Riparian forest buffer systems slow the flow of sediment and nutrients from cultivated lands.

By George Vellidis, Matt Smith, and Richard Lowrance

Agricultural runoff is surface water leaving farm fields because of excessive precipitation, irrigation, or snowmelt. In the early 20th century, there was considerable concern about erosion of farm fields caused by rainfall washing away valuable topsoil from the fields and resulting in loss of productivity. With the passage of the Federal Water Pollution Control Act Amendments of 1972, the potential for pollution of surface waters from agricultural runoff was officially recognized and an assessment of the nature and extent of such pollution was mandated (USEPA, 2002a; Stewart et al., 1976).

Agricultural runoff is grouped into the category of nonpoint-source pollution because the potential pollutants originate over large areas and the point of entry into water bodies cannot be precisely identified. Nonpoint sources of pollution are particularly problematic because it is difficult to capture and treat the polluted water before it enters a stream. Point sources, such as municipal sewer systems, usually enter the water body via pipes, and it is comparatively easy to collect that water and run it through a treatment system before releasing it into the environment. Because agricultural runoff is considered nonpoint-source pollution, efforts to minimize or eliminate pollutants focus on practices applied on or near farm fields. In other words, we usually seek to prevent the pollution rather than treating the polluted water.

Because of the great successes in reducing point-source pollution, the relative significance of pollution from agricultural runoff has increased, and it is now considered the primary source of pollutants to United States streams and lakes. It is also the third leading source of pollution in US estuaries (USEPA, 2002b). Pollutants in agricultural runoff include eroded soil particles (sediments), nutrients, pesticides, salts, viruses, bacteria, and organic matter.

Agricultural Runoff Quantity

Agricultural runoff occurs when the precipitation rate exceeds the infiltration rate of the soil. Small soil particles that have been dislodged by raindrops can fill and block soil pores, resulting in a decrease in infiltration. As excess precipitation builds up on the soil surface, it flows in thin layers from higher areas to lower. This diffuse surface runoff quickly starts to concentrate in small channels called rills. The concentrated flow generally has a higher velocity than the flow in thin films over the surface. The concentrated flow velocity might become rapid enough to cause scouring of the soil that makes up the channel sides and bottom. The dislodged soil particles can then be carried by the flowing water to distant locations in the same field or be carried to a receiving water body. If the quantity and velocity of flow are large enough, the rills can grow so large that the damage cannot be easily repaired by typical earthmoving machinery. When this happens, the rill has become a gully.

The quantity of runoff from agricultural fields is not usually listed explicitly as a concern separate from the quality of the runoff. It should be considered, however, because it transports the pollutants and because excessive flows can cause erosion of receiving streams. Also, if more water is retained on the field, there is likely to be a corresponding reduction in the amount of supplemental water to be added through irrigation. Runoff quantity varies significantly according to soil type, presence of vegetation and plant residue, physical soil structures such as contoured rows and terraces, field topography, and the timing and intensity of the rainfall event.

Some agricultural practices increase the infiltration capacity of the soil while others decrease it. The presence of vegetation and plant residues reduces runoff. Actively growing plants also reduce the amount of water in the soil through evapotranspiration, thus making more room for infiltrating water to be stored in the soil profile. Bare soils increase runoff because there is nothing except the soil surface to absorb the energy of raindrops. The rain, therefore, dislodges soil particles that will tend to seal the surface and reduce infiltration.

Soil Erosion and Associated Pollutants

One of the primary pollutants in agricultural runoff is eroded soil. In 1975, 223 million ac. of cropland produced 3,700 million tons of eroded sediments - an average of nearly 17 tons of soil lost per acre of cropland per year. It is estimated that cropland, pasture, and rangeland contributed more than 50% of the sediments discharged to surface waters in 1977 (Van der Leeden et al., 1990).

Eroded sediments also represent a loss of fertile topsoil from the field, which can reduce the productivity of the field. Soil formation is an extremely slow process occurring over periods ranging from decades to centuries (Foth, 1990). Excessive erosion can cause farmers to use more fertilizer and water, plant more tolerant crops, or abandon fields for agricultural production.

