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. Hydrological models can provide estimates of streamflow pre- andpost-observations, which enable greater understanding of past hydrologicalbehaviour, and potential futures. In this paper, a new multi-objectivecalibration method was derived and tested for 303 catchments in the UK, andthe calibrations were used to reconstruct river flows back to 1891, in orderto provide a much longer view of past hydrological variability, given thebrevity of most UK river flow records which began post-1960. A Latinhypercube sample of 500 000 parameterisations for the GR4J model for eachcatchment were evaluated against six evaluation metrics covering all aspectsof the flow regime from high, median, and low flows. The results of the topranking model parameterisation (LHS1), and also the top 500 (LHS500), foreach catchment were used to provide a deterministic result whilst alsoaccounting for parameter uncertainty. The calibrations are generally good atcapturing observed flows, with some exceptions in heavily groundwater-dominated catchments, and snowmelt and artificially influenced catchmentsacross the country. Reconstructed flows were appraised over 30-year movingwindows and were shown to provide good simulations of flow in the earlyparts of the record, in cases where observations were available. To considerthe utility of the reconstructions for drought simulation, flow data for the1975–1976 drought event were explored in detail in nine case study catchments.The model's performance in reproducing the drought events was found to varyby catchment, as did the level of uncertainty in the LHS500. TheStandardised Streamflow Index (SSI) was used to assess the modelsimulations' ability to simulate extreme events. The peaks and troughs ofthe SSI time series were well represented despite slight over- orunderestimations of past drought event magnitudes, while the accumulateddeficits of the drought events extracted from the SSI time series verifiedthat the model simulations were overall very good at simulating droughtevents. This paper provides three key contributions: (1) a robustmulti-objective model calibration framework for calibrating catchment modelsfor use in both general and extreme hydrology; (2) model calibrations for the303 UK catchments that could be used in further research, and operationalapplications such as hydrological forecasting; and (3)  ∼ 125  years of spatially and temporally consistent reconstructed flow datathat will allow comprehensive quantitative assessments of past UK droughtevents, as well as long-term analyses of hydrological variability that havenot been previously possible, thus enabling water resource managers tobetter plan for extreme events and build more resilient systems for thefuture.

A multi-objective ensemble approach to hydrological modelling in the UK: an application to historic drought reconstruction