Defining Criteria for the Creation and Maintenance of Riparian Buffers: A Review
Heidi Moltz, Steve Ventura, and Cynthia Stiles
Email: hlmoltz@wisc.edu
Phone: 608-358-4136
Address: 1150 E. Johnson St. #3
Madison, WI 53703
ABSTRACT: Developers of new rules for agricultural non-point source pollution in the state of Wisconsin
sought information on the efficacy of and maintenance requirements for riparian buffers. A literature review
was conducted to learn about: 1) the implications of different buffer designs and vegetation mixes, 2) factors
that compromise established buffers, and 3) long-term buffer maintenance. Buffer establishment practices are
site specific, but trends in the literature provide expected outcomes based on location, size, preparation, and
planting. Maintenance will affect the ability of a buffer to efficiently filter nutrients and toxins for extended
periods of time. Optimizing the factors that enhance and sustain toxin degradation, erosion control, infiltration,
and high net nutrient uptake will aid in the creation of a successful buffer. If riparian buffers are to become a
real component of land use planning, it appears that there must be economic incentives. However, little
quantitative evidence exists about specific on-farm and social costs and benefits of implementing riparian
buffer. This paper offers observations from a broad array of findings that may influence programs designed to
encourage buffer installation and that could influence individual land owners or managers who wish to take
advantage of potential buffer benefits.
Keywords: riparian buffer, filter-strips, literature review
INTRODUCTION
The “Wisconsin Buffers Initiative” (WBI) is a multi-disciplinary science-based effort to develop on site
agricultural non-point source pollution mitigation rules for Wisconsin.
Riparian buffers appear to be an
important option for controlling loss of sediments and nutrients from fields to receiving bodies of water. Many
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questions about the suitability, installation, maintenance, efficacy, and economics of agricultural buffers have
arisen in discussions within an advisory body representing a broad range of agricultural and environmental
interests, regulatory agencies, and scientists. Some of these questions are being addressed through a series of
interlinked research projects at the University of Wisconsin-Madison. Remaining questions, particularly those
related to installation, maintenance, and costs and benefits have been addressed through an extensive review of
the existing literature on buffers. This paper provides a summary of key findings from this literature review.
Although the impetus has been the Wisconsin setting, many of the observations are derived from research
across the nation. Examples may be oriented to the upper Midwest, but the underlying principles are more
broadly applicable.
The primary focus of this review is riparian buffers, though the WBI and others recognize that these
components are part of a comprehensive strategy for nutrient and sediment control. Lowrance et al. (2002)
recommends a combination of in-field, edge-of-field, and streamside buffers. Combining different buffer types
can help control both local and regional nonpoint source pollutants.
Although buffers seem to operate
effectively in mitigating the effects of agriculture and industry, the best way to reduce contamination to our
water sources is to control the generation of pollutants at their source (Barling and Moore 1994).
Purpose of Riparian Buffers
Riparian buffers placed between agricultural lands and waterways can have an important impact on
water quality, quantity, stream bank health, and local biota including fish, birds, insects, amphibians, reptiles,
and small mammals (Schultz et al., 1995c).
Gilliam (1994) called buffers “the most important factor
influencing non-point source pollutants entering surface water.” Many researchers agree with Gilliam that
riparian buffers are a viable option for reducing sediment runoff from fields and removing nutrients and
pollutants from surface and groundwater (e.g., Dosskey, 2001; Osborne and Kovacic, 1993; Schultz et al.,
1995a,b,c; Schultz et al. 1997; Lowrance et al., 1984). In addition, buffers can stabilize stream banks (Dosskey,
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2001; Zaimes et al., 2004; Schultz et al., 1997), affect local fauna (Allan et al., 1997; Naiman and Decamps
1997; Schultz et al. 1995b), moderate flooding, help recharge underground water supplies, and provide land
owners with valuable biomass, timber, and nut crops (Schultz et al., 1997).
BUFFER EFFECTS
After the installation of plants between agricultural fields and a waterway, buffers begin to influence
physical, chemical, biological and ecological processes. The extent and rate of effects depends on local
conditions, buffer types, and maintenance procedures.
