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 TOPM ODEL is applied within the Generalised Likelihood Uncertainty Estimation (G LUE) framework to the River Wye catchment, Plynlimon, Wales, using a 21-yea r period of rainfall and discharge observations. Rejection of non-behaviour al 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 pres cribed limits, TOPMODEL can adequately achieve both flood frequency and con tinuous simulation modelling goals. Extension of the flood frequency estima tions 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 rai nfall model is conditioned on the observed rainfall frequency statistics fo r different storm duration classes, also within the GLUE framework. (C) 199 9 Elsevier Science B.V, All rights reserved.