The role of buffer features in the rural landscape is to reduce the impact of agricultural pollutants (including sediment) on the aquatic envi- ronment. However, a conflict exists between environmental benefits and economic goals of the farmer. Presently, there are some economic incen- tives for farmers to incorporate buffer strips into their farming system, including financial incentives within agri-environment schemes and reduced risk of prosecution. Yet rising crop prices may well lead to a decline in uptake of agri-environment schemes in the future. To maintain a balance between economic goals of productivity and environmental quality, it is important that features, such as vegetated buffer strips, are effectively designed and placed within the landscape, in order to maximize sediment
Table 6 Buffer Selection Table (BST), for the optimal buffer width selection based on soil type and ground cover fraction of buffer. The values represent buffer width (m) needed to reduce soil loss from a 1ha field to less than 2t ha1yr1
Field classification Buffer condition (% ground cover)
Class Soil <20% 20–40% 40–60% 60–80% 80–100%
Heavy C 137 63 47 30 24
ZC 99 53 28 20 14
Medium ZCL 67 29 18 12 8
SCL 51 24 14 10 6
Light ZL 8 4 2 2 2
SL 4 2 2 2 2
and sediment-associated pollutant capture, and to minimize the area of land taken out of production.
A DSS based on the integration of informed decisions founded on available literature, field and landscape observations, and model-aided design decisions will help maximize environmental protection while limit- ing land taken out of production (Dosskeyet al., 2008). From the informa- tion gathered in the Parrett catchment, literature survey, and MMF model simulations, a DSS for buffer placement was developed: the Buffers DSS (see Fig. 4). The Buffers DSS based on the Parrett catchment data set has seven steps. Steps 1–4 and Steps 6–7 rely on field and landscape observa- tions; while model-aided buffer design is incorporated into Step 5. The following sections describe these steps.
Step 1, identify fields with soil erosion problems: in the first step, target fields are identified where erosion is known to be a problem or risk. This can be achieved through local knowledge and experience of relative susceptibility for the land to erode (Table 7), or by other means, such as soil erosion models, farm advisors, or other expert guidance. There may already be local government requirements to collect such data, for example, under current
Step 1: Identify fields with soil erosion problem
Step 4: Identify methods of preventing soil loss
Step 5: Identify appropriate width and shape of filter strip
Step 7: Check for damage and maintain
Step 6:
• Consider additional function, for example, habitat
and biodiversity Step 6:
• Does the design meet specification
• Density and species composition
• Linear feature For example, vegetated buffer strip
• Variable width No
Yes
New Existing
Soil erosion reduced
Step 3:
Do you want to prevent soil loss?
Step 2: Identify the surface flow pathways and locate field boundaries towards which sediment
is likely to be transferred
Figure 4 Schematic diagram of BUFFERS Decision Support System for the effective design and placement of vegetated buffers in a field system.
UK agri-environment schemes there is already an obligation for farms entered into the Entry Level Scheme (ELS) or Organic Entry Level Scheme (OELS) for Environmental Stewardship to prepare a Farm Environment Record. This record requires farmers to identify fields at high risk of soil erosion (see, e.g.,Fig. 5).
As part of Step 1 it is important to consider how management factors affect soil erosion. Since field management not only affects erosion potential from year to year but also within each season, it is important to identify the time when the greatest likelihood of erosion will occur. Erosion should be managed assuming the worst-case management scenario so that any tempo- ral changes in condition are within the limits of any control measure put in place. Ultimately, the allocation of target fields will depend on the tolerance to erosion included in the decision-making and connectivity, as defined in Step 2.
Table 7 Relative susceptibility for land to erode for a range of land-use, soil cultivation, and weather conditions (modified from “Controlling soil erosion,”Defra, 2005a)
Relative
susceptibility Land use Cultivation Weather
High Late sown winter cereals Fine seed beds Wet Potatoes
Sugar beet Field vegetables
Miscanthus (during rhizome harvesting)
Outdoor pigs Bare land after root crop harvesting Grass re-seeds
Forage maize Out wintering stock Grazing forage crops in
autumn or winter
Rough ploughed/
cultivated land Moderate Early sown winter cereals
Oilseed rape—winter and spring sown
Spring sown cereals Spring sown linseed Short rotation coppice/
Miscanthus
Cereal stubble Long grass leys
Permanent grass
Low Woodland (excluding short term coppice)
Land with good crop/
vegetation cover
Dry
Step 2, identify sediment flow pathways and target location of buffer feature: in Step 2 the risk of diffuse pollution is evaluated. For all target fields identified in Step 1 (e.g.,Fig. 5), direction of sediment flow pathways and connectiv- ity are determined. This can simply be achieved by sketching local knowl- edge (e.g., historical, or recently observed instances of runoff), onto a topographic map, or using more sophisticated methods, such as interpreta- tion of potential flow pathways using a geographical information system (GIS) interpretation of a digital terrain model. Tools such as the BZIEF would also help provide such information. Additionally, boundaries inter- cepting flow pathways are identified during this step, marking the location where buffers should be placed.