Physical and Chemical Processes
Benefits from buffers accrue primarily from their ability to trap and retain sediments and nutrients. As
an indicator of particle trapping, an increase in soil organic matter (SOM) is possible after a relatively short
amount of time following buffer installation. SOM increases the ability of buffer soils to process non-point
source pollutants (Marquez et al., 1998, 1999). Within two to three years of buffer installation, run-off
reductions have reached optimal levels, although reductions in run-off volume have been noted soon after buffer
installation (Udawatta et al., 2002). According to Bharati et al. (2002), optimum conditions occur in an
established multi-species riparian buffer after six years, based on using bulk density as a proxy for infiltration.
Riparian buffers have the ability to filter groundwater nitrate (NO3-) effectively through conversion of
NO3- to N2 gas (denitrification) (Hill, 1996; Dhondt et al., 2002; Fennessy and Cronk, 1997; Hedin et al., 1998;
Jacobs and Gilliam, 1985). According to Fennessy and Cronk (1997), this process depends on NO3- loading,
carbon availability from litter, and moisture status. Accordingly, the ability of a riparian buffer strip to support
denitrification varies strongly with vegetation composition, soil type and pH (Groffman et al., 1991). Riparian
zones have a greater affect on groundwater NO3- if the groundwater interacts with vegetation and organic rich
soil (Hill, 1996). This can be accomplished by choosing hydrogeologic systems where a shallow groundwater
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flow system channels water directly through the riparian buffer at relatively low velocities that allow for
denitrification to occur (Simpkins et al., 2002).
The amount of nitrogen (N) accumulation and release also varies temporally and spatially (Steinheimer
et al., 1998; Dhondt et al., 2002). Denitrification occurs throughout the year as long as subsurface hydrology is
active, whereas plant uptake of N is limited to seasonal removal (Fennessey and Cronk, 1997). Dhondt et al.
(2002) reported that denitrification is increased in all seasons by ambient conditions which increase water
retention and biotic activity in the root zone.
Riparian buffer strips have also been shown to effectively protect streams from phosphorus (P) pollution
(Reed and Carpenter, 2002; Cooper and Gilliam, 1987). According to Cooper and Gilliam (1987), buffers can
serve as sinks for P in several ways, including sorption of P from the throughflow water by soil and sediments,
deposition of enriched sediment, and plant uptake of P. If plants are harvested and removed from the site, P can
be removed and exported from the system. Reed and Carpenter (2002) showed the importance of riparian
buffers as moderators of P flux from upland agricultural lands into streams. In this study, up to 50% of the P
leaving agricultural fields appeared to have been removed from the runoff water in riparian areas. Cooper and
Gilliam (1987) found total P in the sediments near streams greater than the concentration in nearby fields.
When plants are senescing, buffers could be sources of nutrients instead of sinks. Dillaha et al. (1989)
found that soluble nutrients in filter outflow are sometimes greater than the incoming soluble nutrient load,
presumably due to lower removal efficiencies for soluble nutrients and the release of nutrients previously
trapped in the filters. Phosphorus concentrations in flood plain sediments have been reported to be two to three
times higher than concentrations in the overlying water during high flow stages of winter and spring. Soluble P
can be released from the buffer sediments during these periods of high flow (Cooper and Gilliam, 1987). For
example, forested ephemeral channels have little vegetation and are effective sediment sinks during the dry
season but are ineffective during large storm events because of the small resistance to flow (Daniels and Gilliam
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1996). According to Barling and Moore (1994), the potential for flushing sediment and phosphorus during
large runoff events and the resultant impact on downstream ecology is largely unexplored.
According to Cooper and Gilliam (1987), riparian areas can function as long-term sinks for total P even
though soluble forms are released during periods of increased discharge. Mineralization of biologically bound
P and N and subsequent transport to the stream channel also occurs seasonally. The buffer strip acts as a
nutrient sink for much of the year, but also releases accumulated nutrients during the remaining portion of the
year. Periodic harvesting of plant biomass at critical times of the year may reduce the amount of P released
during the dormant season (Osborne and Kovacic, 1993).
Plants can reduce pesticide contamination of soil or water through direct metabolism, stimulation of
microbial activity in the root zone, and extraction of contaminated water. Some plants have the ability to
metabolize pesticides and other toxins into non-toxic secondary metabolic products while others have the ability
to filter the pesticides out of the water and store them by sequestration (Karthikeyan et al., 2004). Pesticides
strongly absorbed to soil particles will tend to remain in the sediment of the riparian zone, where they may be
absorbed by vegetation and organic debris over long periods of time.
