Riparian Buffer Functions and Benefits

 Last updated: 7/17/14

This list includes resources for understanding the functions and benefits of riparian buffers, especially those functions and benefits seen in different widths (for example, a 50 foot versus a 300 foot buffer).

Baker, ME, Weller, DE, Jordan, TE. 2006. Improved methods for quantifying potential nutrient interception by riparian buffers. LANDSCAPE ECOLOGY, 21 (8):1327-1345.

Abstract: Efforts to quantify the effects of riparian buffers on watershed nutrient discharges have been confounded by a commonly used analysis, which estimates buffer potential as the percentage of forest or wetland within a fixed distance of streams. Effective landscape metrics must instead be developed based on a clear conceptual model and quantified at the appropriate spatial scale. We develop new metrics for riparian buffers in two stages of increasing functional specificity to ask: (1) Which riparian metrics are more distinct from measures of whole watershed land cover? (2) Do functional riparian metrics provide different information than fixed-distance metrics? (3) How do these patterns vary within and among different physiographic settings? Using publicly available geographic data, we studied 503 watersheds in four different physiographic provinces of the Chesapeake Bay Drainage. In addition to traditional fixed-distance measures, we calculated mean buffer width, gap frequency, and measures of variation in buffer width using both “unconstrained” metrics and “flow-path” metrics constrained by surface topography. There were distinct patterns of relationship between watershed and near-stream land cover in each physiographic province and strong correlations with watershed land cover confounded fixed-distance metrics. Flow-path metrics were more independent of watershed land cover than either fixed-distance or unconstrained measures, but both functional metrics provided greater detail, interpretability, and flexibility than the fixed-distance approach. Potential applications of the new metrics include exploring the potential for land cover patterns to influence water quality, accounting for buffers in statistical nutrient models, quantifying spatial patterns for process-based modeling, and targeting management actions such as buffer restoration.

Carline, RF, Walsh, MC. 2007. Responses to riparian restoration in the Spring Creek watershed, central Pennsylvania. RESTORATION ECOLOGY, 15 (4): 731-742.

Abstract: Riparian treatments, consisting of 3- to 4-m buffer strips, stream bank stabilization, and rock-lined stream crossings, were installed in two streams with livestock grazing to reduce sediment loading and stream bank erosion. Cedar Run and Slab Cabin Run, the treatment streams, and Spring Creek, an adjacent reference stream without riparian grazing, were monitored prior to (1991–1992) and 3–5 years after (2001–2003) riparian buffer installation to assess channel morphology, stream substrate composition, suspended sediments, and macroinvertebrate communities. Few changes were found in channel widths and depths, but channel-structuring flow events were rare in the drought period after restoration. Stream bank vegetation increased from 50% or less to 100% in nearly all formerly grazed riparian buffers. The proportion of fine sediments in stream substrates decreased in Cedar Run but not in Slab Cabin Run. After riparian treatments, suspended sediments during base flow and storm flow decreased 47–87% in both streams. Macroinvertebrate diversity did not improve after restoration in either treated stream. Relative to Spring Creek, macroinvertebrate densities increased in both treated streams by the end of the posttreatment sampling period. Despite drought conditions that may have altered physical and biological effects of riparian treatments, goals of the riparian restoration to minimize erosion and sedimentation were met. A relatively narrow grass buffer along 2.4 km of each stream was effective in improving water quality, stream substrates, and some biological metrics.

Clinton, BD, et al. 2010. Can structural and functional characteristics be used to identify riparian zone width in southern Appalachian headwater catchments? CANADIAN JOURNAL OF FOREST RESEARCH, 40 (2): 235-253.

We characterized structural and functional attributes along hillslope gradients in headwater catchments We endeavored to identify parameters that described significant transitions along the hillslope On each of four catchments, we installed eight 50 m transects perpendicular to the stream. Structural attributes included woody and herbaceous vegetation: woody debris and forest floor mass. nitrogen (N) and carbon (C), total soil C and N. litterfall amount and quality by species: and microclimatic conditions Functional attributes included litter decomposition, soil microarthropods. soil CO2 evolution, soil solution chemistry, and soil extractable N Forest floor mass, N and C, and soil depth increased with distance from the stream and transitioned between 10 and 20 m In contrast, litterfall N rate (kilograms of nitrogen per hectare per day), downed woody debris, soil A-horizon C and N. and soil solution NO3 concentration all decreased with distance, and exhibited significant transitions. Certain overstory species were more abundant in the uplands than near the stream Herbaceous diversity and richness were similar across the hillslope, but species distributions varied in response to hillslope moisture content. Taken together, these results suggest that at 10-20 m from the stream, transitions occur that separate riparian from upland conditions and may provide valuable insight into riparian zone definition

Costello, D,Lamberti, G. 2008. Non-native earthworms in riparian soils increase nitrogen flux into adjacent aquatic ecosystems. OECOLOGIA, 158 (3): 499-510.

Abstract: Riparian zones are an important transition between terrestrial and aquatic ecosystems, and they function in nutrient cycling and removal. Non-native earthworms invading earthworm-free areas of North America can affect nutrient cycling in upland soils and have the potential to affect it in riparian soils. We examined how the presence of earthworms can affect riparian nutrient cycling and nutrient delivery to streams. Two mesocosm experiments were conducted to determine how (1) the biomass of earthworms and (2) earthworm species can affect nutrient flux from riparian zones to nearby streams and how this flux can affect streamwater nutrients and periphyton growth. In separate experiments, riparian soil cores were amended with one of four mixed earthworm biomasses (0, 4, 10, or 23 g m−2 ash-free dry mass) or with one of three earthworm species (Aporrectodea caliginosa, Lumbricus terrestris, L. rubellus) or no earthworm species. Riparian soil cores were coupled to artificial streams, and over a 36-day period, we measured nutrient leaching rates, in-stream nutrient concentrations, and periphyton growth. Ammonium leaching increased with increasing biomass and was greatest from the A. caliginosa treatments. Nitrate leaching increased through time and increased at a greater rate with higher biomass and from cores containing A. caliginosa. We suggest that the overall response of increased nitrate leaching [90% of total nitrogen (N)] was due to a combination of ammonium excretion and burrowing by earthworms, which increased nitrification rates. During both experiments, periphyton biomass increased through time but did not differ across treatments despite high in-stream inorganic N. Through time, in-stream phosphorus (P) concentration declined to <5 μg l−1, and periphyton growth was likely P-limited. We conclude that activities of non-native earthworms (particularly A. caliginosa) can alter biogeochemical cycling in riparian zones, potentially reducing the N-buffering capacity of riparian zones and altering stoichiometric relationships in adjacent aquatic ecosystems.

DeWalle, DR. 2010. Modeling Stream Shade: Riparian Buffer Height and Density as Important as Buffer Width. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2): 323-333.

A theoretical model was developed to explore impacts of varying buffer zone characteristics on shading of small streams using a path-length form of Beer's law to represent the transmission of direct beam solar radiation through vegetation. Impacts of varying buffer zone height, width, and radiation extinction coefficients (surrogate for buffer density) on shading were determined for E-W and N-S stream azimuths in infinitely long stream sections at 40 degrees N on the summer solstice. Increases in buffer width produced little additional shading beyond buffer widths of 6-7 m for E-W streams due to shifts in solar beam pathway from the sides to the tops of the buffers. Buffers on the north bank of E-W streams produced 30% of daily shade, while the south-bank buffer produced 70% of total daily shade. For N-S streams an optimum buffer width was less-clearly defined, but a buffer width of about 18-20 m produced about 85-90% of total predicted shade. The model results supported past field studies showing buffer widths of 9-11 m were sufficient for stream temperature control. Regardless of stream azimuth, increases in buffer height and extinction coefficient (buffer density) were found to substantially increase shading up to the maximum tree height and stand density likely encountered in the field. Model results suggest that at least 80% shade on small streams up to 6-m wide can be achieved in mid-latitudes with relatively narrow 12-m wide buffers, regardless of stream azimuth, as long as buffers are tall (approximate to 30 m) and dense (leaf area index approximate to 6). Although wide buffers may be preferred to provide other benefits, results suggest that increasing buffer widths beyond about 12 m will have a limited effect on stream shade at mid-latitudes and that greater emphasis should be placed on the creation of dense, tall buffers to maximize stream shading.

