However, the scale of the restoration effort needs to match the scale of the problem. A small urban park, even one with a big tree will do little to restore the water cycle to a big city (Figure 46). Larger scale efforts are usually needed. Storm water modeling with CITYgreen© software (American Forests 1996) demonstrates the scale of coverage needed with its expected water quantity benefits (Figure 47).

Figure 46. 

Increasing or preserving tree coverage in an urban watershed can have water quantity and quality benefits. However, the scale of the restoration effort needs to match the scale of the problem.

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Figure 47. 

Computer models such as CITYgreen© software (American Forests 1996) can demonstrate the value of the ecological services that trees provide.


Illustrations from CITYgreen

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Their model predicts increasing tree coverage on an example residential development will reduce storm water runoff and save money. With a 30% tree cover the model predicts a 5 % decrease in runoff volume, a 9% decrease in peak flow and a 15 acre feet/square mile increase in water storage. Potential storm water storage treatment savings were estimated to be about $120,000/square mile. When the tree canopy coverage is increased to 70%, the model predicts a 17% decrease in runoff volume, a 27% decrease in peak flow and a 48 acre feet/square mile increase in storage. Potential storm water storage treatment savings were estimated to be about $390,000/square mile.

There are some issues that must be considered when evaluating the water quantity and quality benefits of tree cover. The first is the timing of benefits. Storm water engineers must design new developments so that they meet hydrological specifications for the first storm, not how the development will respond many years later when the canopy has grown to significant coverage. The development must also continue to meet hydrological specifications in winter when deciduous trees have lost their cover. Storm water engineers also know that canopy storage will be quickly filled by the large storms that cause flooding events. However, canopy storage can reduce the runoff of the frequent smaller storms, and thus has the potential to reduce pollutant loading to aquatic ecosystems.

Riparian Forest Buffers

Riparian forest buffers have the potential to reduce the amount of runoff and pollutants washing into riparian ecosystems. They also stabilize stream banks and moderate water temperatures. Preserving or restoring forested riparian buffers also preserves some of their ecological functions such as providing terrestrial and aquatic habitats, and supplying the source for detrital food webs. Many forested riparian areas also contain flood plains and wetlands that provide additional water quantity and quality benefits. Forested riparian buffers are aesthetically beautiful areas and can provide some forms of low impact recreation.

There are three functional zones comprising a well designed forested riparian buffer (Figure 48). Zone 3 is a flat grassy area about 10m wide at the urban-buffer interface (Figure 49). Its major function is to convert channelized urban flow into sheet flow and slow water velocity to less than 0.3 m/sec. Zone 3 performs some settling, filtering, and infiltration.

Figure 48. 

There are three functional zones comprising a well designed forested riparian buffer. The zones are designed to spread out and infiltrate storm water, assimilate nutrients, and preserve the aquatic habitat.

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Figure 49. 

Zone 3 is a flat grassy area about 10m wide at the urban/buffer interface. Its major function is to convert channelized urban flow into sheet flow and slow water velocity to less than 0.3 m/sec. Zone 3 performs some settling, filtering, and infiltration.


Photos are of the "Difficult Run" urban riparian project, courtesy of Judy Okay, Virginia Department of Forestry.

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Zone 2 is a vigorously growing forest with a width of 15 to 150m (Figure 50). The required width depends on the load amount and the buffer slope, soils, vegetation and level of allowed disturbance. The major function of Zone 2 is to provide the environment and contact time (at least 9 minutes) for pollutant removal through sedimentation, filtration, cation exchange, and plant uptake. In forest and agricultural situations, selective removal of trees from Zone 2 is recommended. Tree removal removes nutrients and keeps the forest in a vigorous growth stage.

Figure 50. 

This photo was taken down slope from Figure 49 and shows the establishment of a zone 2 managed forest. The left side is at planting and the right side is three years after planting.


Judy Okay, Virginia Department of Forestry.

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Zone 1 is the mature forest at the land-water interface and it controls the physical, chemical, and trophic status of the stream (Figure 51). Zone 1 should be at least 10m wide. The major water quality functions of Zone 1 are to stabilize the stream bank and to shade and stabilize water temperatures. Anoxic (without oxygen) organic soils in this zone can remove nitrogen by the process of denitrification, but uptake of other nutrients may be balanced by litter fall. Zone 1 also provides detritus for the aquatic food web and large woody debris for critical aquatic habitat.

Figure 51. 

Zone 1 is the mature forest at the land/water interface. It most directly controls the physical, chemical, and trophic status of the stream.


Judy Okay, Virginia Department of Forestry.

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Forested riparian buffers have their limits (Herson-Jones et al. 1995). Pollutant removal effectiveness is poor when the slopes are greater than 10% and with soils that have infiltration rates less than 0.64 cm/hour. Disturbance (many recreational activities) will greatly reduce their effectiveness. The scale of the buffer needs to match the scale of the source area. Poor performance can be expected with high rates of channelized flow from large impervious areas. Upstream Best Management Practices (BMPs) may be required to scale the load to match the buffer's capacity. Even under good conditions total suspended solid removal is estimated to be 50%.

Source Control

The United States has 30 million acres of lawn. On these lawns over 100 million tons of fertilizer and 80 million pounds of pesticides are applied annually (Borman et al. 1993) (Figure 52). This rate of application is ten times the rate chemicals are used per acre on US farms. The importation and concentration of chemicals in urban watersheds saturates and overwhelms our efforts to treat polluted non-point source runoff. In an effort to reduce harmful impacts to our aquatic ecosystems, many new programs are focused on reducing the sources of non-point pollution. These programs encourage landscaping that uses and exports less water and chemicals. Some examples of these types of programs are BayScaping in the Chesapeake Bay area (, Nature Scaping in the Portland, Oregon area (, Florida Yards and Neighbors (, and EPA's Green Communities (

Figure 52. 