Biological and Ecological Processes
The installation of riparian buffers affects more than the physical actions of run-off, erosion, and
nutrient filtering. Biological and ecological processes in the riparian zone are also affected by buffers.
Lowrance et al. (2002) proposed that it may take between 10 and 15 years for a buffer to become as fully
functional as a native riparian ecosystem in removing pollutants, although improvements compared to cropland
are immediately noticeable. Trees require more time than shrubs and ground cover to produce desired effects
such as woody habitat creation and filtration of groundwater (Belt et al. 1992).
Biological processing in buffers is not only a function of the vegetation, but also of the microbial
community within the undisturbed soils of the buffers. Schultz et al. (1995c) notes that microbial processes in
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buffers are important for the reduction of nonpoint source pollution. Microbes will assimilate and immobilize
pollutants, though their rapid turnover and relatively small biomass make the microbes themselves a minor sink.
The ability of buffer systems to detoxify and process excess nutrients into less available forms is influenced by
the diversity and vigor of the microbial community.
Ecological communities in and near buffers are influenced by buffer type and efficacy. In streams
surrounded by buffers, Stauffer et al. (2000) and Stewart et al. (2001) found that local factors dominate regional
or landscape level factors in regards to fish community composition. The shade provided by riparian trees and
shrubs affects stream temperature, an important habitat criterion for salmon, trout, and char (Belt et al., 1992).
Sediment reduction from buffers may also be significant. Zimmerman et al. (2003) found a 98% decrease in
“lethal” concentrations of suspended sediment for fish with an increase in conservation tillage, riparian buffers,
and permanent vegetation cover.
The vegetation of riparian areas provides habitat for many different levels of faunal communities. For
example, when compared with an uncut control area and clearcuts, riparian buffer strips have the highest bird
species richness and diversity (Triquet et al., 1990). Filter strips have been found to provide better habitat for
birds than crop species like corn (Gillespie et al., 1995). Insectivorous birds also increased in number in
riparian areas due to a concordant increase in insect populations in riparian zones (Whitaker et al., 2000).
Buffers may enhance movement and act as corridors for juvenile birds while maintaining the movement rates of
adults (Machtans et al., 1996). Grazing in riparian areas can provide habitat for birds, including grassland bird
species of management concern such as the Savannah Sparrow, Eastern Meadowlark, and Bobolink (Renfrew
and Ribic, 2001).
Amphibians and reptiles actively use the lands adjacent to waterways, making buffers a necessity for
sustainable populations. Core terrestrial habitat ranges from 159 to 290m for amphibians and from 127 to 289m
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for reptiles from the edge of the aquatic site (Semlitsch and Bodie, 2003). Turtles lay eggs in the sandy areas
adjacent to waterways making riparian zones important for the continuance of the turtle life cycle.
Chapman and Ribic (2002) found that buffer strips supported a particularly rich and abundant small
mammal community in southwestern Wisconsin. Total small mammal abundance is greater near the stream
than away from the stream. Intensive Rotational Grazing (IRG) increases small mammal abundance indirectly
by causing an increase in the prevalence of pasture in the agricultural landscape.
Guidelines for Creating and Maintaining Riparian Buffers
To ensure that benefits from pollution reduction and ecological enhancement accrue, the design,
installation, and maintenance of buffers should be planned. The most important decisions in design and layout
of buffers involve spatial configuration and species selections. Installation and maintenance requires additional
thought to ground preparation, vegetation establishment and management, associated flora and fauna, sediment
and pollutant fluxes, equipment, and annual cycles.
Layout and Establishment
While the general concept for a riparian buffer is a simple strip or strips adjacent to a waterway, there
are numerous design combinations that may provide more effective function for specific sites (Schultz et al.,
2004). A buffer should be created with its intended use in mind. Preliminary results from the Wisconsin
Buffers Initiative indicate that, if the goal is minimizing sediment delivery, a strip running along the edge of a
field may be effective for diffuse flow patterns while a dense, strategically-located buffer may be the most costeffective way to reduce sediment transport at critical points of flow concentration more typical of rolling and
hilly terrain. Large blocky buffers can be used to provide optimal wildlife habitat and groundwater clean up
(Lowrance et al. 2002), optimized by the “corridor” nature of the buffer strip design.