Diebel, M et al. 2009. Landscape Planning for Agricultural Nonpoint Source Pollution Reduction III: Assessing Phosphorus and Sediment Reduction Potential.  ENVIRONMENTAL MANAGEMENT, 43 (1): 69-83.

Abstract: Riparian buffers have the potential to improve stream water quality in agricultural landscapes. This potential may vary in response to landscape characteristics such as soils, topography, land use, and human activities, including legacies of historical land management. We built a predictive model to estimate the sediment and phosphorus load reduction that should be achievable following the implementation of riparian buffers; then we estimated load reduction potential for a set of 1598 watersheds (average 54 km2) in Wisconsin. Our results indicate that land cover is generally the most important driver of constituent loads in Wisconsin streams, but its influence varies among pollutants and according to the scale at which it is measured. Physiographic (drainage density) variation also influenced sediment and phosphorus loads. The effect of historical land use on present-day channel erosion and variation in soil texture are the most important sources of phosphorus and sediment that riparian buffers cannot attenuate. However, in most watersheds, a large proportion (approximately 70%) of these pollutants can be eliminated from streams with buffers. Cumulative frequency distributions of load reduction potential indicate that targeting pollution reduction in the highest 10% of Wisconsin watersheds would reduce total phosphorus and sediment loads in the entire state by approximately 20%. These results support our approach of geographically targeting nonpoint source pollution reduction at multiple scales, including the watershed scale.

Dosskey, MG, et al. 2010. The Role of Riparian Vegetation in Protecting and Improving Chemical Water Quality in Streams. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2): 261-277.

We review the research literature and summarize the major processes by which riparian vegetation influences chemical water quality in streams, as well as how these processes vary among vegetation types, and discuss how these processes respond to removal and restoration of riparian vegetation and thereby determine the timing and level of response in stream water quality. Our emphasis is on the role that riparian vegetation plays in protecting streams from nonpoint source pollutants and in improving the quality of degraded stream water. Riparian vegetation influences stream water chemistry through diverse processes including direct chemical uptake and indirect influences such as by supply of organic matter to soils and channels, modification of water movement, and stabilization of soil. Some processes are more strongly expressed under certain site conditions, such as denitrification where groundwater is shallow, and by certain kinds of vegetation, such as channel stabilization by large wood and nutrient uptake by faster-growing species. Whether stream chemistry can be managed effectively through deliberate selection and management of vegetation type, however, remains uncertain because few studies have been conducted on broad suites of processes that may include compensating or reinforcing interactions. Scant research has focused directly on the response of stream water chemistry to the loss of riparian vegetation or its restoration. Our analysis suggests that the level and time frame of a response to restoration depends strongly on the degree and time frame of vegetation loss. Legacy effects of past vegetation can continue to influence water quality for many years or decades and control the potential level and timing of water quality improvement after vegetation is restored. Through the collective action of many processes, vegetation exerts substantial influence over the well-documented effect that riparian zones have on stream water quality. However, the degree to which stream water quality can be managed through the management of riparian vegetation remains to be clarified. An understanding of the underlying processes is important for effectively using vegetation condition as an indicator of water quality protection and for accurately gauging prospects for water quality improvement through restoration of permanent vegetation.

Dosskey, MG, Helmers, MJ, Eisenhauer, DE. 2008. A design aid for determining width of filter strips. JOURNAL OF SOIL AND WATER CONSERVATION, 63 (4): 232-241.

Abstract: Watershed planners need a tool for determining width of filter strips that is accurate enough for developing cost-effective site designs and easy enough to use for making quick determinations on a large number and variety of sites. This study employed the process-based vegetative Filter Strip Model to evaluate the relationship between filter strip width and trapping efficiency for sediment and water and to produce a design aid for use where specific water quality targets must be met. Model simulations illustrate that relatively narrow filter strips can have high impact in some situations, while in others even a modest impact cannot be achieved at any practical width. A graphical design aid was developed for estimating the width needed to achieve target trapping efficiencies for different pollutants under a broad range of agricultural site conditions. Using the model simulations for sediment and water, a graph was produced containing a family of seven lines that divide the full range of possible relationships between width and trapping efficiency into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering field length and cover management, slope, and soil texture. Relationships for sediment-bound and dissolved pollutants are interpreted from the modeled relationships for sediment and water. Interpolation between lines can refine the results and account for additional variables, if needed. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. This design aid strikes a balance between accuracy and utility that fills a wide gap between existing design guides and mathematical models.

Ficetola, GF, Padoa-Schioppa, E., De Bernardi, F. 2009. Influence of Landscape Elements in Riparian Buffers on the Conservation of Semiaquatic Amphibians. CONSERVATION BIOLOGY, 23 (1): 114-123.

Abstract: Studies on riparian buffers have usually focused on the amount of land needed as habitat for the terrestrial life stages of semiaquatic species. Nevertheless, the landscape surrounding wetlands is also important for other key processes, such as dispersal and the dynamics of metapopulations. Multiple elements that influence these processes should therefore be considered in the delineation of buffers. We analyzed landscape elements (forest cover, density of roads, and hydrographic network) in concentric buffers to evaluate the scale at which they influence stream amphibians in 77 distinct landscapes. To evaluate whether our results could be generalized to other contexts, we determined whether they were consistent across the study areas. Amphibians required buffers of 100–400 m of suitable terrestrial habitat, but interspecific differences in the amount of habitat were large. The presence of amphibians was related to roads and the hydrographic network at larger spatial scales (300–1500 m), which suggests that wider buffers are needed with these elements. This pattern probably arose because these elements influence dispersal and metapopulation persistence, processes that occur at large spatial scales. Furthermore, in some cases, analyses performed on different sets of landscapes provided different results, which suggests caution should be used when conservation recommendations are applied to disparate areas. Establishment of riparian buffers should not be focused only on riparian habitat, but should take a landscape perspective because semiaquatic species use multiple elements for different functions. This approach can be complex because different landscape elements require different spatial extents. Nevertheless, a shift of attention toward the management of different elements at multiple spatial scales is necessary for the long-term persistence of populations.

Ghermandi, A et al. 2009. Model-based assessment of shading effect by riparian vegetation on river water quality. ECOLOGICAL ENGINEERING, 35 (1): 92-104.

Abstract: Shading by riparian vegetation affects incident solar radiation and water temperature in small- and moderate-size streams, and is thus an important component in the influence of forested riparian buffers on streams. The water quality effects of riparian shading are largely unknown. A simulation study was carried out to evaluate the effect of shading on six water quality variables in a moderate-size Belgian river stretch. A dynamic modelling approach making use of the River Water Quality Model No. 1 was chosen to represent the system. The scenarios developed indicate that shading may be an effective tool in controlling stream eutrophication (44% reduction in phytoplankton productivity in the simulated stretch) but has a limited effect on dissolved oxygen, chemical oxygen demand, nitrates, ammonium nitrogen, and phosphates. Results suggest that shading can effectively be implemented as a direct management strategy to improve water quality conditions in small and moderate-size watercourses that are exposed to excessive algal growth during summer periods.

