A modified circular flume was developed to measure flow in subsurface drains that may be subjected to outlet submergence. Two sizes of these flumes (203 mm and 152 mm) were tested over a wide range of flow rates and levels of downstream submergence and the resulting data were used to construct rating curves. With these rating curves, it is possible to quickly and easily determine flow rate through a similarly-sized circular flume using the upstream depth and the depth of flow in the receiving water body. The modified circular flume performed well at all levels of outlet submergence. Under unsubmerged conditions the coefficients of determination of the equations fitted to the data were 0.99 and 0.98 for the 203 mm and 152 mm flumes, respectively.