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Buffer strips may have different widths that can be adapted to fit each site and land setting. Standards
for buffer width are usually set on a “rule-of thumb” basis which is unable to reflect regional variations in
physical conditions (Xiang, 1993). A literature search performed by Castelle et al. (1994) found a range of
buffer widths from 3 to 200 m to be effective, depending on site-specific conditions and buffer goals.
According to Schultz et al. (1995a), if a buffer strip is designed for sediment removal, a width of 50 feet may be
sufficient on slopes of 0-5%. If nutrient removal is also important, a width of 66-100 feet would be necessary.
If the slope, intensity of land use, or total area of land producing nonpoint source pollution increases, or as soil
permeability decreases, a larger area is required for a buffer. Table 1 provides an overview of suggested buffer
widths.
Alberts et al. (1981) found that reductions in sediment and nutrient discharges with increasing length
and percentage cover of the residue strips were almost proportional. Fennessy and Cronk (1997) report that a
buffer zone 20 to 30 m wide can remove up to 100% of incoming NO3-. Jacobs and Gilliam (1985) found that
buffer strips of greater than 16m were effective for inducing significant losses of NO3- before drainage water
reached the stream. Other research suggests that a narrow near-stream region is functionally the most important
location for NO3- consumption by denitrification (Hedin et al., 1998).
Vegetation
The usefulness of the different plants for use in buffers depends on four main criteria: 1) their ability to
filter nutrients and sediments, 2) adaptation to the local environment 3) metabolism, uptake and tolerance of
pesticides and other toxins (Karthikeyan et al., 2004), and 4) potential for economic benefits from the plants.
Trees and Shrubs
The tap and feeder roots of woody plant such as trees stabilize nearly vertical streambanks while
providing a large nutrient sink that can be removed for fiber products via the aboveground biomass. They also
shade streams and provide large woody debris for habitat and channel control (Schultz et al., 2004). Rapidly
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growing trees with large canopies and high transpiration rates are well suited as biological pumps for pesticides
in the groundwater (Karthikeyan et al., 2004).
Forested buffer areas favor NO3- removal in areas with
subsurface flow in contrast to grass buffer areas (Fennessy and Cronk, 1997) even in winter months because
herbaceous vegetation has no active role in retaining NO3- in the winter (Haycock and Pinay, 1993). Contrary
to this, Komor and Magner (1996) found no evidence that trees close to streams that take up groundwater
through their roots also take up NO3- from the groundwater. However, Tomer and Burkart (2003) noted that
determining ground water quality responses to new agricultural practices may take decades in some watersheds
and that effects of buffers would take summarily as long to evaluate.
Shrubs may be added to multi-species buffers because of their permanent root systems and because they
add biodiversity and understory wildlife habitat (Schultz et al., 2004; Schultz et al., 1997; Lee et al., 2000). The
root systems also provide bank stability to relatively steep banks. The multiple stems of shrubs provide an
excellent trap for flood water debris and some detritus for the stream ecosystem while providing little shade for
prairie streams (Schultz et al., 2004). The multiple stems found on shrubs also function to slow flood flows.
Deep rooted woody plants are effective in trapping fine textured sediments and soluble nutrients (Lee et al.,
2000).
Grasses
Grasses and herbaceous forbs provide bank stability, trap sediment from surface runoff, provide
significant organic C to soil, improve soil structure and provides wildlife habitat (Schultz et al., 2004; Lyons et
al., 2000; Nerbonne and Vondracek, 2001). A benefit of the grass component in buffer strips is the ability of
grass to increase hydraulic roughness because of greater stem density, subsequently decreasing flow velocity
and sediment carrying capacity (Osborne and Kovacic, 1993). For this reason, grassy riparian areas may be
more effective in reducing bank erosion than wooded areas (Lyons et al., 2000; Nerbonne and Vondracek,
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2001). In addition, grass buffers also allow for greater access to the stream, if necessary (Osborne and Kovacic,
1993).
In the upper Midwest, switchgrass (Panicum virgatum), reed canary grass (Phalaris arundinacea), tall
fescue (Festuca arundinacea), and soft rush (Juncus effusus) are often integral members of a grassy buffer zone.
