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Beneficial management practices for reducing runoff and soil loss

An analysis of diversion terraces and grassed waterways at the Black Brook Watershed proved that this beneficial management practice (BMP) was effective in reducing runoff and soil loss.

Diversion terraces and grassed waterways

Diversion terraces (sometimes called contour terraces) are BMPs that break up a long, sloping field into a series of fields with shorter slopes in order to divert runoff and encourage infiltration. This is done to conserve water for plant growth and to reduce runoff related soil erosion. Excess runoff drains into grassed waterways or other suitable outlets.

Grassed waterways are another BMP that can be used alone, or in conjunction with, diversion terraces, and are typically constructed in natural depressions where water would normally flow and collect. The reduced grade and increased surface roughness in grassed waterways slow down runoff, filter out sediment, and reduce erosion.

Findings from the Black Brook Watershed in New Brunswick, and from other nearby study sites, indicated that a combination of diversion terraces and grassed waterways BMPs can reduce surface runoff, soil erosion, and pesticide concentrations in waterways. But this combination may contribute to excess nutrient loadings via increased water infiltration to groundwater. Results from the upper watershed, however, often paint a different picture from that of the combined flow. This is an indication that local variability, such as BMP location, weather, or landscape characteristics, may influence net BMP effect.

Surface water monitoring has also shown that a trade-off to the potential benefit of these BMPs may be an overall increase in soluble nutrient leaching to groundwater, as reflected in degraded stream water quality during low flow periods. A separate study has also confirmed that these BMPs can significantly reduce water contamination by pesticides.

The soils and landscapes in this watershed are typical of those underlying many of the potato producing regions in north-eastern North America. Research findings from this watershed may be applicable to these areas as well.

Streamflow and sediment

Runoff reductions were very apparent at the bottom of the two sub-watersheds, where the average yearly streamflow decreased by 24%. Total yearly sediment loads were reduced by over half from pre-BMP implementation. The average sediment concentration in the stream also decreased from 0.7 to 0.4 grams per litre.

By comparison, post-BMP changes in streamflow and sediment were minimal at the upper, smaller sub-watershed location. Although average yearly streamflow declined by 11%, total yearly mass loading of sediment to the stream did not decline. The average concentration in the stream water samples slightly increased from 0.9 to 1.3 grams per litre.

Aerial view of diversion terraces and grassed waterways in the Black Brook Watershed.

More than half of the farmland in the Black Brook Watershed is now protected by diversion terraces and grassed waterways.

Nutrients

Although diversion terraces and grassed waterways appear to contribute to reductions in surface runoff and soil erosion, there is evidence that nutrients may be increasing in surface waters.

In the combined flow from the two sub-watersheds, average nitrate concentrations increased by 28% (from 4.8 to 6.2 milligrams per litre), whereas total nitrate loading (2.5 tonnes per year) remained unchanged. At the same time, while the average soluble phosphorus concentration at the lower outlet almost doubled (from 0.045 to 0.09 milligrams per litre), the total annual mass loading of phosphorus at the lower outlet also remained unchanged (30 kilograms per year).

Meanwhile, at the upper sub-watershed, a 16% increase in nitrate concentration (from 6.8 to 8.0 milligrams per litre) and a doubling of phosphorus concentrations (from 0.045 to 0.09 milligrams per litre), were similar to the trends noted at the lower location. However, in contrast to unchanging nutrient mass loadings at the lower outlet, the upper watershed had a 17% decrease in nitrate loading (from 1.3 to 1.1 tonnes per year), whereas phosphorus loading at the upper watershed increased three-fold (from 4.8 to 15.2 kilograms per year).

Researchers speculated that the noted rise in soluble nutrients may be due to increases in groundwater contribution to streamflows, resulting from improved water infiltration following BMP implementation. Although decreases in surface runoff will reduce the transport of sediment-bound nutrients to waterways, the increased infiltration caused by these same runoff waters being retained in terraces may allow nutrients to reach groundwater, and hence surface waters, by a different means.

Pesticides

Results indicated that, overall, diversion terraces and grassed waterways decreased pesticide concentrations by almost 50% compared to the site without these BMPs. During low flow conditions, pesticide concentrations were generally less than the water quality guidelines for aquatic life. Although, concentrations of selected pesticides sometimes exceeded guidelines during heavy rainfall.

Economic considerations

Hydrologic modelling results indicated that the implementation of diversion terraces and grassed waterways at the Black Brook Watershed contributed to a 20% reduction in water runoff, and a 56% reduction in sediment loadings. However, the benefit of sediment reduction decreased as the percentage of land protected by diversion terraces within the watershed increased. The associated marginal costs also increased as the number of diversion terraces went up. In other words, it became increasingly more expensive to further reduce sediment loading as the area of protected land expanded within the watershed.

Economic analysis showed that these BMPs have high installation and annual maintenance costs. While they do not appear to negatively affect production yields, the extent of their potential economic returns has yet to be fully determined. Additional on-farm and off-farm benefits have yet to be assessed. On-farm benefits may include the value of keeping the soil profile from eroding away, along with the elimination of erosion sediment loads that interfere with cropping operations, and the prevention of erosion gullies. Off-farm benefits may include cost reductions associated with removing sediment from road ditches, and the public good achieved through reduced runoff and cleaner water.

Potato yield and quality surveys conducted over four crop years revealed no consistent improvements resulting from terrace or contour cropping. Because crop response may vary under different potato varietals and weather patterns, the surveys were unable to show that yield increase alone will consistently pay for the cost of BMP application, at least in the short term.

Landowners would normally apply conservation measures on a priority basis to those lands subject to the most critical soil erosion. The implementation of diversion terraces and grassed waterways on the remaining, less vulnerable lands would result in less proportional sediment reduction response. Lands that do not exhibit critical soil erosion problems may benefit from the implementation of other BMPs.

The soils and landscapes in this watershed are typical of those underlying many of the potato producing regions in north-eastern North America. Research findings from this watershed may be applicable to these areas as well.

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