Many of these sediments are heavy and will settle in slow-moving portions of streams or in reservoirs, dramatically altering the ecology of the streambed. Aquatic plants, insects, and fish all have specific requirements related to composition of the streambed for them to live and reproduce (Gordon et al., 1992). Sediments in reservoirs reduce the volume of water that can be stored. This might result in reduced production of hydroelectric power, reduced water availability for municipal supply, interference with navigation and recreation, and increased dredging to maintain harbor navigability.

Eroded sediments can transport other pollutants into receiving waters. For example, the plant nutrient phosphorus, used as fertilizer, is most often transported from fields by chemically bonding to clay minerals. Many agricultural pesticides also bond to eroded clays and organic matter. Once these chemicals have entered the aquatic ecosystem, they can be released from their sediment carriers. Phosphorus, when released, can contribute to the eutrophication of lakes and reservoirs. Pesticides and their degradation products can be toxic to aquatic life and must be removed from municipal water supplies.

Erosion from animal agriculture, such as feedlots and pastures, can result in the transport of sediments composed of animal manures, which can transport significant quantities of potential pathogens. Animal manures are primarily organic in nature and can serve as a food source for natural bacteria in the receiving water. When these naturally occurring bacteria begin to utilize the organic matter in this way, they might deplete the water of dissolved oxygen as they respire and multiply, a process known as biochemical oxygen demand (BOD). High BOD can reduce stream oxygen levels enough that fish and other organisms that require dissolved oxygen suffer, die, or relocate, when possible, to more suitable habitats (Gordon et al., 1992).

Dissolved Pollutants

Agricultural runoff can carry with it many dissolved pollutants, including plant nutrients, pesticides, and salts. Because they are dissolved, control measures are most often aimed at reducing the volume of agricultural runoff or making the pollutants less available to be dissolved.

Nitrogen is one of the major pollutants of concern in agricultural runoff. A relatively cheap component of most fertilizers and necessary for plant growth, nitrogen in the form of nitrate is highly soluble in water. Just as it does in an agricultural field, nitrogen can promote growth of vegetation in aquatic environments. Excess nitrogen and phosphorus in runoff can lead to the eutrophication of lakes, reservoirs, and estuaries. Nitrogen in the form of ammonia can be dissolved in runoff from pastures and feedlots. Ammonia is toxic to many aquatic organisms, thus it is important to it ammonia in runoff (Abel, 1996).

Many agriculturally applied pesticides are also soluble in water. If they get into aquatic ecosystems, there is a potential for toxic effects. These pesticides must also be removed from drinking-water supplies, and if concentrations are high or persistent, such treatment can be difficult and expensive. Stable, persistent pesticides can bioaccumulate in the food chain, and people who eat fish from contaminated waters might be exposed to higher concentrations than exist in the water (USEPA, 2002c).

Runoff from agricultural fields can contain significant concentrations of dissolved salts, which originate in precipitation, irrigation water, fertilizers and other agricultural chemicals, and from soil minerals. Plants generally exclude ions of chemicals that they do not need, so dissolved salts in irrigation water, for example, can be concentrated in the root zone of the growing crop. Runoff can redissolve these salts and transport them into aquatic ecosystems where some - naturally occurring selenium, for example - can be toxic to fish and other wildlife (US Geological Survey, 2002).

Transport of fertilizers and pesticides from their point of application can result in significant environmental costs. This transport, or loss from the field, can have substantial negative economic impacts on the grower. Fertilizers lost from fields are not available to promote crop growth. Agricultural chemicals lost from the field are not available to protect the plants from pests and diseases. In both cases the grower is paying for expensive inputs and paying to apply them. It is always in the growers' and the environment's best interests, therefore, to keep agricultural chemicals in the field where they are needed.

