SMARTER_BufferZ - Specific Management And Robust Targeting of Riparian Buffer Zones

Grass Buffer Strips at Watercourse

November 16th, 2021

Key Concepts

Often considered to be a very effective edge of field measure for separating watercourses from agricultural activity and intercepting overland flow to reduce run-off and pollutants into the stream from upslope fields. Several decades of evidence have developed on grass edge of field filter strips and these are very popular as agri-environment measures in riparian contexts. The buffer zone can be a physical break from cultivation activities (fertiliser and agrochemical use near the watercourse), or with a fence, from direct pressures on watercourses by livestock. The buffers can stabilise banks and roughen the riparian zone Width can vary depending on setting but it is increasingly recognised that functions of grass buffers are limited compared to inclusion of other specific aspects and that narrower (several metres) buffers are very limited. There is limited ability for anything other than quite wide buffers to function well for trapping and retaining dissolved nutrients, especially groundwater nitrate flows, when only grass is used. It is well understood that subsurface artificial drainage pathways can readily pass through buffers and undermine their effectiveness.

Summary of Evidence on Functions

There is a wealth of evidence on functioning for sediment, total P and to a lesser extent N and pesticides that shows the general effectiveness of grass buffers from relatively narrow widths. This is associated with roughness and infiltration that traps particles. Dissolved N and P functioning is worse and more complex to predict at a site. The functionality for faecal indicator organisms (FIO) retention in grassland requires a fenced buffer and this brings management issues for providing off-stream animal watering. Terrestrial habitat functions are potentially low although increasing roughness with tussocky grassland could improve habitat for small mammals and associated hunting birds. Biomass harvesting is counter-effective due to reducing hydraulic roughness and worsened run-off control, although it was recognised biomass harvesting can remove P. Occasional access by animals was considered appropriate but use for vehicular access was likely to be negative.

Technical Evidence References

Arora K et al. Review of Pesticide Retention Processes Occurring in Buffer Strips Receiving Agricultural Runoff' Journal of the American Water Resources Association, 2010. 46(3), 618-647;

Schoumans O et al. Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: A review' Science of the Total Environment, 2014. 468,1255-1266;

Collins AL, Blackwell MSA, Critchley N, Zhang YS and others. 'Developing The Evidence Base on Riparian Buffer Strips and Other Options for Sediment Loss From Agriculture' Defra Report, project WQ0208, 2012;

Dorioz JM et al.The effect of grass buffers on phosphorus dynamics - A critical review and synthesis as a basis for application in agricultural landscapes in France. Agriculture, Ecosystems and Environment, 2006. 117, 4-21;

Hoffmann CC et al. Phosphorus retention in riparian buffers: review of their efficiency' Journal of Environmental Quality, 2009. 38(5), 1942-55;

Newell-Price JP et al. 'An Inventory of Mitigation Methods and Guide to Their Effects on Diffuse Water Pollution, Greenhouse Gas Emissions and Ammonia Emissions from Agriculture: User Guide' Defra Project Report WQ0106, 2011;

Muscutt AD and others. 'Buffer zones to improve water quality: a review of their potential use in UK agriculture' Agriculture, Ecosystems and Environment, 1993. 45, 59-77;

Main Countries Where This Is Practiced

U.S., across Europe, Australia, Canada, New Zealand

Challenges and Limitations, Including Site-specific Factors, Planning Requirements and Particular Suitability to Farming System

Need time to establish grasses and stiffer vegetation to form a barrier to surface runoff. If placed in a livestock field these features need to be fenced. Reduces the size of field. Could impact on eligibility of land for Pillar I payments in CAP. Flow can often bypass the grass filter strip area. Vehicle compaction can make buffers less effective. This includes where sediment builds up on the buffer surface and needs removal or ground reprofiling. Overgrown grass and weeds, especially in nutrient rish, former cropland soils, can increase nutrient cycling and lead to poor biodiversity, necessitating managamant (cutting) of vegetation.

Ability to Modify It For Site Requirements

Grazed buffers offer a form of land management but exclusion of cattle (for microbial contamination of watercourses and bank erosion) is best practice. Temporary sheep grazing can help grass management and resources for manual cutting and removal. The measure is flexible thus lends itself to shorter term agri-environment measures (e.g. proposed Ecoschemes under Pillar I).

Critical Factors in Establishment and Ongoing Management

This measure requires relatively low management inputs but it still requires a certain level of effort to establish the buffer, for example to establish a grass sward. Sward should be dense and permanent to combat year-round surface flows. Buffer topsoil compaction and rills or gulleys should be removed. Heavy seeding machinery should be avoided (risk of compaction and bank collapse) and is not allowed in UK within 2 metres of the bank. Grass may be cut several times over first 2-years to promote density and control weeds. Once established, the level of maintenance required is potentially low. However, cutting and removing vegetation improves the nutrient retention capabilities and may guard against P leaching, but requires effort. Controlled use of herbicides (spot-treat and weed-wipe only) can remove aggressive weeds or non-native invasive plants. If the buffer is located within a livestock field or if intensive grassland is part of the rotation, then fencing will be required. Avoid poaching by livestock near to the fence. Don't cut vegetation from start of March to end of August to avoid disturbing nesting birds. Avoid trafficking the buffer with machinery, especially in wet weather.

Guidance and Other Resources

The Diffuse Pollution Measures User Guide report;

Farm Wildlife has guidance on creation of grass margins as buffer strips for wildlife with several case studies at https://farmwildlife.info/how-to-do-it-5/field-boundaries/grass-margins-as-buffer-strips/

Scotland's farm advisory service video at https://www.youtube.com/watch?v=NeuVKTx8U68

Championing the Farm Environment (CFE) have guidance including margin types for differing slopes, establishment and management advice at https://www.cfeonline.org.uk/advice-training/grass-buffer-strips-next-to-a-watercourse-or-pond/

Voluntary Initiative specifically guide on buffers to stop pesticide losses to waters at https://voluntaryinitiative.org.uk/media/2106/13055cpa-oilseedrapeherbicides_infosheet9.pdf

Swarm Hub have good guidance on buffer creation with links to workshop outputs and technical documents at https://www.swarmhub.co.uk/water-master/buffer-strips/

Riparian Measure Effectiveness

The effectiveness of the 16 short-listed riparian measures is based on analysis of expert opinion (covering a range of stakeholders). The measures are assessed based on their potential to deliver various ecosystem services relating to:

Soil loss control and sediment retention
Phosphorus capture and retention
Nitrogen capture, uptake and transformations
Pesticide/herbicide capture and breakdown

Faecal Indicator Organisms, barrier and retention
Benefits to aquatic processes (shade, leaf litter)
Terrestrial habitat diversity
System C retention (biomass, soil)
Hydromorphic and geomorphic improvement

Reduction in runoff response speed and volume including flooding management

Benefits to agronomic field processes: pollinators, pests
Production of biomass: food, fuel, green manure
Visual landscape enhancement
Integration with access and recreation
Hydromorphic and geomorphic improvement

The effectiveness score ranges from 1 (low) to 5 (high) and is subject to the appropriate siting and management of the measure. The assessment also included confidence in scoring (covering Low, Medium and High) and is based on the current pool of evidence on benefits of the measure.