Gorsevski, PV, et al. 2008. Dynamic riparian buffer widths from potential non-point source pollution areas in forested watersheds. FOREST ECOLOGY AND MANAGEMENT, 256 (4): 664-673.

Abstract: Efforts to manage National Forests in the USA for wood production, while protecting water quality, are currently constrained by models that do not address the temporal dynamics of variable non-point source (NPS) areas. NPS areas are diffuse sources of contaminants contributed mostly by runoff as a result of different land use activities. Riparian vegetative buffers are often used to control contaminants from NPS areas but defining suitable widths require different policy considerations. In this study, the approach for defining suitable buffer widths is to apply a distributed process-based model that predicts potential NPS areas prone to generating runoff in relation to overland flow distances. A case study of the concept was applied to the 72 km2 Pete King watershed located in the Clearwater National Forest (CNF) in central Idaho, USA. This grid modeling approach is based on a Geographic Information System (GIS) and it integrates the soil moisture routing (SMR) model with probabilistic analysis. The SMR model is a daily water balance model that simulates the hydrology of forested watersheds using real or stochastically generated climate data, a digital elevation model, soil, and land use data. The probabilistic analysis incorporates the variability of soil depth and accounts for uncertainties associated with the prediction of NPS areas using Monte Carlo simulation. A 1-year simulation for the case study location was performed to examine the spatial and temporal changes in NPS areas prone to generating runoff. The results of the simulation indicate that the seasonal variability of saturated areas determines the spatial dynamics of the potential NPS pollution. Use of this model for the design of riparian buffer widths would increase the effectiveness of decision-making in forest management and planning by mapping or delineating NPS areas likely to transport contaminants to perennial surface water bodies.

Hairston-Strang, A. 2010. Assessing forest buffer functions after five years : final report. Maryland Department of Natural Resources, Forest Service. Available online at http://www.dnr.state.md.us/irc/docs/00016298.pdf

Hazlett, P et al. 2008. The importance of catchment slope to soil water N and C concentrations in riparian zones: implications for riparian buffer width. CANADIAN JOURNAL OF FOREST RESEARCH, 38(1): 16-30.

Abstract: Buffer zones are an important component of forest-management strategies and are thought to reduce the impact of nutrients released after harvesting on water quality. Conceptually, steep slopes have shorter water residence times than shallow slopes, have a reduced capacity to moderate water quality, and therefore, require wider buffers. Carbon and N concentrations in riparian zone shallow soil water at 30 cm depth and lake water were measured on shallow and steep slopes at the Esker Lakes Research Area in northeastern Ontario to determine if nutrient concentrations were correlated to catchment terrain attributes. Field measured slope, slope class obtained from a triangular irregular network model, and upslope contributing area and topographic index calculated from a digital elevation model were calculated for each sampling location. Modeled terrain properties, including those currently used during forest-management planning, were not significantly correlated with soil water N and C concentrations, whereas only dissolved organic carbon levels were significantly greater on field measured steep slopes. Forest species composition and soil N levels were positively correlated with soil water N concentrations. These results from the undisturbed boreal ecosystem highlight the potential limitation of using only catchment slope as a tool for prescribing riparian buffers during harvesting when considering terrestrial nutrient export.

Horwitz, RJ et al. 2008. Effects of riparian vegetation and watershed urbanization on fishes in streams of the mid-Atlantic piedmont (USA). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 44(3): 724-741.

Abstract: The joint influences of riparian vegetation and urbanization on fish assemblages were analyzed by depletion sampling in paired forested and nonforested reaches of 25 small streams along an urbanization gradient. Nonforested reaches were narrower than their forested counterparts, so densities based on surface area differ from linear densities (based on reach length). Linear densities (based on number or biomass of fish) of American eel, white sucker and tesselated darter, and the proportion of biomass of benthic invertivores were significantly higher in nonforested reaches, while linear densities of margined madtom and the number of pool species were significantly higher in forested reaches. Observed riparian effects may reflect differences in habitat and algal productivity between forested and nonforested reaches. These results suggest that relatively small-scale riparian restoration projects can affect local geomorphology and the abundance of fish. Dense vegetative cover in riparian zones and similar or analogous habitats in both forested and nonforested reaches, the relatively small scale of the nonforested reaches, and the low statistical power to detect differences in abundance of rare species may have limited the observed differences between forested and nonforested reaches. There was a strong urbanization gradient, with reductions of intolerant species and increases of tolerant species and omnivores with increasing urbanization. Interactions between riparian vegetation type and urbanization were found for blacknose dace, creek chub, tesselated darter, and the proportion of biomass of lithophilic spawners. The study did not provide consistent support for the hypotheses that responses of fish to riparian vegetation would be overwhelmed by urban degradation or insignificant at low urbanization.

Hunt, PG, Matheny, TA, Ro, KS. 2007. Nitrous oxide accumulation in soils from riparian buffers of a coastal plain watershed-carbon/nitrogen ratio control. JOURNAL OF ENVIRONMENTAL QUALITY, 36 (5): 1368-1376.

Abstract: Riparian buffers are used throughout the world for the protection of water bodies from nonpoint-source nitrogen pollution. Few studies of riparian or treatment wetland denitrification consider the production of nitrous oxide (N2O). The objectives of this research were to ascertain the level of potential N2O production in riparian buffers and identify controlling factors for N2O accumulations within riparian soils of an agricultural watershed in the southeastern Coastal Plain of the USA. Soil samples were obtained from ten sites (site types) with different agronomic management and landscape position. Denitrification enzyme activity (DEA) was measured by the acetylene inhibition method. Nitrous oxide accumulations were measured after incubation with and without acetylene(baseline N2O production). The mean DEA (with acetylene) was 59 microg N2O-N kg(-1) soil h(-1) for all soil samples from the watershed. If no acetylene was added to block conversion of N2O to N2, only 15 microg N2O-N kg(-1) soil h(-1) were accumulated. Half of the samples accumulated no N2O. The highest level of denitrification was found in the soil surface layers and in buffers impacted by either livestock waste or nitrogen from legume production. Nitrous oxide accumulations (with acetylene inhibition) were correlated to soil nitrogen (r2=0.59). Without acetylene inhibition, correlations with soil and site characteristics were lower. Nitrous oxide accumulations were found to be essentially zero, if the soil C/N ratios>25. Soil C/N ratios may be an easily measured and widely applicable parameter for identification of potential hot spots of N2O productions from riparian buffers.

Kline, M, Cahoon, B. 2010. Protecting River Corridors in Vermont. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2):227-236.

The Vermont Agency of Natural Resources' current strategy for restoring aquatic habitat, water quality, and riparian ecosystem services is the protection of fluvial geomorphic-based river corridors and associated wetland and floodplain attributes and functions. Vermont has assessed over 1,350 miles of stream channels to determine how natural processes have been modified by channel management activities, corridor encroachments, and land use/land cover changes. Nearly three quarters of Vermont field-assessed reaches are incised limiting access to floodplains and thus reducing important ecosystem services such as flood and erosion hazard mitigation, sediment storage, and nutrient uptake. River corridor planning is conducted with geomorphic data to identify opportunities and constraints to mitigating the effects of physical stressors. Corridors are sized based on the meander belt width and assigned a sensitivity rating based on the likelihood of channel adjustment due to stressors. The approach adopted by Vermont is fundamentally based on restoring fluvial processes associated with dynamic equilibrium, and associated habitat features. Managing toward fluvial equilibrium is taking hold across Vermont through adoption of municipal fluvial erosion hazard zoning and purchase of river corridor easements, or local channel and floodplain management rights. These tools signify a shift away from primarily active management approaches of varying success that largely worked against natural river form and process, to a current community-based, primarily passive approach to accommodate floodplain reestablishment through fluvial processes.