A minimum of 20 feet of switchgrass or reed canary grass is recommended because it produces a uniform cover
and has dense, stiff stems that provide a highly frictional surface to intercept surface runoff containing coarse
sediment and sediment-bound nutrients (Schultz et al., 1995c; Lee et al., 2000). Other permanent warm season
grasses, such as Indian grass and big bluestem, and native perennial forbs may also be included in the
community mixture (Schultz et al., 1995a), particularly if palatable grasses are desired for planned grazing.
Multi-Species Riparian Buffers
Multi-species riparian buffer (MSRB) strips of trees, shrubs, and grasses are well suited to the
agroecosystems of the Midwest and eastern Great Plains (Isenhart et al., 1995). Schultz et al. (1997) calls these
multi-component buffers “the most effective riparian buffer strip.” Buffers containing dense, stiff, native warm
season grasses and woody vegetation improve the efficacy of the system to removal non-point source pollutants
from agricultural areas (Lee et al., 2003). Although the construction of MSRBs can take several forms,
scenarios created by Isenhart et al. (1995), Schultz et al. (1995b), and Schultz et al. (1995c) call for a three-zone
construct similar to a natural riparian community. The first zone (closest to the stream) is comprised of a multispecies community containing several rows of rapidly growing trees like willows (Salix spp.) or cottonwoods
(Populus deltoides). The second zone contains one or two rows of shrubs and the third zone (farthest away
from the stream) contains native, warm-season grasses. Schultz et al. (1997) suggest that a well designed buffer
may also include plantings that stabilize the streambank along with wetlands constructed at field tile outlets to
treat drainage water.
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Non-native and Invasive Plants
When selecting plants for a riparian buffer, it may be beneficial to consider the implications of planting
non-native and/or invasive species because of their ability to significantly alter the physical environment.
Aggressive invasive species tend to consistently exceed natives and less invasive species in biomass
productivity, standing crop, length of growing season, standing leaf area, leaf longevity, and total chlorophyll.
In some cases, invasive species are also known to accrue nutrients more efficiently (Farnsworth and Meyerson,
2003).
The success of invasive species has been attributed to the ability to displace other species by direct
competition. In some cases, species that grow at the same rates when planted alone have significantly different
growth rates when planted in communities with other plants.
These enhanced growth characteristics in
situations of competition are common to some invasive species (Barrat-Segretain and Elger, 2004). Other
studies have shown, however, that it is not always the case that invasive populations should out-compete
ecologically similar natives in a common environment (Bossdorf et al., 2004). In highly disturbed environments
such as edges of agricultural fields, invasive species can dominate. If the only goal of a riparian buffer is
nutrient and sediment control, this may be a desirable trait. If multiple goals are identified, including habitat
creation for native plant and animal species, invasive species must be controlled.
Maintenance
Maintaining the effectiveness of riparian buffers requires management plans. Guides to buffer creation
and maintenance procedures, including details such as types of ground preparation and appropriate spacing
between plants, have been presented by Blinn and Kilgore (2001); Lee et al. (2004); Olaughlin and Belt (1995).
Maintenance plans requires consideration of the following: 1) geomorphological characteristics of the
watershed; 2) vegetation community composition; 3) site access conditions, including seasonal limitations; 4)
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spacing and arrangement of plants; and 5) availability of appropriate equipment and labor. This section
provides a review of some key guidance criteria appropriate for the development of buffer management plans.
Intensity of buffer maintenance depends on the planned function of the system, with some systems
requiring more care than others. For example, high diversity, low biomass buffers may require continuing
management and are difficult to create (Weiher et al. 1996) while forested buffers may only require harvesting
every few years. Most of the chemical transformation and sediment retention occurring in a buffer happens
within the near surface soil/sediment interface with plant tissue. Buffers with dense stands of vegetation and
large volumes of decaying plant litter will thus have a greater capacity for pollution removal. Management
techniques that accelerate vegetation establishment or litter buildup improve chemical retention and favor
degradative processes (Schultz et al., 1995a; Jin et al., 2002; Lowrance et al., 2002).
Vegetative communities in buffers require different management tasks. Maintenance of grassy riparian
vegetation usually requires active management like mowing, burning, herbicide treatments, and grazing.