Controlling Agricultural Runoff

Rill erosion

Algal growth as a result of eutrophication

One of the most direct methods of controlling pollution by agricultural runoff is to minimize the potential for runoff. Other methods or best management practices (BMPs) can be employed to reduce the amounts of sediments and dissolved chemicals in runoff. Often practices aimed at controlling one aspect of agricultural runoff are also effective at reducing other components because of the interrelationships between runoff volume, erosion, transport, dissolution, and delivery.

Maintaining good soil tilth and healthy vegetation can minimize runoff by promoting increased infiltration. Terracing, contour plowing, and use of vegetated waterways to convey runoff can result in decreased runoff by slowing the water leaving the field and allowing more time for infiltration to occur. Construction of farm ponds to receive runoff can result in less total runoff from the farm, lowered peak rates of runoff, and storage of runoff for use in irrigation or livestock watering (Stewart et al., 1976).

Control of water pollution in agricultural runoff is most effectively achieved by reducing erosion from the field, and the primary method of doing this is by maintaining a vegetative or plant residue cover on the field at all times or minimizing bare areas of the field. Techniques include conservation tillage, strip tillage, and the use of cover crops. Additional measures that can be employed at the edge of the field, or off-site, include vegetative filter strips and farm ponds.

Loss of nitrogen and other plant nutrients can be reduced if nitrogen is applied in the quantity required and when the crop needs it. This requires multiple applications and can be difficult for tall crops. For this reason, most or all of the nitrogen required by a crop is often applied at planting. Because nitrogen fertilizers are relatively inexpensive, growers tend to overapply rather than underapply. Soil tests can tell a grower how much nitrogen is already in the soil and how much needs to be applied for a specific crop. Efforts have been made to make the nitrogen less soluble by changing the form applied to the field so that it becomes available to the plants (and, thus, available for loss in runoff) more slowly (Owens, 1994).

Another method of controlling the loss of agricultural chemicals is to minimize their use through such programs as integrated pest management, under which some crop damage is allowed until it becomes economically justified to apply pesticides (US Department of Agriculture, 2002). And a third approach is to make the chemicals more easily degraded so that they are less likely to stay around long enough to be influenced by runoff-producing rainfall events.

Riparian Forest Buffer Systems

Sediment accumulating along the perimeter of an RFBS after a spring storm

In contrast to the aforementioned practices, riparian forest buffer systems (RFBSs) are streamside ecosystems that can be managed to reduce nonpoint-source pollution after it leaves the field but before it reaches the stream. Riparian forests have been found to reduce delivery of nonpoint-source pollution to streams and lakes in many types of watersheds . Vellidis et al. (2002, 2003), Lowrance et al. (1983, 1984a, 1984b, 1985a, 1985b, 1997), and Peterjohn and Correll (1984) demonstrated that riparian forest ecosystems are excellent nutrient and herbicide sinks that reduce the pollutant discharge from surrounding agroecosystems. The use of RFBSs is relatively well established as a BMP for water-quality improvement in forestry practices (Comerford et al., 1992) but has been much less widely applied as a BMP in agricultural areas or in urban or suburban settings. RFBSs 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 US (Leopold et al., 1964). Riparian vegetation has well-known beneficial effects on the bank stability, biological diversity, and water temperatures of streams (Karr and Schlosser, 1978).

Compared to other nonpoint-source pollution control measures, RFBSs can lead to longer-term changes in the structure and function of agricultural landscapes. To produce long-term improvements in water quality, RFBSs must be designed with an understanding of the processes that remove or sequester pollutants entering the riparian buffer system, the effects of riparian management practices on pollutant retention, the effects of such forest buffers on aquatic ecosystems, the recovery time after harvest of trees or reestablishment of riparian buffer systems, and the effects of underlying soil and geologic materials on chemical, hydrological, and biological processes.

Experiment designed to evaluate RFBS management practices near Tifton, GA

The US Department of Agriculture has used the research cited above to develop a general riparian forest buffer system specification for controlling nonpoint-source pollution from agriculture and improving general water quality. The specification calls for a three-zone buffer system, with each zone having specific purposes but also interacting with the adjacent zones to provide the overall RFBS function (Figure 1).