Knight, KW, et al. 2010. Ability of Remnant Riparian Forests, With and Without Grass Filters, to Buffer Concentrated Surface Runoff.JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2):311-322.

Riparian forest buffers established according to accepted conservation practice standards have been recommended as one of the most effective tools for mitigating nonpoint source pollution. The midwestern United States is characterized by many kilometers of narrow, naturally occurring forests along streams. However, little is known about the relative effectiveness of these remnant forests compared with these newly established buffers. This study compared the ability of naturally occurring remnant forests with and without adjacent planted grass filters to buffer concentrated flow paths (CFPs) originating in crop fields along first- and second-order streams in three northeast Missouri watersheds. Remnant forests breached by runoff through CFPs were narrower than those that dispersed 100% of the CFPs. Remnant forests with adjacent grass buffers were nearly twice the width as those without grass filters. We also found that CFPs, which developed within remnant forests and at the base of in-field grass waterways, were potential sources of sediments to streams. Methods to mitigate these CFPs warrant further investigation. Our study suggests that although these natural remnant forests provide substantial buffering capacity, both improved management and/or the addition of an adjacent grass filter would improve water quality by reducing sediment loss to streams. Inferences can be used to inform the design and management of similar conservation buffer systems within the region.

Liu, XM, Mang, XY, Zhang, MH. 2008. Major factors influencing the efficacy of vegetated buffers on sediment trapping: A review and analysis.  JOURNAL OF ENVIRONMENTAL QUALITY, 37 (5):1667-1674.

Abstract: Sediment is a major agricultural pollutant threatening water quality. Vegetated buffers, including vegetative filter strips, riparian buffers, and grassed waterways, are best management practices (BMPs) installed in many areas to filter sediments from tailwaters, and deter sediment transport to water bodies. Along with reducing sediment transport, the filters also help trap sediment bound nutrients and pesticides. The objectives of this study were: (i) to review vegetated buffer efficacy on sediment trapping, and (ii) to develop statistical models to investigate the major factors influencing sediment trapping. A range of sediment trapping efficacies was found in a review of over 80 representative BMP experiments. A synthesis of the literature regarding the effects of vegetated buffers on sediment trapping is needed. The meta-analysis results based on the limited data showed that buffer width and slope are two major factors influencing BMPs efficacy of vegetated buffers on sediment trapping. Regardless of the area ratio of buffer to agricultural field, a 10 m buffer and a 9% slope optimized the sediment trapping capability of vegetated buffers.

Liu, Y, Yang, W, Wang, X. 2007. GIS-based integration of SWAT and REMM for estimating water quality benefits of riparian buffers in agricultural watersheds. TRANSACTIONS OF THE ASABE, 50 (5):1549-1563.

Abstract: The Soil and Water Assessment Tool (SWAT) is a process-based, watershed-scale model with a hydrologic response unit (HRU) as the basic computation element, which makes it difficult to accurately represent riparian buffers using their physical parameters (e.g., vegetation structure). On the other hand, the field-scale Riparian Ecosystem Management Model (REMM) provides the opportunity to consider details of hydrologic processes within a riparian buffer zone. However, the runoff and its associated constituents from the upland area that is hydraulically connected to the riparian buffer zone must be provided as inputs into REMM. The rationale proposed here is that the integration of SWAT and REMM would improve the assessment of riparian buffers, which is vital to watershed management but which has not been described in the literature. The objective of this study was to develop a GIS interface that integrated SWAT and REMM for estimating water quality benefits of riparian buffers in agricultural watersheds. For modeling purposes, the interface subdivided a watershed into a number of sub-basins, each of which was further subdivided into drainage areas of isolated impoundments (e.g., wetlands), concentrated flow, and riparian buffers using available GIS data. As a result, riparian buffers received runoff and associated pollutants from corresponding contribution areas to mimic actual field conditions. The interface facilitated transferring the SWAT outputs into REMM and computing the site characteristic parameters (e.g., length and width) of the riparian buffers. The outputs from subsequent REMM runs were in turn taken as inputs into SWAT for channel routing and further simulation. The interface was used to assess water quality benefits of riparian buffers in the Lower Canagagigue Creek watershed located in southern Ontario, Canada. The results indicated that the existing riparian buffer system achieved a 27.9% abatement in sediment and a 37.4% reduction in total phosphorus. The model runs demonstrated that the GIS interface was easy to use and could serve as a protocol for integrating models with distinctly different spatial scales.

Madden, SS, Robinson, GR, Arnason, JG. 2007. Spatial variation in stream water quality in relation to riparian buffer dimensions in a rural watershed of eastern New York state. NORTHEASTERN NATURALIST, 14 (4): 605-618.

Abstract: Studies of forested rural watersheds provide estimates of background contamination for comparison with streams and rivers in other settings. We performed a landscape analysis and measured major dissolved ions and benthic macroinvertebrates for a small rural watershed in Albany County, NY, to determine spatial variation in water quality. An estimated 73% of the surface cover is post-agricultural forest, with only 2.3% of the watershed covered by roads and other impervious surfaces. Although water quality was consistently high in most of the creek, we detected three relatively distinct zones separated by impoundments; zonation was most apparent in relative concentrations of major ions, less so with benthic macroinvertebrate community similarity. At ten sample stations, buffer size, measured as upstream land cover and distance to nearest road, did not correlate well with chemical water quality indicators. In particular, we found the highest levels of chloride, indicative of road-salt contamination, in areas of maximum forest buffer. Small feeder creeks that drain nearby roads may function as “leaks” in otherwise well-buffered watersheds with low road densities.

Mankin, KR et al. 2007. Grass-shrub riparian buffer removal of sediment, phosphorus, and nitrogen from simulated runoff.  JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 43 (5): 1108-1116.

Abstract: Riparian buffer forests and vegetative filter strips are widely recommended for improving surface water quality, but grass-shrub riparian buffer system (RBSs) are less well studied. The objective of this study was to assess the influence of buffer width and vegetation type on the key processes and overall reductions of total suspended solids (TSS), phosphorus (P), and nitrogen (N) from simulated runoff passed through established (7-year old) RBSs. Nine 1-m RBS plots, with three replicates of three vegetation types (all natural selection grasses, two-segment buffer with native grasses and plum shrub, and two-segment buffer with natural selection grasses and plum shrub) and widths ranging from 8.3 to 16.1 m, received simulated runoff having 4,433 mg/1 TSS from on-site soil, 1.6 mg/1 total P, and 20 mg/1 total N. Flow-weighted samples were collected by using Runoff Sampling System (ROSS) units. The buffers were very efficient in removal of sediments, N, and P, with removal efficiencies strongly linked to infiltration. Mass and concentration reductions averaged 99.7% and 97.9% for TSS, 91.8% and 42.9% for total P, and 92.1% and 44.4% for total N. Infiltration alone could account for >75% of TSS removal, >90% of total P removal, and >90% of total N removal. Vegetation type induced significant differences in removal of TSS, total P, and total N. These results demonstrate that adequately designed and implemented grass-shrub buffers with widths of only 8 m provide for water quality improvement, particularly if adequate infiltration is achieved.

