Estimation of recharge to groundwater is an important component in the characterization of groundwater bodies for the Water Framework Directive (WFD). The time scale allowed for the WFD initial characterization required a pragmatic methodology for efficient assessment of recharge to each delineated groundwater body. An assessment methodology was developed through a project funded by the Scotland and Northern Ireland Forum for Environmental Research, although it has application throughout the UK. Few recharge calculations have previously been attempted in Scotland. The environmental influences on recharge processes differ from much of the rest of the UK and the ability of the groundwater body to accept recharge is a key criteria.
The method proposed is based on a water budget approach which estimates direct and indirect recharge flow components. It includes estimation of effective rainfall, actual evapotranspiration, surface run-off and interflow in addition to the potential impacts of snow-melt, bypass flows, urban recharge and surface water leakage where appropriate. This is calculated on a daily or monthly time step with the use of a spreadsheet software tool. The degree of uncertainty in the recharge estimation increases with the scarcity of data available to the groundwater body. Many recharge estimations in Scotland will rely on national-scale mapping of climatic, land use, soil and aquifer properties. It is important that the confidence in the estimation is balanced against the perceived risk of stress to the groundwater body. Validation of the recharge calculations is ongoing and the assessment of bypass flow and interflow has been determined as critical to the accuracy of the calculation.