The current understanding of the groundwater flow regime in the Dumfries Basin aquifer has been developed over a number of years and encapsulated in a conceptual model. A numerical modelling study has been undertaken to test the confidence in this understanding. This study has included three activities: recharge estimation, water balance calculations and steady-state groundwater flow modelling. It has been confirmed that the highest recharge is found in the NW of the basin and that the peat and marine clay deposits limit direct recharge in the east and south. The water balance is dominated by surface water flow, in particular the River Nith brings over 3200 Ml day−1 onto the basin. Conceptualized flow directions were confirmed by the steady-state modelling. The understanding and model validity are severely constrained by a lack of flow data in the lower River Nith and scarcity of water level and aquifer property information. These knowledge gaps could be filled by flow gauging just above the tidal limit of the river and additional drilling and testing.