Marczak, Laurie B., Takashi Sakamaki, Shannon L. Turvey, Isabelle Deguise, Sylvia L. R. Wood, John S. Richardson. 2010.  Are forested buffers an effective conservation strategy for riparian fauna? An assessment using meta-analysis. Ecological Applications. 20(1): 126-134.

Abstract: Historically, forested riparian buffers have been created to provide protection for aquatic organisms and aquatic ecosystem functions. Increasingly, new and existing riparian buffers are being used also to meet terrestrial conservation requirements. To test the effectiveness of riparian buffers for conserving terrestrial fauna, we conducted a meta-analysis using published data from 397 comparisons of species abundance in riparian buffers and unharvested (reference) riparian sites. The response of terrestrial species to riparian buffers was not consistent between taxonomic groups; bird and arthropod abundances were significantly greater in buffers relative to unharvested areas, whereas amphibian abundance decreased. Edge-preferring species were more abundant in buffer sites than reference sites, whereas species associated with interior habitat were not significantly different in abundance. The degree of buffer effect on animal abundance was unrelated to buffer width; wider buffers did not result in greater similarity between reference and buffer sites. However, responses to buffer treatment were more variable in buffers <50 m wide, a commonly prescribed width in many management plans. Our results indicate that current buffer prescriptions do not maintain most terrestrial organisms in buffer strips at levels comparable to undisturbed sites.

Mayer, P.M., S.K. Reynolds, M.D. McCutchen, and T.J. Canfield. Riparian buffer width, vegetative cover, and nitrogen removal effectiveness: A review of current science and regulations. EPA/600/R-05/118. Cincinnati, OH, U.S. Environmental Protection Agency, 2006. Available online.

Riparian zones, the vegetated region adjacent to streams and wetlands, are thought to be effective at intercepting and controlling nitrogen loads entering water bodies. Buffer width may be positively related to nitrogen removal effectiveness by influencing nitrogen retention through plant sequestration or removal through microbial denitrification. We surveyed peer-reviewed scientific literature containing data on riparian buffers and nitrogen concentration in streams and groundwater of riparian zones to identify causation and trends in the relationship between buffer width and nitrogen removal capacity. We also examined Federal and State regulations regarding riparian buffer widths to determine if such legislation reflects the current scientific understanding of buffer effectiveness. Nitrogen removal effectiveness varied widely among riparian zones studied. Subsurface removal of nitrogen was efficient but did not appear to be related to buffer width. Surface removal of nitrogen was partly related to buffer width and was generally inefficient, removing only a small fraction of the total nitrogen flowing through soil surface layers. While some narrow buffers (1-15 m) removed significant proportions of nitrogen, narrow buffers actually contributed to nitrogen loads in riparian zones in some cases. Wider buffers (>50 m) more consistently removed significant portions of nitrogen entering a riparian zone. Buffers of various vegetation types were equally effective at removing nitrogen in the subsurface but not in surface flow. The general lack of vegetation type or buffer width effects on nitrogen removal, especially in the subsurface, suggests that soil type, watershed hydrology (e.g., soil saturation, groundwater flow paths, etc.), and subsurface biogeochemistry (organic carbon supply, high nitrate inputs) may be more important factors dictating nitrogen concentrations due to their influence on denitrification. State and Federal guidelines for buffer width also varied widely but were generally consistent with the peer-reviewed literature on effective buffer width, recommending or mandating buffers ~7-100 m wide. Proper design, placement, and protection of buffers are critical to buffer effectiveness. To maintain maximum effectiveness, buffer integrity should be protected against soil compaction, loss of vegetation, and stream incision. Maintaining buffers around stream headwaters will likely be most effective at maintaining overall watershed water quality while restoring degraded riparian zones, and stream channels may improve nitrogen removal capacity. Riparian buffers are a “best management practice” (BMP) that should be used in conjunction with a comprehensive watershed management plan that includes control and reduction of point and non-point sources of nitrogen from atmospheric, terrestrial, and aquatic inputs.

McCarty, GW et al. 2008. Water quality and conservation practice effects in the Choptank River watershed. JOURNAL OF SOIL AND WATER CONSERVATION, 63 (6): 461-474.

Abstract: The Choptank River is an estuary, tributary of the Chesapeake Bay, and an ecosystem in decline due partly to excessive nutrient and sediment loads from agriculture. The Conservation Effects Assessment Project for the Choptank River watershed was established to evaluate the effectiveness of conservation practices on water quality within this watershed. Several measurement frameworks are being used to assess conservation practices. Nutrients (nitrogen and phosphorus) and herbicides (atrazine and metolachlor) are monitored within 15 small, agricultural subwatersheds and periodically in the lower portions of the river estuary. Initial results indicate that land use within these subwatersheds is a major determinant of nutrient concentration in streams. In addition, the 18O isotope signature of nitrate was used to provide a landscape assessment of denitrification processes in the presence of the variable land use. Herbicide concentrations were not correlated to land use, suggesting that herbicide delivery to the streams is influenced by other factors and/or processes. Remote sensing technologies have been used to scale point measurements of best management practice effectiveness from field to subwatershed and watershed scales. Optical satellite (SPOT-5) data and ground-level measurements have been shown to be effective for monitoring nutrient uptake by winter cover crops in fields with a wide range of management practices. Synthetic Aperture Radar (RADARSAT-1) data have been shown to detect and to characterize accurately the hydrology (hydroperiod) of forested wetlands at landscape and watershed scales. These multiple approaches are providing actual data for assessment of conservation practices and to help producers, natural resource managers, and policy makers maintain agricultural production while protecting this unique estuary.

McIlroy, SK et al. 2008. Identifying Linkages Between Land Use, Geomorphology, and Aquatic Habitat in a Mixed-Use Watershed. ENVIRONMENTAL MANAGEMENT, 42 (5): 867-876.

Abstract: The potential impacts of land use on large woody debris (LWD) were examined in Sourdough Creek Watershed, a rapidly growing area encompassing Bozeman, Montana, USA. We identified six land classes within a 250 m buffer extending on either side of Sourdough Creek and assessed aquatic habitat and geomorphologic variables within each class. All LWD pieces were counted, and we examined 14 other variables, including undercut bank, sinuosity, and substrate composition. LWD numbers were generally low and ranged from 0 to 8.2 pieces per 50 m of stream. Linear regression showed that LWD increased with distance from headwaters, riparian forest width, and sinuosity in four of the six land classes. Statistically significant differences between land classes for many aquatic habitat and geomorphologic variables indicated the impacts of different land uses on stream structure. We also found that practices such as active wood removal played a key role in LWD abundance. This finding suggests that managers should prioritize public education and outreach concerning the importance of in-stream wood, especially in mixed-use watersheds where wood is removed for either aesthetic reasons or to prevent stream flooding.

Newbold, JD et al. 2010. Water Quality Functions of a 15-Year-Old Riparian Forest Buffer System. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2): 299-310.