Understanding connectivity in relation to erosion risk in a field is important because fields with high erosion rates may not represent an environmental risk if they are not directly connected to a watercourse (cf.
Walling and Zhang, 2004). Similarly, fields with low erosion risk may represent a higher environmental risk if they are directly connected to the watercourse. Tolerance will vary depending on acceptable levels set by policy at the time of implementation.
High erosion Moderate erosion Low erosion
To Luton To Bedford
The maples
To Ampthill
Figure 5 Example soil erosion map following a risk analysis per field.
Step 3, decide if it is necessary to prevent soil loss from each field: Step 3 considers the problem of soil erosion in the context of the “whole-farm.” It may not always be necessary or desirable to prevent or reduce soil loss from each field.
It is important to consider the impact of the soil loss from one field on the sediment balance across all fields. Therefore, from a diffuse pollution perspec- tive it may not be necessary to place a buffer preventing sediment loss from one field to the next if the field is at a distance from a watercourse. In this case, sediment will deposit naturally in the downslope field(s) and may even compensate for any losses downslope from these fields, maintaining a sedi- ment balance. However, fields that are directly connected with a watercourse or drainage system may require a buffer even if they are considered to be of moderate or low erosion risk, because the likelihood of sediment and asso- ciated pollutants entering the wider river network at this point is high.
Step 4, decide what the best options are to prevent sediment transfer: Step 4 questions how soil erosion can be prevented or contained within the field system. The prevention of soil erosion should always be considered the first priority before the use of buffers. So-called “source control” includes considering alternative management methods that reduce the risk of erosion (Defra, 2005a), for example, ground cover during susceptible periods and permanent grass. Methods that prevent the build-up of surface runoff by breaking slope length or improving infiltration, can all act to reduce or prevent soil erosion (Morgan, 1980).
Having considered source control, if erosion is still occurring and sediment is being transferred out of the field then buffers may offer a solution. Duzant (2008) identifies 19 different types of buffer including vegetated filter strips, hedges, and ponds (Table 1). The merits of each option, either individually or in combination, should be considered includ- ing effectiveness, practicality, and cost.
Once the need for a buffer feature has been identified, and the best buffer or combination of buffers has been selected, then the Buffers DSS continues to the next step. In the example shown here (Fig. 4), we suggest a vegetated buffer as this was the most common form of buffer observed in the Parrett catchment and is often found in other UK agricultural areas.
Step 5, consider the most appropriate width of vegetated buffer: Step 5 considers the design and placement of buffer features. There is currently a lack of available tools for guiding vegetated buffer design and placement in the UK, although this problem has been considered in other countries (see, e.g., Dosskeyet al., 2008; Liaet al., 2008). A range of buffer widths (2, 4, and 6 m) is suggested under present UK agri-environment schemes. Presently, farmers are advised to choose the width that is most suitable to them and their machinery (Defra, 2005b). With little other guidance, this choice of buffer width may be inappropriate for worst-case scenarios of sediment loss (Liaet al., 2008).
The BST (Table 6) is a key part of the Buffers DSS. It provides a quick and easy method of determining the required buffer width to control sediment and associated pollutant loss from a field, given the soil texture and expected ground cover for the proposed vegetated buffer. A similar look-up tool is proposed byDosskeyet al. (2008)based on US soils and land management.
Present advice given to farmers and land owners often seems to assume an idealized slope form with equal potential for sediment to arrive at any point along the downslope edge (Fig. 6A). As a consequence most options for establishing vegetated buffers lead to semi-permanent strips of uniform width. Vegetated buffers work best when overland flow arrives uniformly across the buffer (Barfieldet al., 1979). However, in reality, slope form can often be complex and different levels of protection may be needed along the downslope edge, for example, where slopes converge and concentrate surface runoff (Fig. 6B).
