Riparian forest buffers: hype? Or the silver bullet for NPS pollution control?
What are riparian forest buffers?
Riparian forest buffer systems (RFBS) are streamside ecosystems that can be managed to reduce NPS pollution after it leaves the source area but before it reaches the stream (Figure 1). Our research and that of others around the world has demonstrated that RFBS are excellent sinks of nutrients, herbicides, and sediments. In a recently completed study, we found that an RFBS down-slope from a field receiving regular applications of liquid dairy manure from a lagoon reduced annual nitrogen and phosphorus loads in water passing through the buffer by 59 percent and 66 percent, respectively, over a period of 10 years.
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The use of RFBS is relatively well established as a BMP for water quality in forestry practices but has been much less widely applied as a BMP in agricultural areas or in urban or suburban settings. RFBS are especially important on small streams where intense interaction between terrestrial and aquatic ecosystems occurs. First- and second-order streams comprise nearly three-quarters of the total stream length in the United States (Figure 2).
To produce long-term improvements in water quality, RFBS must be designed with an understanding of: 1) the processes that remove or sequester pollutants entering the riparian buffer system, 2) the effects of riparian management practices on pollutant retention, 3) the effects of riparian forest buffers on aquatic ecosystems, 4) the time to recovery after harvest of trees or re-establishment of riparian buffer systems, and 5) the effects of underlying soil and geologic materials on chemical, hydrological, and biological processes.
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The research discussed above has been used by the USDA to develop a general Riparian Forest Buffer System specification for controlling NPS pollution from agriculture and improving general water quality. The specification calls for a three-zone buffer system, with each zone having specific purposes as well as interactions with the adjacent zones to provide the overall RFBS function (Figure 3).
Zone 1 of the RFBS is an area of permanent forest vegetation immediately adjacent to the stream channel and encompassing at least the entire stream channel system. Zone 2 is an area of managed forest, upslope from Zone 1. Zone 2 is managed for control of pollutants in subsurface flow and surface runoff through biological and chemical transformations, storage in woody vegetation, infiltration, and sediment deposition. When trees in Zone 2 reach maturity, they can be harvested and the zone replanted with seedlings (Figure 4). Zone 3 is a grass or other herbaceous filter strip upslope from Zone 2. Zone 3 is managed to provide spreading of concentrated flow into sheet flow and to remove sediment and sediment-associated pollutants.
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Riparian forest buffer systems provide four important functions. The first is control of sediment and sediment-borne pollutants carried in surface runoff. Properly managed, RFBS should provide a high level of control of sediment and sediment-borne chemicals regardless of physiographic region. Research shows that forests are particularly effective in filtering fine sediments and promoting co-deposition of sediment as water infiltrates. The slope of the RFBS is the main factor limiting the effectiveness of the sediment removal function. In all physiographic settings, it is important to convert concentrated flow to sheet flow in order to optimize RFBS function. Conversion to sheet flow and deposition of coarse sediment, which could damage young vegetation, are the primary functions of Zone 3, the grass vegetated filter strip.
The second function of RFBS is to control nitrate in shallow groundwater moving toward streams. When groundwater moves in short, shallow flow paths through the RFBS, 90 percent of the nitrate input may be removed (Figure 5). In contrast, nitrate removal may be minimal in areas where water moves to regional groundwater. In these regions, high-nitrate groundwater may emerge in stream channels as base flow and bypass most of the RFBS (Figure 6). In the areas where this occurs or where high-nitrate water moves out in seepage faces, deeply rooted trees in Zone 1 or in seepage areas are essential. The degree to which nitrate (or other groundwater pollutants) will be removed in the RFBS depends on the proportion of groundwater moving in or near the biologically active root zone and on the residence time of the groundwater in these biologically active areas.
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The third function of RFBS is control of dissolved phosphorus (P) in surface runoff or shallow groundwater. Control of sediment-borne P is generally effective. In certain situations, dissolved P can contribute a substantial amount of total P load. Most of the soluble P is bioavailable, so the potential impact of a unit of dissolved P on aquatic ecosystems is greater. Some studies indicate that natural riparian forests may have very low net dissolved P retention. In managing for increased P retention, effective fine sediment control should be coupled with use of vegetation, which can increase P uptake into plant tissue.
The final function of RFBS is to provide control of the stream environment. These functions include modifying stream temperature and controlling light quantity and quality, enhancing habitat diversity, modifying channel morphology, and enhancing food webs and species richness (as shown on this magazine's cover). All of these factors are important to the ecological health of a stream and are best provided by an RFBS that includes a Zone 1 that approximates the original native vegetation. These functions occur along smaller streams regardless of physiographic region. These functions are most important on smaller streams, although they are important for bank and near-shore habitat on larger streams. RFBS contribute to bank stability and thus minimize sediment loading due to instream bank erosion. Depending on bank stability and soil conditions in Zone 1, management of Zone 2 for long-term rotations may be necessary for sustainability of the stream environment functions of Zone 1.
The bottom line
Riparian forest buffer systems can be used to reduce NPS pollution from farms, urban and suburban areas, construction sites, and forestry operations. However, RFBS should be considered insurance policies and should never be the only BMP used to control NPS pollution. Finally, RFBS will work to fullest capacity only under the proper hydrologic and topographic conditions.
ASAE member George Vellidis is a professor of biological and agricultural engineering at the University of Georgia, Tifton, GA 31793-0748 USA; 229-386-3377, fax 229-386-3958, email@example.com.
Richard Lowrance is an ecologist with the USDA-ARS Southeast Watershed Research Laboratory, Tifton, GA 31793-0946 USA; firstname.lastname@example.org.
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|Author:||Vellidis, George; Lowrance, Richard|
|Publication:||Resource: Engineering & Technology for a Sustainable World|
|Date:||Dec 1, 2004|
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