We monitored long-term water quality responses to the implementation of a three-zone Riparian Forest Buffer System (RFBS) in southeastern Pennsylvania. The RFBS, established in 1992 in a 15-ha agricultural (row crop) watershed, consists of: Zone 1, a streamside strip (similar to 10 m wide) of permanent woody vegetation for stream habitat protection; Zone 2, an 18- to 20-m-wide strip reforested in hardwoods upslope from Zone 2; and Zone 3, a 6- to 10-m-wide grass filter strip in which a level lip spreader was constructed. The monitoring design used paired watersheds supplemented by mass balance estimates of nutrient and sediment removal within the treated watershed. Tree growth was initially delayed by drought and deer damage, but increased after more aggressive deer protection (1.5 m polypropylene shelters or wire mesh protectors) was instituted. Basal tree area increased similar to 20-fold between 1998 and 2006, and canopy cover reached 59% in 2006. For streamwater nitrate, the paired watershed comparison was complicated by variations in both the reference stream concentrations and in upslope groundwater nitrate concentrations, but did show that streamwater nitrate concentrations in the RFBS watershed declined relative to the reference stream from 2002 through the end of the study in early 2007. A subsurface nitrate budget yielded an average nitrate removal by the RFBS of 90 kg/ha/year, or 26% of upslope subsurface inputs, for the years 1997 through 2006. There was no evidence from the paired watershed comparison that the RFBS affected streamwater phosphorus concentration. However, groundwater phosphorus did decline within the buffer. Overland flow sampling of 23 storms between 1997 and 2006 showed that total suspended solids concentration in water exiting the RFBS to the stream was on average 43% lower than in water entering the RFBS from the tilled field. Particulate phosphorus concentration was lower by 22%, but this removal was balanced by a 26% increase in soluble reactive phosphorus so that there was no net effect on total phosphorus.

Shandas, V and Alberti, M. 2009.  Exploring the role of vegetation fragmentation on aquatic conditions: Linking upland with riparian areas in Puget Sound lowland streams. LANDSCAPE AND URBAN PLANNING, 90 (1-2): 66-75.

Abstract: A controversial issue in managing urbanizing watersheds is determining the scale at which conservation measures should be implemented. Current “best practices” suggest establishing riparian buffers along stream corridors and limiting impervious surfaces to prevent degradation of instream biological conditions. While there is increasing evidence that the amount of land covers (e.g., impervious surface, vegetation) has an impact on instream aquatic conditions, the effect of upland vegetation fragmentation on aquatic conditions requires further study. By using landscape metrics to quantity vegetation amount and distribution at the riparian and watershed scales, and a macroinvertebrate index to describe aquatic conditions, this study presents empirical evidence about the interactions between riparian and upland vegetation as they affect instream biological condition of 51 nested watersheds in the Puget Sound lowland. We ask if the fragmentation of vegetation within a watershed helps predict instream biological condition. In addition, we hypothesize that the fragmentation of vegetation at the riparian and watershed scales affects instream biological condition. Using parametric and non-parametric statistical analyses to test relationships, our findings suggest that the fragmentation of upland vegetation and the total amount of riparian vegetation explain the greatest amount of variation in aquatic conditions. These results help frame a management approach for conserving upland areas of vegetation through the use of land use planning techniques.

Smith, TA, Osmond, DL, Gilliam, JW. 2006.Riparian buffer width and nitrate removal in a lagoon-effluent irrigated agricultural area. JOURNAL OF SOIL AND WATER CONSERVATION, 61 (5): 273-281.

Abstract: The ideal buffer width required to maximize water quality benefits while minimizing unnecessary land utilization is difficult to determine. This study examined the effect of increasing buffer width on nitrate (NO3-N) reduction in shallow groundwater in the middle Coastal Plain of North Carolina. The study site was a streamside buffer zone where previous research determined the buffer did not sufficiently reduce NO3-N discharge to stream water. Buffer width was increased from 9 to 30 m (30 to 98 ft) by fencing out cattle and allowing volunteer vegetation to emerge, thus forming a grass and shrub buffer along the newly widened area. Buffer functions were assessed by comparing groundwater NO3-N concentrations from this new system with the same area prior to widening. A significant increase in percent NO3-N reduction was observed in shallow (0.8 to 1.2 m; 2.6 to 4.0 ft) groundwater on the east (34.7 percent to 95.0 percent; p = 0.02) side of stream, most likely due to an increase (p = 0.01) in dissolved organic carbon (C) at this depth. Reduction at the 30-m (98-ft) width in shallow groundwater at the west side of stream was measured at 93.3 percent, although this was not a statistically significant increase from the 9-m (30-ft) width (53.1 percent; p = 0.2). Percent NO3-N reduction was less efficient in deep (2.4 to 4.5 m; 7.9 to 14.5 ft) groundwater, although a significant increase (-37.8 percent to 39.2 percent; p = 0.003) at the east side of stream was observed. Reduction on the west side at this depth significantly decreased (19.5 percent to -5.1 percent; p = 0.05) suggesting that the deeper confining layer along this side of stream allowed groundwater to bypass the zone of high dissolved organic C enhanced by riparian vegetation, thus minimizing the effects of buffer widening in this area. Stream sampling indicated no significant difference in upstream and downstream NO3-N between the two buffer widths (p = 0.61). Although increasing the buffer width did improve reduction in some areas, the severity of the groundwater NO3-N situation at this site was not completely resolved by increasing the buffer width to 30 m (98 ft).

Speiran, GK.2010. Effects of Groundwater-Flow Paths On Nitrate Concentrations Across Two Riparian Forest Corridors. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2):246-260.

Groundwater levels, apparent age, and chemistry from field sites and groundwater-flow modeling of hypothetical aquifers collectively indicate that groundwater-flow paths contribute to differences in nitrate concentrations across riparian corridors. At sites in Virginia (one coastal and one Piedmont), lowland forested wetlands separate upland fields from nearby surface waters (an estuary and a stream). At the coastal site, nitrate concentrations near the water table decreased from more than 10 mg/l beneath fields to 2 mg/l beneath a riparian forest buffer because recharge through the buffer forced water with concentrations greater than 5 mg/l to flow deeper beneath the buffer. Diurnal changes in groundwater levels up to 0.25 meters at the coastal site reflect flow from the water table into unsaturated soil where roots remove water and nitrate dissolved in it. Decreases in aquifer thickness caused by declines in the water table and decreases in horizontal hydraulic gradients from the uplands to the wetlands indicate that more than 95% of the groundwater discharged to the wetlands. Such discharge through organic soil can reduce nitrate concentrations by denitrification. Model simulations are consistent with field results, showing downward flow approaching toe slopes and surface waters to which groundwater discharges. These effects show the importance of buffer placement over use of fixed-width, streamside buffers to control nitrate concentrations.

Stutter, MI, Langan, SJ, Lumsdon, DG. 2009. Vegetated Buffer Strips Can Lead to Increased Release of Phosphorus to Waters: A Biogeochemical Assessment of the Mechanisms. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 43 (6): 1858-1863.

Abstract: Establishing vegetated buffer strips (VBS) between cropland and watercourses is currently promoted as a principal control of diffuse pollution transport. However, we lack the mechanistic understanding to evaluate P retention in VBS and predict risks of P transport to aquatic ecosystems. We observed that VBS establishment led to enhanced rates of soil P cycling, increasing soil P solubility and the potential amount leached to watercourses. Soil in VBS, relative to adjacent fields, had increased inorganic P solubility indices, dissolved organic P, phosphatase enzyme activity, microbial diversity, and biomass P. Small relative increases in the pool of soil P rendered labile had disproportionate effects on the P available for leaching. We propose a mechanism whereby the establishment of VBS on previous agricultural land causes a diversifying plant-microbial system which can access previous immobilized soil P from past fertilization or trapped sediment P. Laboratory experiments suggested that sediment-P inputs to VBS were insufficient alone to increase P solubility without biological cycling. Results show that VBS management may require strategies, for example, harvesting vegetation, to offset biochemical processes that can increase the susceptibility of VBS soil P to move to adjoining streams.

Sutton, AJ, Fisher, TR, Gustafson, AB. 2010. Effects of Restored Stream Buffers on Water Quality in Non-tidal Streams in the Choptank River Basin. WATER AIR AND SOIL POLLUTION, 208 (1-4): 101-118.