As part of the DSS it is suggested that within-field variability of topog- raphy should be considered. An approach is suggested (but not tested) following the principles proposed in a review by Dosskey et al. (2005).
Using the concept of “precision conservation” proposed by Dosskeyet al.
A B
C D
Figure 6 Flow of sediment toward a buffer strip considering topography and shape requirements of buffer, modified afterDosskeyet al. (2005): (A) uniform runoff flow to a uniform-width buffer; (B) converging flow and contributing runoff areas to a uniform-width buffer; (C) nonuniform runoff areas and the corresponding variable- width buffer areas into which they flow; (D) smoothed edged alternative to (C) (Wood et al., 2007).
(2005), the downslope field boundary is divided into segments and buffer width requirements assessed for each segment separately (Fig. 6C). Once the area contributing to surface runoff in each segment is identified, the sedi- ment loads leaving the area and the required buffer width to achieve a tolerable soil loss are predicted using the modified MMF model. To ensure the resultant shape is practical to achieve and manage, the final shape of the combined segments would be smoothed (Fig. 6D). As no one field is exactly the same a simplified look-up table is not possible. The procedure has to be conducted on a field by field basis.
Step 6, buffer establishment: Following on from the previous steps, once fields, boundaries, and buffer designs have been selected the next step is to identify what is already in place. There may be an existing feature that can act as a buffer and it may be less expensive to improve the properties of this existing feature than to remove it and install a new one. Existing features need assessing to see if their design meets the specified requirements: are they the required width? Is plant coverage within the buffer sufficient to meet the requirements?
If no buffering feature is present, then the design criteria identified in the previous steps can be implemented. The opportunity also exists to incorpo- rate other design features at this stage. This may include additional func- tionality of the buffer, for example, habitat and biodiversity benefits.
Step 7, buffer maintenance: Vegetated buffers are a dynamic system vul- nerable to change due to external pressures. Their effectiveness in trapping and retaining sediment and associated pollutants depends on: buffer condi- tion (plant diameter and density) and damage leading to bypass. Once established there are no guarantees that functionality will be maintained without regular maintenance (Liaet al., 2008). Frequent observations of the buffer feature are needed to identify any changes that may reduce efficiency.
Using a scheme such as the BZIEF survey would build a temporal record that could effectively be used to assess changes in the buffer. Typically, these changes may include: gaps and channels that appear in the buffer, potentially from sediment inundation of vegetation; compaction, caused by trafficking the buffer, reducing infiltration, and affecting sediment deposition; changes in plant coverage due to compaction, age, drought, or disease; steps, created by tillage practice between the front edge of the buffer and the field, deflecting sediment transfer; and animal burrows that provide a by-pass pathway circumventing the buffer (e.g., seeOwenset al., 2007). Remedial action may include redesigning the buffer to prevent inundation by con- centrated flow, filling in gaps/holes, subsoiling the buffer strip to reduce compaction, and reseeding to increase ground coverage.
5. Conclusions
The purpose of a DSS is to aid decision making. As with any DSS, its aim is to reduce time and effort wasted due to trial and error. A simple, user friendly, support tool has been proposed that will aid effective placement of vegetated buffers for specific sites. The system has no extensive data require- ments and can be performed as a paper-based exercise in the field. However, the Buffers DSS also has the potential to offer more sophisticated site specific assessment. This is the first system of its type to incorporate soil texture and vegetation cover within guidance on vegetated buffer design and placement for UK conditions, based on parameters measured for specific fields.
Linear buffer features are not necessarily the most efficient use of land.
Precision conservation will require some level of automated modeling. The modified MMF model within the Buffers DSS is a simple user friendly model that can be run after limited training. This design will enable farm advisors, if not farmers, to calculate site specific buffer width requirements.
Incorporated into a GIS this would offer a very powerful tool. The Buffers DSS is very versatile and could easily be adapted/modified for use in other agricultural systems outside the UK. That said, it is important to recognize any specific differences between the environment upon which the DSS has been developed and that which it is transferred to (such as the occurrence of frozen ground for long periods of time, and contrasting hydro-climatic and vegetative conditions) as this may limit its applicability.
ACKNOWLEDGMENTS
We thank the Department for Environment, Food and Rural Affairs (Defra) for funding and support of the larger Buffers project (PE0205). Thanks are extended to the steering group who Beta tested the DSS and to the farmers who allowed access to their land. We would also like to acknowledge the guidance and support offered by Prof. Roy Morgan during the preparation of this chapter.
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