Most Roman aqueducts were flat-bottomed, arch-section conduits that ran 0.5 to 1 m beneath the ground surface, with inspection-and-access covers at regular intervals. Conduits above ground level were usually slab-topped. Early conduits were ashlar-built but from around the late Republican era, brick-faced concrete was often used instead. The volume of water transported within depended on the catchment hydrology ะ rainfall, absorption, and runoff ะ the cross section of the conduit, and its gradient; most ran about two-thirds full. The conduit's cross section was also determined by maintenance requirements; workmen must be able to enter and access the whole, with minimal disruption to its fabric.