Flood frequency estimation by continuous simulation for a gauged upland catchment (with uncertainty)

This paper explores the possibility of deriving frequency distributions of extreme discharges by continuous simulation. The rainfall-runoff model TOPMODEL is applied within the Generalised Likelihood Uncertainty Estimation (GLUE) framework to the River Wye catchment, Plynlimon, Wales, using a 21-year period of rainfall and discharge observations. Rejection of non-behavioural parameter sets is achieved through an evaluation of both annual maximum discharge and continuous hydrograph simulation. Annual maximum peak timings and rankings are also considered. It is demonstrated that, within the prescribed limits, TOPMODEL can adequately achieve both flood frequency and continuous simulation modelling goals. Extension of the flood frequency estimations beyond the upper limit of the observed series is attained through the coupling of behavioural TOPMODEL sets with those of a stochastic rainfall generator for 1000-year simulation periods using hourly time steps. The rainfall model is conditioned on the observed rainfall frequency statistics for different storm duration classes, also within the GLUE framework. (C) 1999 Elsevier Science B.V, All rights reserved.