Otherwise successional processes will tend to ultimately favor woody vegetation (Lyons et al., 2000). After the
first five years, the grass zone of a buffer can, and probably should, be cut or burned on an annual or biannual
basis. Regular removal of biomass promotes the dense upper plant and root re-growth needed to improve soil
quality and permeability, and thus enhance the ability of the buffer to filter pollutants. If the grasses cannot be
harvested, some of the biomass can be removed by short, controlled grazing, using fences to restrict livestock
access to the stream (Schultz et al., 1997). In multi-species riparian buffers, tree and shrub rows should be
mowed once or twice during the season to maintain defined the planting rows and to discourage rodent
problems (Schultz et al., 1995c). Periodic tree harvesting is also necessary to keep forests highly productive
where net nutrient uptake is high (Lowrance et al., 1985). Uptake of N and P by young trees is significantly
higher than in older forests (Mander et al., 1997). The fast growing trees should be harvested every 8-12 years.
If harvesting is done with a minimum of soil disturbance during the winter or dry season, it will have little
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detrimental effect on pollution control by riparian systems (Lowrance et al., 1985; Gregory and Ashkenas,
1990).
Weed control is extremely important during the first few years of establishment. During this early cycle,
the buffer should be inspected frequently and appropriate herbicides or mowing used if needed (Schultz et al.,
1995c). Mowing is useful for areas that cannot be burned or sprayed and have large populations of competing
annual weeds. Weeds can also be sprayed with appropriate selective, short-lived herbicides to keep growth at a
minimum. Two main options for spraying, according to Schultz et al. (2002), are backpack spraying and
general broadcast spraying. Backpack units are particularly useful for areas where precision is needed or heavy
equipment can not be used. These also minimize pesticide presence in the system. General broadcast should be
limited to situations where large areas are to be treated.
Regularly scheduled maintenance should begin immediately after the buffer has been planted (Schultz et
al., 1995a; Schultz et al., 1997). Inspection of the buffer system should occur at least once annually to
determine if the buffer is functioning within the original plan. Inspection must also take place after major storm
events to determine if there is damage and to ascertain repair requirements. These repairs should be completed
as soon as possible to maintain proper buffer functions (Schultz et al. 2002).
Pesticides, concentrated flow patterns, and excessive nutrient loading can negatively influence a buffer’s
ability to function properly. Pesticide laden runoff can kill buffer vegetation and harm overall functioning by
selectively impairing or eliminating portions of the biotic community. Karthikeyan et al. (2004) provided an
extensive review of non-target plant sensitivity to agriculture pesticides. Where pesticide contamination is
likely, tolerant species need to be chosen for the riparian buffer. For example, atrazine applied at the labeled
rate to row crops would kill the C3 grasses such as brome grass but would only slightly stunt the C4 grasses such
as big and little bluestem (Karthikeyan et al. 2004).
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Concentrated flow of water through riparian buffers can be substantial and may greatly limit the filtering
effectiveness (Dosskey et al., 2002; Osborne and Kovacic, 1993; Boyd et al., 2003). By diffusing and retaining
agricultural runoff, a buffer’s capacity to remove sediments and nutrients increases (Qiu and Prato, 2001). High
volume flows commonly overwhelm grass and multi-species riparian filters next to cultivated fields (Cooper et
al., 1987), the latter because preferential flow channels can rapidly develop under high flow volumes. The
sediment trapping performance of a riparian buffer decreases as the sediment particle size decreases as nutrients
are often preferentially attached to fine sediment. The result is that buffer strips are better filters of sediment
than of nutrients and buffer strips are most effective when flow is relatively low volume and enters the buffer
strip uniformly along its length (Barling and Moore, 1994). Table 2 shows a summary of flow characteristics
and impacts.
ECONOMICS
Buffers are obviously helpful to ecosystem health and land management in the long-run, but a decision
on whether individuals install them will generally depend on cost and benefits accruing to land owners or
managers, not society as a whole. If riparian buffers in agricultural lands in general are to become a reality, onfarm benefits will need to outweigh the costs, including whatever social incentives are provided to farmers.
Thus, a brief overview of economic issues is pertinent.
On-Farm Costs and Benefits
The costs of implementation and management have been compiled or estimated for several
circumstances, but the ranges are quite wide because of differences in buffer layout and development, and in
assumptions about land costs, foregone production, and capital investments. For example, Isenhart et al.
