AbstractThe impact of an arterial drainage scheme on the water balance of a 200 km2catchment in Northern Ireland is quantified. Monitored changes in groundwater storage act as an independent check on conclusions about storage change derived from measurements of surface hydrology. Three subcatchments of the study area and one other independent experimental catchment constitute controls against which the impact of arterial drainage can be assessed. Fiver years' predrainage data and three years' post‐drainage data are examined.Stream flow from the floodplain area of the catchment has increased, especially low flows. Catchment water‐tables, however, as monitored in floodplain transects and disused farm wells have not been drawn down. Any immediate drainage benefits deriving directly from arterial drainage are difficult to detect. Groundwater evidence indicates that the likely source of the excess discharge is an underlying gravel aquifer that has been punctured by the deeper arterial chan