Buffer strips around cropland are typically viewed as “filters,” taking up excess nutrients before they can reach waterways. Some provincial governments are considering legislation requiring farmers to leave buffer strips. However, new work from the University of Manitoba questions their effectiveness.
Dr. David Lobb, senior research chair for the Watershed Systems Research Program and a U of M soil science professor, says riparian buffer strips are “highly inefficient” when it comes to filtering nutrient runoff from agricultural land in the Prairies, and in fact might contribute nutrients to runoff.
Three years ago, Lobb and colleagues completed a study on water quality for Environment Canada under the Lake Winnipeg Stewardship Fund that examined the effectiveness of riparian buffers on multiple sites. The report received little attention, and its recommendations have yet to turn into policy. This year, Minnesota passed a buffer strip requirement into law requiring “perennial vegetation buffers of up to 50 feet along rivers, streams, and ditches that will help filter out phosphorus, nitrogen, and sediment.”
Lobb says there are many benefits to riparian buffer strips. They improve wildlife habitats, promote stream health, and offer a setback for additional farm safety near waterways.
“But we don’t know the optimum width for corridors or habitats. We don’t know what the minimum should be. Many jurisdictions, including Manitoba, have promoted buffers as a filter — that’s a problem. They aren’t. They’re so inefficient that it would be ridiculous for a farmer to propose establishing a riparian buffer for the purpose of protecting water quality,” says Lobb.
Lobb says there are two reasons riparian buffers are not effective in “filtering” nutrients in the Prairies. First, in northern environments, the heaviest runoff events occur in the spring with snowmelt, when soils and vegetation are frozen and infiltration is extremely limited or nonexistent. “The vegetation is likely contributing nutrients to any runoff that’s occurring,” says Lobb. About 80 per cent of the runoff and nutrient losses occur during spring snowmelt in this region.
The second reason has to do with basic hydrology. When water runs off the land it concentrates. “Water passing through the riparian buffer will pass through maybe one per cent of the area,” he explains. “Ninety-nine per cent of the buffer does not intercept runoff.”
Lobb says vegetative filter strips were developed as an engineered technology in which runoff passes through the vegetated area during growing season as a sheet of water in a controlled manner, and the vegetation is harvested to remove the nutrients. “Now we’re looking at undulating land, where water runs through an unmanaged riparian area, and through only a very small percentage of that riparian buffer. Based on that fact alone, riparian buffers are highly, highly inefficient, and more likely to cause a water quality problem than to solve one.”
To make them more effective, Lobb says buffer zones should be “shaped” and smoothed out to promote evenly dispersed flow through more of the riparian areas and to detain runoff and retain sediments. And, they must be designed so the vegetation can be harvested and the accumulating nutrients can be removed.
“In low-lying areas where runoff occurs, producers should put in a broad grassed waterway and harvest the grass. This means that you put vegetation where the flow is occurring and then harvest that vegetation. That’s the best thing producers can do,” he says.
“If you don’t remove the vegetation, it senesces and dies and releases those nutrients into the runoff and waterways. There’s a problem with a strict naturalist approach to managing riparian areas. If we don’t manage the riparian areas, they will not be effective as buffers.”
Sandi Riemersma, an environmental biologist with Palliser Environmental Services, says the effectiveness of buffers depends on several factors, including the slope of land, soil characteristics, buffer width, vegetation, season and management.
“A riparian buffer strip is a good tool to reduce sediment transport and often can reduce particulate phosphorus mobility,” she says. “But buffers are not effective in winter and early spring when vegetation is dormant, soils are frozen and microbial activity is low or absent,” she says.
Riemersma emphasizes nutrient application management as an essential aspect of protecting waterways from nutrient runoff. In addition, she says permanent cover should be maintained near waterways, steep slopes and on erodible and saline soils. “Riparian buffers help to maintain stable streambanks, thereby reducing soil erosion and associated sediment and nutrient transport in waterways,” she says.