(1995), Schultz et al. (1995a), and Schultz et al. (1995c) estimate the installation cost of a three zone multispecies riparian buffer to be $875 per hectare in 1995 dollars ($350 per acre). This includes plant purchases,
site preparation, planting, labor, and maintenance costs in the first year. They project that for three to four
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years, annual buffer maintenance should be around $50 per hectare in 1995 dollars ($20 per acre). On-farm
benefits include grazing, logging, and feedlots. The probability of buffer creation increases when on-farm profit
exceeds the costs outlined above. For example, land owners can use buffers to generate revenue from haying,
grazing and logging. Buffers may also provide cost avoidance compared to more expensive alternatives to
meeting water quality, nutrient management, or erosion control statutes.
Grazing in riparian buffers requires more intense management scheduling. Although grazing generates
near-term on-farm economic benefits, care should be exercised as continuous grazing causes extensive
degradation of riparian habitats (Chapman and Ribic, 2002). Grazing in the riparian zone should be limited
during the times when streambank soils are moist and most susceptible to compaction and collapse, particularly
during early spring following snow melt and early spring rains. Grazing pressure should be controlled to allow
desirable plants time to re-grow and compete with undesirable species (Schultz et al., 1995c; Clary and
Leininger, 2000), and to allow effective buffer sediment control. Primary variables that influence sediment
filtration are predominantly stem density and to a lesser extent surface random roughness. Gillen et al. (1991)
report that cattle grazing can reduces stem density by 40%. Monitoring stem density should aid in regulating
cattle use of riparian areas (McEldowney et al., 2002).
Intensive rotational grazing (IRG), which is more compatible with buffer objectives, has been proposed
for some Wisconsin settings. Lyons et al. (2000) evaluated riparian IRG as a stream rehabilitation practice. In
this study, bank erosion, fish habitat characteristics, trout abundance, and an index of biotic integrity were
compared along 23 trout stream reaches in southwest Wisconsin influenced by different land management
practices. Amongst the observed management practices, IRG was found to have the least bank erosion and fine
substrate in the channel. For this reason and others, well managed rotational grazing may be a practical use of
or alternative to buffer strips (Paine and Ribic, 2002).
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Animal feedlots are required to control polluted runoff and riparian buffers may find good use in these
settings. Several reports suggest that vegetative filter strips are effective for the removal of sediment and other
suspended solids contained in the surface runoff from feedlots if runoff is low-volume and uniform across the
drainage. Effectiveness of the filter strip decreases with sediment accumulation within the filters (Dillaha et al.,
1988; Fajardo et al., 2001). It is noted, however, that coliform bacteria are not adequately removed in the runoff
by the actions of riparian buffers (Fajardo et al. 2001). In particular, grass may not be an equally effective
control for fecal bacteria as it is for soil erosion in surface runoff (Coyne et al., 1995). Although it may
minimize sediment loss and fecal bacteria, a grassy filter strip does not reduce fecal contamination from feed
lots sufficiently to meet existing water quality standards (Coyne et al., 1998). Comparisons of feedlot runoff
control with buffers versus other approaches have not been reported.
Logging of buffer strips changes riparian vegetation to earlier successional stages (Idaho Forest,
Wildlife and Range Policy Analysis Group). Hardwood tree species can provide additional income to farm
owners as well as site protection (Gillespie et al. 1995).
Social Costs and Benefits
Although buffer adoption decisions may hinge on benefits and costs of buffers to individual land-owners
or managers, buffers create economic costs and benefits for society as a whole. The social aspects of buffer
economics are not widely documented. According to Klapproth (1999), the social benefits of buffers are
extensive. The benefits include reducing the costs of water treatment, reducing flood damage to communities
and croplands, and improving the quality of groundwater supplies, commercial fisheries and agriculture by
reducing the amount of sediment, nutrients, and other contaminants that reach the streams and lakes.
The cost of reducing soil erosion with riparian buffers is lower when buffers are installed in
conventionally tilled fields and the costs of buffers are comparable to the costs of no-till (Nakao and Sohngen,
2000). Other estimates suggest that using a land rental rate of $210 per ha ($85 per acre) in 1993 dollars, a filter
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strip program costs $100 per metric ton ($91 per ton) of sediment abated (Pritchard et al. 1993). Williams and
Nicks (1993) suggest that buffers are more cost effective on some drainage areas than on others. They found
optimum drainage areas ranging from 2.4 and 8.1 ha (0.96 and 3.24 acres respectively). In addition, the
economic value of buffers is significantly affected by some soil properties such as texture and drainage, stream
length, and cropland percentage in the watershed (Qiu and Prato, 2001).
