GSI Groundwater Recharge Mapping

The national Groundwater Recharge Map depicts estimated recharge to the deep groundwater system, i.e. groundwater that can be tapped as the sustainable resource.

The main hydrogeological controls on groundwater recharge include subsoil permeability, subsoil thickness, saturated soils, and the ability of the underlying aquifer to accept percolating waters. Estimated groundwater recharge is lowest in areas overlain by thick, low permeability clay. Where lower productivity aquifers underlie the land surface, a recharge cap is applied to simulate rejected recharge, even where subsoils are thin. This reflects the limited ability of these aquifers to accept and transmit recharging waters.

The map is derived from existing hydrogeological and meteorological 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.

Utilities of the map include: aquifer water balance assessments using simple lumped models or distributed numerical groundwater models; assessment of the impact of groundwater abstractions as required under the European Water Framework Directive; and delineation of source protection zones.

Outer and inner ranges for the groundwater recharge coefficients are defined, to be used depending on the hydrogeology of the study area. Users should be aware that the map uses the central value of the inner recharge coefficient range, and the 30 year average effective rainfall (1971-2000, Met Eireann). Thus, 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; that karst aquifer resources may be overestimated due to low storage within these aquifers; that only diffuse recharge is modelled and point and allogenic recharge are not accounted for in the groundwater recharge map; that the influence of the water table depth and ground slope are not accounted for.