How Effective is Riparian Management for Protecting Waterway Health?
Concern about the health of streams in farming areas has lead farmers to adopt new technologies and practices to improve stream health. Riparian management through creating buffer zones is one of the tools that farmers can use to help them farm sustainably. But how effective are Riparian Buffer Zones? How much of their success is dependent on site-specific factors? Does one size fit all?
Earlier this year, MAF commissioned NIWA to review Riparian Buffer Zone effectiveness (report available on the MAF website1). MAF sought to know how effective buffer zones are at removing sediment and nutrients in farm runoff before these contaminants enter streams and whether buffer zones can enhance biological diversity. The review confirms that managing a small proportion of productive land alongside waterways can provide substantial environmental benefits to streams.
As indicated in the box, there are many different forms of riparian management practiced today. The review considered these forms to analyse effectiveness where appropriate studies were available.
There is clear evidence, from the review, that riparian buffer zones can be effective at removing nutrients and sediment from surface and subsurface flow paths. Buffer zones can remove nutrient and sediment inputs to streams by restricting the direct use of land beside the stream (by excluding stock) and by processing water as it travels through the riparian zone. Fenced riparian buffers stop erosion and impaction of soils caused by stock and exclude the direct input of faeces and nutrients to streams. The dense vegetation associated with a buffer reduces the velocity of surface flow, promoting infiltration of water and associated contaminants into the soil and deposition of sediment and particulate nutrients. Wetlands alongside the stream, characterised by low-oxygen, organic-rich soils, promote the removal of nitrate through the process of denitrification where microbes convert nitrate to nitrogen gases. Furthermore, plants themselves take up dissolved nutrients to grow.
Several studies have shown impressive removal rates of over 90 percent of the soluble nitrate travelling through buffer zones. However, the review revealed that the effectiveness of contaminant removal does differ according to the type and width of the buffer zone, characteristics of the local hydrology, soils, and vegetation, and the mode of contaminant transport to streams. Studies comparing multiple width buffers in the same location have shown that, while sediment and total phosphorus removal rates increase with increasing buffer width, substantial sediment removal occurs within only a few metres of the upslope boundary. Grass filter strips, in particular, have proven very effective at trapping sediment particles. Much of the coarse fraction of sediment may be removed within 5 metres of grass buffer, but finer particles require a greater buffer width. Buffer effectiveness can be minimal on steep slopes where flow convergence occurs, and the width of buffer zones may need to extend into these areas.
Nitrate removal from subsurface flows is considered to be higher in forested rather than grassed buffers, partly through uptake by plants and denitrification. Riparian wetlands have been shown to remove ³ 90 percent of nitrate if the hydrological residence times in the wetland are sufficiently long. Planted buffer zones have been shown to be most effective for dissolved nutrient removal when subsurface flow paths cross the root zone of trees before reaching the stream. This nitrate removal can continue indefinitely, provided that there is an ongoing supply of organic carbon (e.g. from tree roots and plant litter in the soil). Farming on land drained by artificial subsurface drains, or very free draining soils, can cause agricultural pollutants to bypass beneath buffers.
Some researchers suggest that buffer zones may have a limited life span where they can continue to be effective for the removal of some contaminants. For example, over time they may become saturated with phosphorus, pore spaces in soils may clog with sediments, or dissolved nutrient uptake by plants may be greatest during early growth phases and decline as vegetation matures. Buffers may need to be wider than early stage studies suggest, or sustainable methods of plant (and thus nutrient) removal may need to be investigated, to retain their uptake capacity long term.
Many of the existing planted buffers in New Zealand are at an early stage of development. While biodiversity improvements in buffers have been less well studied, it is evident that restoring native vegetation to the riparian zone increases terrestrial biodiversity. Restoration of aquatic biodiversity, however, depends upon ease of colonisation by native plants and animals and hence, the location of buffers relative to undisturbed source areas. Improvements are most likely to occur when buffer zone planting begins in the headwaters (particularly when this is near native forest) and forms a contiguous corridor.
The review highlights the need to account for a number of inter-related factors such as hydrological pathways, soil drainage characteristics, and the effects of shading upon in-stream processes. An integrated, whole catchment approach is required to improve buffer effectiveness and achieve water quality and biodiversity goals. Assessing catchment and regional hydrology, such as soil drainage profiles and mapping of wetlands as hotspots for denitrification are areas of future research and development that could assist resource managers.
Riparian Management can take various forms:
- Grass filter strips: Fenced strip of rank paddock grasses to filter disease causing microbes, nutrients, and sediment.
- Headwater or riparian wetlands: Fenced wetlands as hotspots for nutrient removal.
- Rotational grazing: Filter strips with varied stock grazing practices, such as occasional light grazing by sheep.
- Forested or planted native trees: A buffer of native trees to return ecological function to the stream and provide water quality benefits.
- Production trees or plants: A buffer of forestry trees left unharvested along stream banks, or production trees that are planted in riparian zones for selective harvesting with minimal disturbance (e.g. Tasmanian blackwoods). Plants such as flax for weaving, fruit and nut trees, or high value native tree species that can be selectively harvested, may also provide ecological function, and a mechanism to remove nutrients such as phosphorus from the riparian zone.
- Multi-tier system: A combination of buffers where native forest trees may be used beside the stream to enhance ecological function and biodiversity, a buffer of production trees could occur between the native trees and a grass filter strip beside agricultural land.
| Stephanie Parkyn Freshwater Ecologist, NIWA, Hamilton |
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| Stephanie has been involved in studies of land use effects on waterways at NIWA in Hamilton since 1994, focusing largely on the effects of pastoral farming. She has conducted research on the ecological benefits of riparian management on stream health and presented workshops on riparian management. Stephanie studied at Canterbury University for her undergraduate degree (BSc Hons) and gained a PhD from Waikato University studying freshwater crayfish ecology in streams of pasture and native forest land uses. |
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Contact for Enquiries
Amber Duncalfe
Editor - RM Update
Ministry of Agriculture and Forestry
PO Box 2526
Wellington
NEW ZEALAND
Tel: +64 4 894 0710
Fax: +64 4 894 0745
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