Because of the dearth of information about societal costs and benefits, it will be important for any buffer
program to devise and incorporate on-going evaluations. On the benefit side, this will include measurements
and modeled estimates of the pollution control provided by buffers, and qualitative estimates of habitat
improvement in both riparian corridors and affected streams. Costs potentially borne by public agencies include
research and development of technical standards and best management practices, design and other
implementation assistance, incentives required to enlist participation in buffer programs, and costs of inspection
and enforcement of mandatory regulations if implemented.
CONCLUSION
Riparian buffers, if implemented and maintained properly, can serve many different functions including,
but not limited to, 1) maintaining biodiversity, 2) reducing nonpoint source pollution, and 3) stabilizing
streambanks. Effective functioning of buffers will depend on several factors, including location, size, species
composition, and maintenance. The wide variety of reported observations about these factors suggests that
some region-specific field research, such as the Wisconsin Buffers Initiative, will be necessary to answer
questions related to buffer design and development under differing biophysical conditions and buffer efficacy
goals.
Vegetation can vary in its ability to filter N and P, pesticides, and other toxins. Woody plants and trees
are most successful at filtering groundwater nutrients and pollutants while grasses are more effective at filtering
surface run-off. In addition, some plants are more suited to the commonly flooded riparian environment.
17
Once planted, a riparian buffer needs to be managed to maintain effectiveness. Techniques should be
selected based on site-specific conditions and the desired impacts of the buffer. These techniques are oriented
to maintaining healthy vegetation, eliminating sources of degradation such as flow that is too concentrated or
laden with pesticides. Some management methods may have direct economic benefits.
If riparian buffers are to become a reality, economically feasible buffers are necessary. On-farm
incentives for buffers include grazing, logging, and aiding in the reduction of pollutants from feed lots to meet
water quality standards. These uses have the potential to outweigh the on-farm costs of implementation and
maintenance. Little present research is available describing the social costs and benefits of riparian buffers, but
on-site evaluations and surveys over time will provide valuable future reference data.
Acknowledgements
This research was supported by special appropriation through the United States Department of Agriculture Natural Resources Conservation Service, thanks to the special interest in agriculture and the environment from
U.S. Senator Herb Kohl.
About the authors
Heidi Moltz is a graduate research assistant in the Water Resource Management Program of the Gaylord Nelson
Institute for Environmental Studies at the University of Wisconsin-Madison. Stephen Ventura is professor of
Environmental Studies and Soils Science, chair of the department of Soil Science, and director of the Land
Information and Computer Graphics Facility at the University of Wisconsin-Madison.
Cynthia Stiles is
assistant professor of Soil Science (Pedology) and affiliate faculty with the Gaylord Nelson Institute for
Environmental Studies at the University of Wisconsin-Madison.
18
Investigator
Alberts et al. (1981)
Castelle et al. (1994)
Fennessy and Cronk (1997)
Hedin et al. (1998)
Jacobs and Gilliam (1985)
Schultz et al. (1995)
Schultz et al. (1995)
Buffer
Width
1.8m to 4.6m
>15m
20-30m
"Narrow"
<16m
>15m
20-30m
Slopes
5%
0-5%
0-5%
Benefit
Sediment and nutrient discharges were reduced proportionally
with increasing length and percent cover.
Protects streams under most conditions
Nitrate removal
Nitrate removal
Nitrate removal
Sediment removal
Nutrient removal
Table 1: Benefits provided with diverse buffer widths.
19
Investigator
Location
Vegetation
Measured Parameters
Reduction of sediment
and pesticide transport
with surface runoff
Boyd et al. (2003)
Central Iowa
Brome Grass
Daniels and Gilliam
(1996)
North Carolina
Grass, Forested,
Multi-species
Dillaha et al. (1989)
Virginia
Orchardgrass
Nutrient and sediment
removal
Sediment, nitrogen, and
phosphorus removal from
runoff
Dosskey et al. (2002)
Nebraska
Grass
Percent sediment removed
from field runoff
Table 2: Summary of flow characteristics and impacts.
20
Conclusion
For low flows, runoff, sediment,
and pesticide transport was
effectively reduced
High volume flows commonly
overwhelmed grass and riparian
filters.
Concentrated flow should be
minimized for effectiveness.
Concentrated flow through
riparian buffers may greatly
limit filtering effectiveness
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25