Click here for a large view

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, that 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. Zone 3 is a grass or other herbaceous filter strip upslope from Zone 2 that is managed to spread concentrated flow into sheet flow and to remove sediment and associated pollutants.

RFBSs provide four important functions. The first is control of sediment and sediment-borne pollutants carried in surface runoff. A properly managed RFBS should provide a high level of such control 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. 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 an RFBS is to control nitrate in shallow groundwater moving toward streams. When groundwater moves in short, shallow paths through the RFBS, 90% of the nitrate input may be removed (Figure 2).

In contrast, nitrate removal might be minimal in areas where water moves to regional groundwater. In these regions, high nitrate groundwater might emerge in stream channels as base flow and bypass most of the RFBS. 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 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.

The third function of an 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. It appears that natural riparian forests have very low net dissolved P retention. In managing for increased P retention, effective fine-sediment control should be coupled with the use of vegetation, which can increase P uptake into plant tissue.

The final function of an RFBS is to provide control of the stream environment: modifying stream temperature and controlling light quantity and quality, enhancing habitat diversity, modifying channel morphology, and enhancing food webs and species richness. These factors are important to the ecological health of a stream and are best provided by an RFBS, which 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. RFBSs contribute to bank stability and thus minimize sediment loading caused by instream bank erosion. Depending on bank stability and soil conditions in Zone 1, management of Zone 2 for long-term rotations might be necessary for sustainability of stream environment functions of Zone 1.

References

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Comerford, N.B., D.G. Neary, and R.S. Mansell. "The effectiveness of buffer strips for ameliorating offsite transport of sediment, nutrients, and pesticides from silvicultural operations." Natl. Coun. Paper Ind. Air & Stream Improvement Tech. Bull. 631. New York, NY. 1992.

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Lowrance, R., J.K. Sharpe, and J.M. Sheridan. "Long‑term sediment deposition in the riparian zone of a coastal plain watershed." J. of Soil and Water Cons., 41:266‑271. 1986.

Lowrance, R., G. Vellidis, R.D. Wauchope, P. Gay, and D.D. Bosch. "Herbicide transport in a riparian forest buffer system in the coastal plain of Georgia." Transactions of the ASAE, 40(4):1047-1057. 1997.

Owens, L.B. Impacts of Soil N Management on the Quality of Surface and Subsurface Water in Soil Process and Water Quality. R. Lal and B.A. Stewards, Eds. Lewis Publishers Inc., Boca Raton, FL. 1994.

Peterjohn, W.T. and D.L. Correll. "Nutrient dynamics in an agricultural watershed: Observations on the role of a riparian forest." Ecology, 65:1466-1475. 1984.

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Van der Leeden, F., F.L. Troise, and D.K. Todd. The Water Encyclopedia. Lewis Publishers, Chelsea, MI. 1990.

Vellidis, G., R. Lowrance, P. Gay, and R.D. Wauchope. "Herbicide transport in a restored riparian forest buffer system." Transactions of the ASAE, 45(1):89-97. 2002.

Vellidis G., R. Lowrance, P. Gay, R.W. Hill, and R.K. Hubbard. "Nutrient transport in a restored riparian wetland." Journal of Environmental Quality, 32(2). 2003.

Webster, J.R., S.W. Golladay, E.F. Benfield, J.L. Meyer, W.T. Swank, and J.B. Wallace. "Catchment disturbance and stream response: an overview of stream research at Coweeta Hydrologic Laboratory." In P.J. Boon, P. Calow, and G.E. Petts (Eds.) River conservation and management, pp. 231-253. John Wiley & Sons, New York, NY. 1992.

George Vellidis and Matt Smith are associate professors of biological and agricultural engineering at the University of Georgia in Tifton, GA. Richard Lowrance is an ecologist with the USDA Agricultural Research Service's Southeast Watershed Research Laboratory in Tifton, GA.

SW - May/June 2003

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