Considering some of the problems of surface reservoirs, such as unsteady stream flow, high evaporation losses and loss of storage owing to sediment entrapment, subsurface dams may be an effective solution in arid–semiarid regions. Combined surface and groundwater dam projects can be developed to withdraw the additional water from the porous alluvial sediments beneath the surface reservoir. We describe the investigation, construction and operation of such a hydraulic structure: the Elmadag dam, near Ankara, Turkey, which stores both surface water and groundwater behind the dam. The height of the Elmadag dam is 26 m but most of the dam wall (15 m) is located beneath the surface, and is constructed in an alluvial sand–gravel aquifer. The surface and subsurface storage capacity of the dam is 3 × 106 m3 and on average 80 l s−1 was pumped between January 2014 and October 2015, which equates to a rate almost equal to the 2015 domestic water demand of Elmadag town. If the surface reservoir dries up temporarily, the dam will continue to provide a supply from groundwater. More importantly, even if the surface reservoir is filled with sediments, the dam will ensure its function by subsurface storage.