Experimental and numerical studies were performed to examine the relationship between the hydraulic properties and physical dimensions of a subsurface weir system and the discharge over it under steady-state conditions. Experimental modelling was based on a laboratory-scale physical sand-tank model, within which a subsurface dam and weir structure was constructed. To evaluate the effect of the dominant parameters on the discharge coefficient, numerical modelling was also carried out. A number of physical and numerical models of subsurface dams and weirs were constructed and the effect of each parameter on the discharge coefficient was studied. The results indicate that the hydraulic conductivity of the aquifer, the head above the weir and the effective width of the aquifer, in addition to the geometry of the subsurface weir, significantly affect discharge efficiency. It was also shown that the Reynolds and Weber numbers only have a small effect on the discharge coefficient. Based on the experimental results, a threshold value is suggested for the subsurface weir submergence. Finally, according to the results of both experimental and numerical models, an equation is derived to estimate the discharge coefficient of subsurface weirs.