What makes an effective riparian buffer?
Summary
The results of the study show that dense grass buffers can be extremely effective riparian buffers in conditions of extreme rainfall and significant erosion such as are found in far north Queensland. However, the authors stress that buffering should only be considered as one element in an integrated approach to water system protection. Crop erosion controls and nutrient management practices are also essential aspects of waterway protection.
Research findings
Farm management can play an important role in protecting water systems. Land managers can help protect the health of our waterways by reducing export of nutrients to water systems. While good fertilizer practices are an important aspect of sus- tainable agricultural management, there are many other ways to protect our water systems.
Zones of vegetation surrounding a waterway, known as ʻriparian buffersʼ, are a particularly important way to minimize erosion and reduce transport of sediment to waterways. Riparian buffers work by slowing the movement of water through the buffer zone; as the water slows, infiltration into soil improves and deposition of sediment increases. This process can also assist to remove nutrients from run-off before it enters a waterway – infiltrating dissolved nutrients into soil, or depositing nutrients that are bundled with sediment particles.
While there is a range of laboratory studies that have examined the effectiveness of riparian buffers, to date there has been little quantitative data collected under natural conditions in the field. Field-based research is particularly important to land man- agers, as experimental designs may not incorporate the range of variables influencing buffer capacity at ʻreal-lifeʼ farming properties.
A recent study undertaken by McKergow et al. (2004) seeks to rectify this situation. Using two banana-farming properties in the Johnstone River Catchment, a region of far north Queensland where erosion has become a significant issue, the study investigated the capacity of four ʻreal-lifeʼ riparian buffers to cope with extremely high rainfall on intensively cropped land. Over a period of four years, the study investigated the effectiveness of a variety of vegetation types and landscape forms to trap sediment and nutrients.
So what factors helped the buffers effectively trap pollutants? From the results of the 2004 study, it appears that, in general, dense signal grass buffers are able to trap a significant amount of bedload eroded from cropped land during rainfall. In fact, McKergow et al. found that, even on slopes with high erosion levels, dense grass buffers were able to trap over 80% of the incoming bedload. However, McKergow et alʼs research indicates that placing buffers on slopes converging toward waterways may compromise their effectiveness. In particular, during the 2004 study, channelling of run-off toward a waterway was observed at a buffer placed on a converging slope. During an extreme rainfall event, a scour through this buffer was created that trans- ported significant sediment into the waterway. McKergow et al. recommend that, to prevent scouring on converging slopes, farmers should ensure that buffers begin as far up-slope as possible, where sediment loads are low and gradients are mild.
Although McKergow et al. stress the importance of native vegetation in maintaining the health of water systems, the results of the 2004 study indicate that rainforest may not be the most effective riparian buffer. In particular, a remnant rainforest buffer investigated during the 2004 study was not successful in trapping sediment. McKergow et al surmise that the lack of under-story in the rainforest buffer hindered its effectiveness – allowing some transport of sediment to the waterway. During heavy rainfall in the rainforest buffer, a series of little channels, or “rills”, formed around tree buttresses – increasing sediment transport to the waterway. In addition, nutrient levels in run-off increased while in the remnant rainforest, indicating that there was a nutrient source within the buffer itself – possibly leaf litter or sediment.
Although a significant amount of bedload deposited in the rainforest was permanently trapped, sediment often ended up being transported to the waterway in subsequent rainfall. By way of contrast, couch grass quickly took seed in sediment deposited in the signal grass buffers. These sediment deposits were, therefore, quickly stabilized and not transported to waterways in subsequent rain events. To maximize the benefits of rainforest buffers, McKergow et al. recommend that grass buffers should be planted upslope to trap as much sediment as possible before run-off enters rainforest.
While the results of the 2004 study show that dense grass buffers can be extremely effective in conditions of extreme rainfall and significant erosion, McKergow et al. stress that buffering should only be considered as one element in an integrated approach to water system protection. Crop erosion controls and nutrient management practices are also essential aspects of waterway protection. In addition, while the study reveals some important factors to consider when designing riparian buffers, many of its results are specific to farming conditions in far north Queensland. It is important that field based studies are conducted in a broad range of conditions so that the farming community can ensure that the buffer they employ will be as effective as possible under conditions specific to their region.
Reference
McKergow LA, Prosser IP, Grayson RB and Heiner D (2004) Performance of grass and rainforest riparian buffers in the wet tropics, Far North Queensland. Australian Journal of Soil Research 42: 485-498. Read Abstract.
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