Appraisal of Rural BMP's for Controlling Diffuse Pollution and Enhancing Biodiversity
- Best Management Practices (BMPs) are available for control of farm derived diffuse pollution (sediment, sediment bound P, soluble nutrients, faecal indicator organisms and pesticides). BMPs have five main points of application: planning/general farm measures, in-field, field margin (or riparian), in-stream and steading measures. BMP efficacy is enhanced if a “treatment train” approach is adopted: firstly reduce inputs, then detain potential runoff water, then prevent soil dislodgement and runoff downslope, then intercept sediments before reaching watercourses.
- Among the most effective BMPs are nutrient budgeting, leaving winter stubble after cereal crops, cultivation after row crops, headland subsoiling, use of buffer strips, restricting water access by livestock, and constructed wetlands. Field margin and watercourse BMPs are not recommended as alternatives to other BMPs, but may be important as additional options within the treatment train.
- A theoretical analysis of buffer strip design for sediment removal suggested that slopes >7° have low efficiency of sediment removal, especially where significant flow concentration occurs. Therefore steeply sloping zones should not be included in the specification of the overall width required for pollution control. Sediment removal was sensitive to flow velocity, but not very sensitive to buffer width.
- The efficacy of the Green’s Burn Buffer strip (Loch Leven catchment) during storm events was assessed. Before buffer strip installation the mean event total P concentration (TP) (for 3 significant events) was 1.1 ±0.6 mg TP/L and after installation (3 significant events) it was 0.5±0.16 mg TP/L. For suspended solids (SS) the values were 233 mg/L ±115 and 94±62 respectively. More data need to be collected before firm conclusions can be drawn.
- Sediment collected in the Green’s Burn buffer strip following a major storm event contained lower P and organic matter content than the field soil, which suggests that the finer colloidal material (which contains a large proportion of the readily available P) is not being collected effectively by the buffer. Most sediment deposition occurred before or within the first few metres of buffer and this suggests that a buffer width of >10m may not be warranted for sediment control alone. Larger buffer strip widths may be warranted for promotion of diverse vegetation and food resources for birds, but this depends on active and appropriate management, to ensure a diverse habitat emerges.
- Invertebrate diversity in the Green’s Burn buffer strip was poor. A greater amount of heterogeneity in vegetation type and structure would be needed to improve biodiversity. To this end, active management, either in the buffer (more frequent cutting, leaving cut vegetation in swaths, some open space), or in field headlands (leaving crops unharvested, sowing of small areas of winter seed bearing crops such as rape or kale, to promote bird activity) could be encouraged.
- The efficacy of fencing to reduce dairy stock access to water was studied in the Cessnock catchment, Ayrshire by event sampling up and downstream of fenced and unfenced reaches of subcatchments. There was a clear impact of the farming on pollutant concentrations in both the fenced and unfenced reaches studied. For all pollutants studied, the dominant factor controlling the pollutant concentration was the scaled discharge, but data were inconclusive in evaluating the effect of fencing on faecal coliform pollution, because of the impact of other sources of pollution such as steading sump overflow.
- Major structural bypass routes (steading sump drains, surface runoff drains from fields) occurred within both arable and dairy farm systems studied, and these should be carefully identified and managed along with diffuse pollution BMP installation. Farm auditing is necessary to identify diffuse pollution sources and is a prerequisite for assessing whether BMPs are being correctly and effectively used by farmers.
- Field plot studies to investigate the relative efficacy of buffer strips for faecal coliforms and inorganic sediments showed that faecal coliform capture is less efficient than inorganic sediment capture. This suggests that fencing improves water quality mainly through reduced bankside access, not through improved filtration of runoff. This means that provision of drinking water supply off stream may be a more cost-effective pollution control measure than fencing. However, as it is not known whether this will significantly reduce stock movement on the banks and into watercourses, further studies are recommended.
- Monitoring of BMPs needs to be event based, and both inlet/upstream and outlet/downstream need to be sampled. We recommend a fixed installation of flow proportional sampling, with routine servicing, discarding of waters when no events occur. For routine analysis we recommend focus on robust determinands (TP, SS, DOC (dissolved organic carbon), turbidity, total N or NH4 (with a nitrification inhibitor, where feasible)), rather than highly labile/degradable determinands (SRP (soluble reactive P), FIOs, nitrates), in order to make assessment cost-effective, timeous and feasible. FIOs and SRP will be useful for more detailed studies.
- Conversion from winter to spring cereals is one potential BMP which has been investigated on a spatial basis across Scotland. For each parish in which arable or mixed cropping is dominant, a probability factor for winter cereals occurring on soils with a moderate or high risk of erosion has been determined. Nearly 70% of land classified as ‘spring and winter sowing dominant’ occurs on land with a medium or high risk of soil erosion. The areas where this factor is highest are along the east side of Scotland, from the Laurencekirk area southwards with the highest probability occurring in a number of parishes in East Lothian.
- Soils within 10 metre buffer strips of watercourses have been aggregated into ‘wet’ and ‘dry’ categories based on their HOST classification. This enabled us to identify the proportion of soils within the 10 metre buffers that have a drainage impedance and therefore likely to have field drains that will reduce their effectiveness in preventing a proportion of leachable pollutants (e.g. nitrates and FIOs) from reaching water courses.
- Areas with predominantly grassland farming have the highest proportion of ‘wet’ potential buffer strips - much of Ayrshire and Central Scotland - and areas with predominantly winter cereals areas also have relatively high proportions, for example the Merse of Berwickshire, the Lothians, Fife, parts of Strathmore and the Peterhead area.
- If all the watercourses in Scotland had 5 or 10 metre buffer strips applied then the ‘loss’ of land from agricultural production would be approximately 1.3 and 2.7 %. Note that this does not equate with loss from agriculture as many of these areas will be in scrub or woodland already. The actual loss of land based on actual implementation is likely to be much less than this.
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