High concentrations of phosphorus and sediment are recognised as a major driver of poor water quality, contributing to eutrophication in water bodies.

As a result, many water bodies fail to meet ecological standards. This loss occurs through overland flow of water which carries sediment and phosphorus into drains and surface water bodies.

Overland flow typically occurs on soils with low permeability or on compacted soils. These poorly drained soils become saturated quickly during prolonged or intense rainfall, resulting in surface runoff.

This runoff can transport both dissolved phosphorus and phosphorus bound to soil particles plus recently applied fertiliser.

Where fields slope towards watercourses, the runoff can rapidly enter drains, ditches, and streams.

Nutrient loss via overland flow doesn’t just occur in fields, it can also happen in farmyards where clean rainwater mixes with soiled areas, along farm roadways, or poorly designed drainage systems.

Nutrient loss can be reduced by breaking the pathway between the source of nutrients and the watercourse.

High-risk areas

At farm scale, areas with high risk of overland flow are called critical source areas (CSAs). They are called “critical” because they combine both a source of phosphorus, and a transport pathway that can move phosphorus into streams or water bodies.

These are often dominated by small areas that contribute a large share of phosphorus losses rather than whole farms. These are often low-lying parts of farms where runoff accumulates.

The identification of these CSAs on a farm is necessary to break the pathway between the source of nutrients and the watercourse. Most of these CSAs are mapped on www.catchments.ie (see image on the right).

Additionally, CSAs on farms can be identified during and after periods of high rainfall as they are saturated with repeat ponding.

Figure 2. Critical source areas mapped on land.

Breaking the pathway between source of nutrients and the water course on farms

1. Buffer strips

Ditches and drains are designed to remove water from fields but can act as corridors and connecting pathways for nutrient loss to water bodies; a buffer strip can be used to avoid these potential losses.

A buffer strip is an area where no fertiliser is spread and which is used for the protection of water from nutrient losses usually sited alongside a water body or critical source areas.

Buffer strips are areas of permanent vegetation, usually grass, wildflowers, shrubs, or trees growing between farmland and a nearby water body.

Buffer strips are a requirement as part of Good Agricultural and Environmental Condition standards and the Nitrates Regulations.

The required buffer strip distances between farming activities and water bodies depend on the farming activity. Required minimum distances are 3m for chemical fertiliser spreading, 5m for organic manure/slurry spreading and 25–200m for drinking water abstraction points/wells.

A 3m uncultivated margin (increased to 6m for late harvested crops such as maize, beet, potatoes) must be adhered to. Buffer strips are a very effective way of minimising nutrient loss to water bodies.

A 5m buffer strip.

2. Riparian margins

Riparian margins are very similar to buffer zones in that the margin is adjacent to a water body.

The objective of a riparian margin is to protect the water body from diffuse phosphorus loss and trap sediment by creating a linear buffer zone between agricultural activity and the water body.

Margins can vary in width; however, in general, the greater the width, the greater benefit to water quality and biodiversity.

The riparian zone is usually fenced to keep stock out and planted with a line of trees or shrubs along the bank of the water body.

The riparian margin improves water quality by trapping sediment, nutrients, fertilisers and manure before these reach water bodies; this provides habitat for insects and other wildlife. These areas are also important for providing shading and cooling the water, which helps aquatic ecosystems.

The image above shows ArraTipp farmer Albert O’Dwyer, Bansha, in his recently fenced off spatially targeted riparian margin, which Albert will plant with trees later in the year.

The reason Albert decided to implement this measure is to mitigate against a CSA.

A riparian buffer strip.

3. Installing earthen bunds

Additionally, installing earthen bund embankments along land contours or field edges can greatly reduce nutrient loss to water bodies.

Bunds slow the flow of surface runoff during heavy rainfall, preventing valuable topsoil from being washed away. The image (right) shows a recently planted hedgerow on an earthen bund along a watercourse.

Additionally, some farmers install earthen bunds to divert storm water away from the farmyard and protect slurry from contamination.

Figure 4. Albert O'Dywer in recently-fenced spatially targeted riparian margin.

4. Woodland creation

Strategic woodland placement in critical source areas can be particularly effective because these locations have a disproportionately high impact on downstream environmental conditions.

This highlights that the location of woodland establishment is often more important than the total amount of woodland area added.

By targeting critical source areas, interventions can focus on areas where runoff, sediment, nutrients, and pollutants are most likely to originate and spread throughout the landscape.

This measure is available through the Woodland for Water Scheme with the Department of Agriculture.

Figure 5. A hedge on earthen mound at planting

Best management

As well as implementing the mitigation strategies outlined above, it is essential to follow best management practices.

These include applying fertiliser at the appropriate time and under suitable conditions, maintaining soil phosphorus levels at Index 3, and avoiding the spreading of fertiliser or slurry on wet or waterlogged ground.

Figure 6b. How a woodland for water habitat matures over time.