Boorman, D.B., Reed, D.W. 1981. Derivation of a catchment average unit hydrograph. Report No. 71, Institute of Hydrology, Wallingford, 50 pp.
A problem that frequently arises in rainfall/runoff modelling is the derivation of a catchment average unit hydrograph from a number of recorded flood events. Two approaches are considered in this report: averaging unit hydrographs derived from individual events and joint analysis of a group of events to determine an average unit hydrograph directly. The problems of instability which often feature in least-squares unit hydrographs derived from individual events are found to have little influence on average unit hydrographs determined for eight trial catchments. Whilst particular techniques have their merits, the main conclusion is that differences are unlikely to be significant in application. Of the methods considered, event superposition followed by a least-squares solution is favoured for computational economy. Replacing the least-squares criterion by an iterative solution of the dominant equations is shown to bring derivation of an average unit hydrograph within the scope of hand calculation.