Restoration of riparian buffers is an important component of nutrient reduction strategies in the Chesapeake Bay watershed. In 1998, Maryland adopted a Conservation Reserve Enhancement Program (CREP), which provides financial incentives to take agricultural land out of production to plant streamside vegetation. Between 1998 and 2005, 1-30% of streamside vegetation (average = 11%), was restored to forest or managed grass in 15 agriculturally dominated sub-basins in the Choptank River basin, a tributary of Chesapeake Bay. Pre-existing forested buffers represented 10-48% of the streamside (average = 33%), for a total of 12-61% buffered streamsides (average = 44%). Using multi-year water quality data collected before and after CREP implementation (1986, 2003-2006), we were unable to detect significant effects of CREP on baseflow nutrient concentrations based on the area of restored buffer, the percentage of restored streamside, or the percentage of total riparian buffer in the sub-basins (p > 0.05). Although CREP increased the average buffered streamside from 33% in the 1990s to 44% by 2005, N and P concentrations have not changed or have increased in some streams over the last 20 years. Reductions may not have occurred for the following reasons: (1) buffer age, width, and connectivity (gaps) between buffers are also important to nutrient reductions; (2) agricultural nutrient inputs may have increased during this period; and (3) riparian buffer restoration was not extensive enough by 2005 to have measurable affects on the stream water quality in these sub-basins. Significant effects of CREP may yet be resolved as the current CREP buffers mature; however, water quality data through 2006 in the Choptank basin do not yet show any significant effects.

Tomer, MD et al. 2009. Methods to prioritize placement of riparian buffers for improved water quality. AGROFORESTRY SYSTEMS, 75 (1): 17-25.

Abstract: Agroforestry buffers in riparian zones can improve stream water quality, provided they intercept and remove contaminants from surface runoff and/or shallow groundwater. Soils, topography, surficial geology, and hydrology determine the capability of forest buffers to intercept and treat these flows. This paper describes two landscape analysis techniques for identifying and mapping locations where agroforestry buffers can effectively improve water quality. One technique employs soil survey information to rank soil map units for how effectively a buffer, when sited on them, would trap sediment from adjacent cropped fields. Results allow soil map units to be compared for relative effectiveness of buffers for improving water quality and, thereby, to prioritize locations for buffer establishment. A second technique uses topographic and streamflow information to help identify locations where buffers are most likely to intercept water moving towards streams. For example, the topographic wetness index, an indicator of potential soil saturation on given terrain, identifies where buffers can readily intercept surface runoff and/or shallow groundwater flows. Maps based on this index can be useful for site-specific buffer placement at farm and small-watershed scales. A case study utilizing this technique shows that riparian forests likely have the greatest potential to improve water quality along first-order streams, rather than larger streams. The two methods are complementary and could be combined, pending the outcome of future research. Both approaches also use data that are publicly available in the US. The information can guide projects and programs at scales ranging from farm-scale planning to regional policy implementation.

Vidon, P. et al. 2010. Hot Spots and Hot Moments in Riparian Zones: Potential for Improved Water Quality Management. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2): 278-298.

Biogeochemical and hydrological processes in riparian zones regulate contaminant movement to receiving waters and often mitigate the impact of upland sources of contaminants on water quality. These heterogeneous processes have recently been conceptualized as "hot spots and moments" of retention, degradation, or production. Nevertheless, studies investigating the importance of hot phenomena (spots and moments) in riparian zones have thus far largely focused on nitrogen (N) despite compelling evidence that a variety of elements, chemicals, and particulate contaminant cycles are subject to the influence of both biogeochemical and transport hot spots and moments. In addition to N, this review summarizes current knowledge for phosphorus, organic matter, pesticides, and mercury across riparian zones, identifies variables controlling the occurrence and magnitude of hot phenomena in riparian zones for these contaminants, and discusses the implications for riparian zone management of recognizing the importance of hot phenomena in annual solute budgets at the watershed scale. Examples are presented to show that biogeochemical process-driven hot spots and moments occur along the stream/riparian zone/upland interface for a wide variety of constituents. A basic understanding of the possible co-occurrence of hot spots and moments for a variety of contaminants in riparian systems will increase our understanding of the influence of riparian zones on water quality and guide management strategies to enhance nutrient or pollutant removal at the landscape scale.

Walter, MT et al. 2009. New Paradigm for Sizing Riparian Buffers to Reduce Risks of Polluted Storm Water: Practical Synthesis. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING-ASCE, 135 (2): 200-209.

Abstract: Riparian buffers are commonly promoted to protect stream water quality. A common conceptual assumption is that buffers “intercept” and treat upland runoff. As a shift in paradigm, it is proposed instead that riparian buffers should be recognized as the parts of the landscape that most frequently generate storm runoff. Thus, water quality can be protected from contaminated storm runoff by disassociating riparian buffers from potentially polluting activities. This paper reviews and synthesizes some simple engineering approaches that can be used to delineate riparian buffers for rural watersheds based on risk of generating runoff. Although reference is made to specific future research that may improve the proposed methods for delineating riparian buffers, the approaches described here provide planners and engineers with a set of currently available scientifically defensible tools. It is recommended that planners and engineers use available rainfall and stream discharge data to parameterize the buffer-sizing equations and use variable-width buffers, based on a topographic index, to achieve a realistic representation of runoff generating areas.

Watson, TK, et al. 2010. Groundwater Denitrification Capacity of Riparian Zones in Suburban and Agricultural Watersheds. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, 46 (2):237-245.

We evaluated the relationship of dominant watershed land use to the structure and nitrogen (N) sink function of riparian zones. We focused on groundwater denitrification capacity, water table dynamics, and the presence and pattern of organically enriched deposits. We used the push-pull method (measurement of 15N-enriched denitrification gases derived from an introduced groundwater plume of 15N-enriched nitrate) to evaluate groundwater denitrification capacity on nine forested wetland riparian sites developed in alluvial or outwash parent materials in southern New England. Three replicate sites were located in each of the three watershed types, those with substantial (1) irrigated agriculture, (2) suburban development, and (3) forest. Soil morphology and water table dynamics were assessed at each site. We found significantly lower mean annual water tables at sites within watersheds with substantial irrigated agriculture or suburban development than forested watersheds. Water table dynamics were more variable at sites within suburban watersheds, especially during the summer. Groundwater denitrification capacity was significantly greater at sites within forested watersheds than in watersheds with substantial irrigated agriculture. Because of the high degree of variability observed in riparian sites within suburban watersheds, groundwater denitrification capacity was not significantly different from either forested or agricultural watersheds. The highly variable patterns of organically enriched deposits and water tables at sites within suburban watersheds suggests that depositional events are irregular, limiting the predictability of groundwater N dynamics in these riparian zones. The variability of riparian N removal in watersheds with extensive suburbia or irrigated agriculture argues for N management strategies emphasizing effective N source controls in these settings.

Wilkerson, E; Hagan, JM; Siegel, D; Whitman, AA. 2006. The effectiveness of different buffer widths for protecting headwater stream temperature in Maine. FOREST SCIENCE, 52 (3): 221-231.

