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Christine Georgakakos1 and Karl Czymmek2
1Biological and Environmental Engineering, 2Animal Science and PRO-DAIRY; Cornell University
Riparian buffers are a popular practice implemented in both animal and crop agriculture. A riparian buffer is an uncultivated area surrounding streams or saturated areas. Saturated areas can include both perennially and temporally wetter-than-average parts of the landscape. We tend to prescribe buffers to span a certain distance from the stream, i.e. a 50 ft buffer along small headwater streams in agricultural lands.
The purpose of a riparian buffer is two-fold. Firstly, buffers filter soil and pollutants from water flowing over the soil surface. Permanent grass and other plant species slow the flow of water by friction. As water moves across a buffer, particles settle out and some water infiltrates into the soil, allowing sorption of pollutants to soil particles or otherwise slowing transport directly to a stream of soluble contaminants. The trapped or settled pollutants can be used by buffer plants in the case of nutrients, or tightly bound by soil particles for some non-nutrient pollutants. Some buffer biomasses are harvested as a way to remove nutrients tied up in the plant material. Secondly, when placed in livestock pastures, buffers are fenced to limit animal access to the stream to eliminate placement of urine and feces directly to surface waters or adjacent areas in addition to the benefits already outlined (Figure 1).
In practice there are a variety of factors that may reduce the water quality gains the buffer may be able to achieve. For optimal effect, many factors need to be considered when installing buffers. Buffers cannot filter water that does not flow over the soil surface or if too much water flows through too rapidly. Surface features and high runoff rates that cause flow to concentrate can overwhelm the ability of a buffer to filter, effectively allowing water to by-pass the buffer. For this reason, some conservation professionals advocate variable width buffers: extending permanent vegetation into fields or pastures to increase contact distance of flowing water within the buffer in areas likely to be saturated. Another means by which buffers can be by-passed is via ditches and subsurface drainage from fields or farmsteads where filtration is limited.
Common point sources tend to be tile line discharges, concentrated flow paths through pastures or fields, and overflow from other water routing structures. Any location where buffers are being planned should be inspected for features that can limit buffer effectiveness so that the system can be most efficiently designed for maximum pollutant removal. Understanding details about each riparian buffer site to assess potential nutrient inputs into streams before implementing a best management practice will help more fully address these issues.
Buffers in pasture systems raise different issues compared to crop fields. In buffer systems where livestock exclusion is involved, fences are typically installed in a straight line(s), simplifying construction and upkeep. Unfortunately, those lines do not always incorporate areas in the landscape that are likely to become saturated. When these wetter than average areas are not included in the buffer system, animals tend to trample the moist, soft soil, reduce infiltration capacity, and generate swampy, wet areas within . These areas then tend to puddle, and form concentrated flow paths through the buffer. (Figure 2) An ideal buffer would take into consideration these areas of higher saturation. Complicating management, these areas are not necessarily perennial, and can change year to year. From a management perspective, it could be potentially difficult as moving fencing to incorporate saturated zones into the buffer is less that desired unless moveable fencing is already included in the management plan. Meandering or movable buffer boundaries may be difficult to implement in many existing management systems, so addressing variable wet areas for animal exclusion would be most convenient if paired with other management practice changes.
Buffers are an effective water quality management tool even if imperfectly implemented. Farmers and planners should strive to identify the areas where buffers can do the most good and locate and eliminate possible by-pass factors for best water quality results. It is important to understand the nutrient inputs to a stream before implementing a riparian buffer so that water quality may be improved as much as possible.
Ideally, when implementing a buffer system, there are a few steps we can take to maximize buffer effectives and minimize short circuiting the buffer: (1) identify and try to address point sources flowing through the buffer system, (2) include areas likely to become saturated within the buffer zone, and (3) be prepared to modify a buffer zone as new saturated areas are developed. These steps should help address the end goal of buffers reducing nutrient pollution to streams.