Over 100 million tons of fertilizer and 80 million pounds of pesticides are applied annually to U.S. lawns.

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The general strategy of these programs is to encourage landscaping that uses less pollutants and produces less runoff. Native vegetation and ground covers are recommended because they generally require less inputs of water and chemicals (Figure 53). In addition, exotic landscaping vegetation can escape and cause hydrological and other ecosystem problems (Figures 54 and 55) (See Chapter 9-Invasive Plants).

Figure 53. 

This is an example of a yard that uses native trees and low maintenance ground cover. Native trees are often adapted to local conditions and require less supplemental inputs of water, fertilizer, and pesticides. In this example the trees also provide pine needles for an attractive and low maintenance ground cover.

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Figure 54. 

Exotic landscape plants can require more water and chemicals and contribute to urban water pollution. In addition, they can invade and damage natural ecosystems. The Salt Cedar (Tamarix sp.) shown on the right in the above photo (Zion National Park) has invaded much of the Southwest altering hydrology and displacing native plants.

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Figure 55. 

Salt Cedar has roots that can reach depths of 30 meters and individual trees can use 800 liters of water per day. Large stands of Salt Cedar can lower the ground water below the level that native vegetation can reach. They also adsorb salts from deeper soil layers and ground water and transport it to their leaves (see above photo). This salt increases the soil salinity above levels that many native plants can tolerate.

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The reduction of impervious surfaces by using gravel driveways (Figure 56) and on-site retention landscaping (Figure 57) are examples of practices that will reduce the export of water and pollutants.

Figure 56. 

Reducing the impervious surfaces at a home by having an attractive gravel driveway instead of an impervious paved one, will significantly reduce the amount of water and pollutants that runoff property. The cumulative impact of many citizens reducing their pollutant load can make the restoration of aquatic ecosystems possible.

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Figure 57. 

Large stormwater treatment facilities often have poor pollutant reduction performance. A better solution is to keep stormwater on site and allow it to be filtered by the soil. This picture shows a "rain garden" where runoff from the roof and driveway will be retained and pollutants filtered out by the soil.


Judy Okay, Virginia Department of Forestry.

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Aquatic Habitat Improvement

The impact of urbanization on aquatic ecosystems goes beyond the damage caused by increased runoff and poor water quality. Frequently, urbanization degrades the physical aquatic habitat by altering its morphology, changing or even paving the bottom substrate, and altering light inputs. Intakes for domestic water supplies and dams will drastically disrupt stream continuity. Aquatic systems are parts of larger ecosystems. Poor urban planning can break links to other systems that provide essential functions to aquatic systems. For example, filling in wetlands and flood plains can eliminate breeding and nursery habitat, and removing upland forests eliminates an important source of energy for detrital food webs. Conversely, forested aquatic ecosystems provide essential elements for upland ecosystems and they often function as crucial corridors necessary for the survival of many species.

Figure 58 shows an urbanized stream that would not function with even the best water quality. Stream morphology has been drastically altered, the bottom substrate paved over, and stream-side communities have been eliminated.

Figure 58. 

Even with the best of water quality this urbanized stream will be a non-functioning ecosystem. The stream morphology has been altered, the bottom substrate paved over, and stream communities have been eliminated.

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In Figure 59 important stream habitat has been restored by importing large woody debris directly into the stream. Large woody debris provides important nesting, cover, and substrate for aquatic life. Stream vegetation has been replanted to provide shade for cooler and more stabilized water temperatures and to provide detritus for food webs.

Figure 59. 

In this stream, important habitat has been restored by importing large woody debris directly into the stream. Large woody debris provides important nesting, cover, and substrate for aquatic life. Streamside vegetation has been replanted to provide shade for cooler and more stabilized water temperatures, and to provide detritus for food webs.

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Engineering is necessary for a city to function properly. Many cities are discovering that with a little extra care, engineering functions can be combined with ecological principles to provide functioning aquatic habitats. For example, retention ponds are used in urban areas to provide storage for increased runoff and to settle out particulate pollutants. Although the pond in Figure 60 may perform some of those functions, it provides little if any aquatic habitat. On the other hand, the detention pond in Figure 61 incorporated wetlands and forests to provide ecological functions as well as engineered treatment of urban storm water.

Figure 60. 

Retention ponds are used in urban areas to provide storage for increased runoff and to settle out particulate pollutants. Although this pond may perform some of those functions, it provides little if any aquatic ecosystem habitat.

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Figure 61. 

On the other hand, this pond was designed to be a functioning ecosystem.

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Urban parks also provide an opportunity for aquatic habitat restoration or preservation. Often urban parks contain a significant amount of impervious area and high maintenance vegetation that can cause degradation of associated aquatic habitat (Figure 62). With careful design forested urban parks can provide recreational opportunities as well as a functional aquatic habitat (Figure 63).

Figure 62. 

Figures 62 and 63 are parks in Mt. Dora, Florida. Although this traditional urban park provides needed recreation activities, the natural habitat has been paved or grassed, and the water features only provide limited aesthetic value.

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Figure 63. 

Nearby Palm Island Park, also at Mt. Dora, Florida, has been left as an intact ecosystem. A board walk allows people to explore the upland/wetland/aquatic wonders with little negative impact to the hydrological cycle and the ecosystems dependent on it.

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This document is FOR95, one of a series of the School of Forest Resources and Conservation Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date August 2001. Revised February 2008. Reviewed November 2012. Visit the EDIS website at


Lawrence V. Korhnak, Senior Biological Scientist, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, PO Box 110410, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.