An improved method for estimating the median annual flood for small ungauged catchments in the United Kingdom

This paper presents a new regression equation to estimate the median annual maximum instantaneous flow (QMED) for small ungauged catchments in the UK. The regression equation utilises catchment descriptors that have been used extensively for flood and low-flow hydrology. To derive this equation, a database of 135 gauging stations that drain areas ranging from 0.5 to 25 km2 has been compiled. This is a larger dataset than has been used in previous equivalent studies. Predictions from the new QMED equation were compared to other widely used flood estimation methods, showing a significant improvement in accuracy in some cases. The merits of including a descriptor quantifying urbanisation in the new regression equation are also discussed.