Abstract: We evaluated the effect of timber harvesting on summer water temperature in first-order headwater streams in western Maine. Fifteen streams were assigned to one of five treatments: (1) clearcutting with no stream buffer; (2) clearcutting with 11-m, partially harvested buffers, both sides; (3) clearcutting with 23-m, partially harvested buffers; (4) partial cuts with no designated buffer; and (5) unharvested controls. Over a 3-year period we measured summer water temperature hourly before and after harvesting, above and below the harvest zone. Streams without a buffer showed the greatest increase in mean weekly maximum temperatures following harvesting (1.4-4.4°C). Streams with an 11-m buffer showed minor, but not significant, increases (1.0-1.4°C). Streams with a 23-m buffer, partial-harvest treatment, and control streams showed no changes following harvest. The mean weekly maximum temperatures never exceeded the thermal stress limit for brook trout (25°C) in any treatment group. The mean daily temperature fluctuations for streams without buffers increased from 1.5°C/day to 3.8°C/day, while with 11-m buffers fluctuations increased nonsignificantly by 0.5-0.7°C/day. Water temperatures 100 m below the harvest zone in the no-buffer treatment were elevated above preharvest levels. We concluded that water temperature in small headwater streams is protected from the effects of clearcutting by an 11-m buffer (with >60% canopy retention).

Wilkerson, E, Hagan, JM, Whitman, AA. 2010. The effectiveness of different buffer widths for protecting water quality and macroinvertebrate and periphyton assemblages of headwater streams in Maine, USA. Canadian Journal of Fisheries and Aquatic Sciences 67(1): 177-190.

Abstract: We evaluated the effect of timber harvesting on water quality and macroinvertebrate and periphyton assemblages in first-order streams in Maine, USA. Fifteen streams were assigned to one of five treatments: clearcutting without a stream buffer, clearcutting with 11m buffers, clearcutting with 23m buffers, partial harvesting with no designated buffer, and unharvested controls. Harvest blocks on both sides of the stream were 6ha and partial harvesting within buffers was allowed. Specific conductivity, pH, dissolved oxygen, turbidity, and soluble reactive phosphorus did not change significantly for 3 years after harvesting in all treatments. Unbuffered streams had significantly elevated concentrations of chlorophylla as well as increased abundance of algal feeding organisms (Diperta Cricotopus and Diptera Psectrocladius). Streams with 11m buffers had substantial (10-fold) but nonsignificant increases in chlorophylla. No other significant changes were detected in other treatment groups. In all treatment groups, the dominant taxa (periphyton Achnanthes minutissimum and macroinvertebrate Chironomidae) are adapted to disturbed environments. We attribute the limited harvest-induced changes to lack of soil disturbance within 8m of the stream, the small (≤40%) proportion of watersheds harvested, and the resilient nature of aquatic organisms. However, small-scale changes may not be detected due to the small sample size, an inherent limitation of field-based studies.

Woodward, KB et al. 2009. Nitrate removal, denitrification and nitrous oxide production in the riparian zone of an ephemeral stream. SOIL BIOLOGY & BIOCHEMISTRY, 41 (4): 671-680.

Abstract: Riparian zones are important features of the landscape that can buffer waterways from non-point sources of nitrogen pollution. Studies of perennial streams have identified denitrification as one of the dominant mechanisms by which this can occur. This study aimed to assess nitrate removal within the riparian zone of an ephemeral stream and characterise the processes responsible, particularly denitrification, using both in-situ and laboratory techniques. To quantify rates of groundwater nitrate removal and denitrification in-situ, nitrate was added to two separate injection–capture well networks in a perched riparian aquifer of a low order ephemeral stream in South East Queensland, Australia. Both networks also received bromide as a conservative tracer and one received acetylene to inhibit the last step of denitrification. An average of 77 ± 2% and 98 ± 1% of the added nitrate was removed within a distance of 40 cm from the injection wells (networks with acetylene and without, respectively). Based on rates of N2O production in the network with added acetylene, denitrification was not a major mechanism of nitrate loss, accounting for only 3% of removal. Reduction of nitrate to ammonium was also not a major pathway in either network, contributing <4%. Relatively high concentrations of oxygen in the aquifer following recent filling by stream water may have reduced the importance of these two anaerobic pathways. Alternatively, denitrification may have been underestimated using the in-situ acetylene block technique. In the laboratory, soils taken from two depths at each well network were incubated with four nitrate-N treatments (ranging from ambient concentration to an addition of 15 mg N l−1), with and without added acetylene. Potential rates of dentrification, N2O production and N2O:N2 ratios increased with nitrate additions, particularly in shallow soils. Potential rates of denitrification observed in the laboratory were equivalent in magnitude to nitrate removal measured in the field (mean 0.26 ± 0.12 mg N kg of dry soil−1 d−1), but were two orders of magnitude greater than dentrification measured in the field with added acetylene. The relative importance of assimilatory vs. dissimilatory processes of nitrate removal depends on environmental conditions in the aquifer, particularly hydrology and its effects on dissolved oxygen concentrations. Depending on seasonal conditions, aquifers of ephemeral streams like the study site are likely to fluctuate between oxic and anoxic conditions; nevertheless they may still function as effective buffers. While denitrification to N2 is a desirable outcome from a management perspective, assimilation into biomass can provide a rapid sink for nitrate, thus helping to reduce short-term delivery of nitrate downstream. Longer-term studies are needed to determine the overall effectiveness of riparian buffers associated with ephemeral streams in mitigating nitrate loads reaching downstream ecosystems.

Yamada, T et al. 2007. Groundwater nitrate following installation of a vegetated riparian buffer. SCIENCE OF THE TOTAL ENVIRONMENT, 385 (1-3): 297-309.

Abstract: Substantial questions remain about the time required for groundwater nitrate to be reduced below 10 mg L− 1 following establishment of vegetated riparian buffers. The objective of this study was to document changes in groundwater nitrate–nitrogen (NO3–N) concentrations that occurred within a few years of planting a riparian buffer. In 2000 and 2001 a buffer was planted adjacent to a first-order stream in the deep loess region of western Iowa with strips of walnut and cottonwood trees, alfalfa and brome grass, and switch grass. Non-parametric statistics showed significant declines in NO3–N concentrations in shallow groundwater following buffer establishment, especially mid 2003 and later. The dissolved oxygen generally was > 5 mg L− 1 beneath the buffer, and neither NO3–N nor DO changed significantly under a non-buffered control area. These short-term changes in groundwater NO3–N provide evidence that vegetated riparian buffers may yield local water-quality benefits within a few years of planting.

Zhang, XY, et al. 2010. A Review of Vegetated Buffers and a Meta-analysis of Their Mitigation Efficacy in Reducing Nonpoint Source Pollution. JOURNAL OF ENVIRONMENTAL QUALITY, 39 (1): 76-84.

Vegetated buffers are a well-studied and widely used agricultural management practice for reducing nonpoint-source pollution. A wealth of literature provides experimental data on their mitigation efficacy. This paper aggregated many of these results and performed a meta-analysis to quantify, the relationships between pollutant removal efficacy and buffer width, buffer slope, soil type, and vegetation type. Theoretical models for removal efficacy (Y) vs. buffer width (w) were derived and tested against data from the surveyed literature using statistical analyses. A model of the form Y = K x (1 - e(-bxw)), (0 < K <= 100) Successfully captured the relationship between buffer width and pollutant removal, where K reflects the maximum removal efficacy of the buffer and b reflects its probability to remove any single particle of pollutant in a unit distance. Buffer width alone explains 37, 60, 44, and 35% of the total variance in removal efficacy for sediment, pesticides, N, and P, respectively. Buffer slope was linearly associated with sediment removal efficacy either positively (when slope <= 10%) or negatively (when slope > 10%). Buffers composed of trees have higher N and P removal efficacy than buffers composed of grasses or mixtures of grasses and trees. Soil drainage type did not show a significant effect on pollutant removal efficacy. Based on our analysis, a 30-m buffer under favorable slope conditions %) removes more than 85% of all the studied pollutants. These models predicting optimal buffer width/slope can be instrumental in the design, implementation, and modeling of vegetated buffers for treating agricultural runoff.

 

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