The selection of catchments was influenced by EU guidelines which suggest that monitoring efforts should be concentrated in “areas of intensive crop and livestock production …with elevated nitrate concentrations... adjacent to existing or projected eutrophication areas…with similar land use, soil type or agricultural practice”.
Using these guidelines a new Geographic Information System (GIS) based selection methodology was developed for the programme. National catchment data was used to generate a list of 1,300 catchments to select from. A range of data covering land use, livestock density, housing density, geology, soils, and nutrient loss risk was used in a Multi-Criteria Decision Analysis (MCDA) approach. Selection criteria were given weightings which reflected how they affected the suitability of the catchments for inclusion in the Programme.
Ranking the Catchments
The catchments were divided into two broad categories - grassland and tillage.
For the criteria shown below the lower the score the higher the ranking for both types of catchment:
- Housing density
- Forestry area
- Area of peat
- Area of non-agricultural land use
For grassland dominated catchments the higher the stocking rate and percentage forage area the higher the ranking while for tillage-dominated catchments a higher ranking was given for higher percentage of tillage area and lower stocking rates.
Risk of nutrient loss to water
Catchments were also ranked by risk of loss of nitrogen or phosphorus to water, based mainly on the drainage characteristics of the soil. Generally more poorly drained soils have a greater risk of phosphorus loss through overland flow or run-off which the more freely drained soils have a greater risk of nitrogen loss through leaching down through the soil.
Six catchments have been selected using this methodology.
Two of these are catchments with a high proportion of tillage:
- A catchment with free-draining soils where the greatest risk is of nitrogen loss through leaching
- A catchment on heavier soils where phosphorus loss through surface run-off is more likely.
There are four grassland-dominated catchments
- One with a high nitrogen loss risk
- Three predominantly at risk of phosphorus loss with varying levels of nitrogen loss risk.
A site on pure limestone geology and dominated by groundwater pathways remains to be selected. The GIS methodology described above was designed for surface water dominated catchments and so doesn’t suit limestone areas. Wide consultation with experts in the field and existing survey data is being used to help identify suitable sites. This selected site is likely to be west of the Shannon in the extensive karst area of Galway/Mayo and will require substantial on-site investigation to delineate its zone of groundwater contribution.
The catchments selected were: