||The groundwater recharge map shows estimated average annual recharge to the deep groundwater system. The ‘deep groundwater’ can be tapped steadily year-round and yields aren’t significantly influenced by seasonal changes. The recharge amount is shown in units of millimetres per year (mm/yr). The amount of recharge was calculated over the period 1981-2010 and then averaged to give a yearly amount.
The main geological controls on groundwater recharge include soil drainage, subsoil type, subsoil permeability, subsoil thickness, and the ability of the underlying aquifer to accept percolating waters.
The amount of rain falling on the land minus how much of that rain is taken up by plants is also a factor that determines how much groundwater recharge there is at a particular location. This is known as the ‘effective rainfall’.
Different combinations of the geological factors give 24 hydrogeological scenarios. There is a ‘recharge coefficient’ for each scenario, which is the percentage of the ‘effective rainfall’ that may become groundwater recharge.
Estimated groundwater recharge is lowest in areas overlain by thick, low permeability clay subsoil. Groundwater recharge is highest where there are coarse sand and gravels, which have high permeability, or where well-drained soils are thin or absent.
Where locally important or poor fractured bedrock aquifers underlie the land surface, a maximum recharge acceptance capacity (‘recharge cap’) is applied, even where soils and subsoils are thin or absent. This is because these bedrock aquifers do not have enough fractures to store or transmit all of the percolating rainwater. In these settings, most of the groundwater recharge stays underground for only a short amount of time before it is ‘rejected’ and flows in nearby streams or ditches.
Users of the map should be aware that for each hydrogeological scenario, the map uses the typical recharge coefficient from an available range. It also uses the 30 year average effective rainfall. This means that groundwater recharge may be over- or under-estimated, depending on local conditions. Users should also be aware that the recharge cap applied to poorly productive aquifers may need further examination for particular studies.
In karst aquifers, groundwater resources may be overestimated due to low storage within these aquifers. Further possible limitations for particular areas are: only diffuse recharge is modelled and point recharge, or sinking streams are not accounted for in the groundwater recharge map; the influence of a shallow water table limiting recharge is not accounted for; ground slope is not accounted for.
The map is derived from existing hydrogeological and hydrometeorological data layers: annual rainfall, annual estimated actual evapotranspiration (AE), soil drainage, subsoil permeability, groundwater vulnerability, peat, sand/gravel aquifer, bedrock aquifer class.
The layers are overlain and interpreted using the guidelines (GW 5) outlined by the Irish Working Group on Groundwater (WGGW, 2005), subsequently revised in Hunter Williams et al. (2011) and Hunter Williams et al. (2013).
The combination of hydrogeological layers gives a particular hydrogeological scenario that is related to a recharge coefficient. The recharge coefficient is the proportion of effective rainfall that can potentially become recharge. The map of recharge coefficients is combined with the effective rainfall map and the recharge cap to produce the groundwater recharge map.
Application of the data: Local details are generalised to fit the original mapping and interpretation scale of 1:40,000. Evaluation of specific sites and circumstances will normally require further and more detailed assessments, and will often require site investigations.
In 2008, Compass Informatics produced the first groundwater recharge map using the principles, hydrogeological scenarios and recharge coefficients outlined in WFD Guidance document GW5 (WGGW, 2005 - http://wfdireland.ie/Documents/Characterisation%20Report/Background%20Information/Review%20of%20Env%20Impacts/Groundwater%20Risk%20Assessment/GW5%20Abstraction%20Pressures.pdf) . The algorithm structure used by Compass Informatics is given in Groundwater Abstraction Pressure Assessment (Dublin City Council/ CDM, February 2009): http://wfdireland.ie/docs/24_Abstractions/Groundwater%20Abstractions%20February%202009%20ISSUE_v2.pdf . The processing was undertaken using ArcGIS and an avenue script. The projection was Irish National Grid (ING).
Input data were: Teagasc/EPA 1:40,000 Subsoil Map; Teagasc/EPA soils 1:40,000; Soil drainage map with 4 classes – wet, dry, peat, made – based on Teagasc/EPA soils map; GSI 1:50,000 Subsoil Permeability Map from county GWPSs; Interim subsoil permeability map based on Teagasc/EPA maps; GSI 1:40,000 interim national groundwater vulnerability map; GSI 1:100,000 Bedrock Aquifer map; GSI 1:50,000 Sand and Gravel Aquifer map; Met Éireann 1971-2000 average annual Rainfall raster; Met Éireann 1971-2000 average annual Actual Evapotranspiration shapefile.
In 2012, Geological Survey Ireland updated the groundwater recharge map to incorporate more extensive groundwater vulnerability and subsoil permeability mapping.
Input data were: Teagasc/EPA 1:40,000 Subsoil Map; Teagasc/EPA soils 1:40,000; Soil drainage map with 4 classes – wet, dry, peat, made – based on Teagasc/EPA soils map; GSI 1:50,000 Subsoil Permeability Map from county GWPSs and NDP mapping; GSI 1:40,000 national groundwater vulnerability map based on county GWPS and NDP mapping; GSI 1:100,000 Bedrock Aquifer map; GSI 1:50,000 Sand and Gravel Aquifer map; Met Éireann 1971-2000 average annual Rainfall raster; Met Éireann 1971-2000 average annual Actual Evapotranspiration shapefile.
Groundwater Recharge map creation technique: Created using tools built though ArcGIS model builder. On a county by county basis. In order for the Recharge map to be created, the recharge coefficient has to be calculated. This calculation depends on a large combination of conditions that are worked out from overlaying the following layers through a combination of unioning, intersecting, adding fields and calculating fields: 1. Teagasc Soils: For indicating areas of Peat and whether soil is wet or dry. 2. Teagasc Subsoils: For indicating sand and gravel soils. 3. Permeability 4. Vulnerability 5. Sand & Gravel Aquifers 6. National Bedrock Aquifer dataset 7. Effective Rainfall (Met Éireann).
The Recharge Map Creation tool goes through several different geoprocessing tasks. For each county: 1. Selecting the county: Union the Teagasc soil, subsoil, Permeability and Vulnerability layers. 2. Unioning and intersecting with Fixed layers: -Intersecting Sand and Gravel Aquifer: This data will be included for analysis along with the Sand and Gravel soils from the Teagasc subsoils layer. -Intersecting National Aquifer: This layer will be used to calculate what cap (if any) will be applied to the potential recharge mm amount. -Aquifers of type Ll, Pu and Pl will entail a capping on this final recharge figure. (100 or 200 mm/yr) 3. A “hydrogeological scenario” category is applied according to combination of values for each record. A Recharge Coefficient is then calculated using the central value of the inner range of the recharge coefficients. 4. The final recharge value is calculated as Effective Rainfall x the %Recharge Coefficient. The projection was Irish National Grid (ING).
For further Information see Hunter Williams et al. (2011) http://www.opw.ie/hydrology/data/speeches/08%20-%20Hunter%20Williams%20-%20National%20Groundwater%20Recharge%20Map.pdf
In 2020, Geological Survey Ireland updated the groundwater recharge map to incorporate updated input maps:
1. hydrometeorological data produced by researchers at the Irish Centre for High End Computing (ICHEC) (Werner et al., 2019)
● MÉRA Daily rainfall in the period 1981-2010. Data source Met Éireann.
● Daily AE in the period 1981-2010. Three soil drainage classes. One reference crop type. Data source ICHEC (2019).
● 30 year average annual effective rainfall (mm/yr) derived from the preceding maps.
2. Teagasc soils drainage maps at 1:250,000 and 1:40,000 were combined
● Teagasc Indicative Soil Drainage map 1:250,000 (Creamer et al., 2016), recategorised to well-drained, moderately-drained and poorly-drained soils was used for the first time. Actual Evapotranspiration was estimated for each of these categories using Schulte et al. (2004) by ICHEC.
● A hybrid map was created by mapping indicative soil drainage categories onto the recategorised 1:40,000 soils map for the wet and dry soil categories.
3. Geological Survey Ireland updated maps:
● 2020 groundwater vulnerability, sand and gravel aquifer and subsoil permeability maps.
In addition, the following improvements were made:
● better representation and improved recharge coefficients for peats (including cut, basin and fen peat);
● better representation of scree;
● additional scenarios for sand and gravel resulting in 24 hydrogeological scenarios. See Hunter Williams et al., upcoming Irish Groundwater Newsletter.
The GIS code was re-written and the main updates are:
● A national grid (20674 grid squares) was created to allow for working on the data on a grid square by grid square basis - this allowed for testing before national rollout.
● Each national input dataset was clipped to each grid square.
● For each grid square, all inputs were unioned together.
● The hydrogeological scenarios and recharge coefficients were then applied to each individual polygon
● Each processed grid square was then merged together to create the national recharge map.
The code was written in the Python language. QGIS provides a python API (Application Programming Interface) - PyQGIS. This API was used to process the data.
Projection = Irish Transverse Mercator (ITM)
Credits and references
Irish WFD Groundwater Working Group (2002-2005) – Geological Survey Ireland: Donal Daly (Convenor), Geoff Wright, Vincent Fitzsimons, Coran Kelly, Taly Hunter Williams, Monica Lee; CDM: Henning Moe; Compass Informatics Ltd.: Paul Mills; Department of the Environment, Heritage and Local Government (DEHLG) Pat Duggan, Jim Ryan (NPWS), Aine O’Connor (NPWS); Environment and Heritage Service/ Geological Survey of Northern Ireland (EHS/GSNI): Peter McConvey; Environmental Protection Agency (EPA) Margaret Keegan, Micheal McCarthaigh; Kirk McClure Morton (KMM): Grace Glasgow, Kieran Fay; O’Callaghan Moran (OCM): Sean Moran, Gerry Baker; O’Neill Groundwater Engineering (OGE): Shane O’Neill; Shannon Pilot River Basin – EPA/TCD Research Fellow Garrett Kilroy; Southeastern River Basin District (SERBD): Colin Byrne; Teagasc: Karl Richards; Trinity College, Dublin (TCD): Paul Johnston, Catherine Coxon.
Fitzsimons, V. and Misstear, B. (2006) Groundwater recharge through tills: uncertainties in applying soil moisture budgeting and river baseflow approaches in Ireland. Hydrogeology Journal, Vol 14, No. 4.
Hunter Williams, N.H., Misstear, B.D.R, Daly, D. and Lee, M. (2013) Development of a of a national groundwater recharge map for the Republic of Ireland. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 46, No. 4.
Schulte, R.P.O., Diamond, J., Finkele, K., Holden, N.M. and Brereton, A.J., 2005. Predicting the soil moisture conditions of Irish grasslands. Irish Journal of Agricultural and Food Research 44: 95–110.
Werner, C., Nolan, P. and Naughton, O. (2019) High-resolution Gridded Datasets of Hydro-climate Indices for Ireland. 2016-W-DS-29. EPA Research Report
Paul Mills, Compass Informatics – Phase 1 GIS
Peter Cooney, GSI – Phase 2 GIS
Shane Carey, GSI – Phase 3 GIS
Robbie Meehan (An Talamh) for advice on optimum use of Teagasc soils maps (Phase 3)
Shane Regan (NPWS) for advice on peat recharge coefficients (Phase 3)