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Hydrological models can provide estimates of streamflow pre- and post-observations, which enable greater understanding of past hydrological behaviour, and potential futures. In this paper, a new multi-objective calibration method was derived and tested for 303 catchments in the UK, and the calibrations were used to reconstruct river flows back to 1891, in order to provide a much longer view of past hydrological variability, given the brevity of most UK river flow records which began post-1960. A Latin hypercube sample of 500 000 parameterisations for the GR4J model for each catchment were evaluated against six evaluation metrics covering all aspects of the flow regime from high, median, and low flows. The results of the top ranking model parameterisation (LHS1), and also the top 500 (LHS500), for each catchment were used to provide a deterministic result whilst also accounting for parameter uncertainty. The calibrations are generally good at capturing observed flows, with some exceptions in heavily groundwater-dominated catchments, and snowmelt and artificially influenced catchments across the country. Reconstructed flows were appraised over 30-year moving windows and were shown to provide good simulations of flow in the early parts of the record, in cases where observations were available. To consider the utility of the reconstructions for drought simulation, flow data for the 1975–1976 drought event were explored in detail in nine case study catchments. The model's performance in reproducing the drought events was found to vary by catchment, as did the level of uncertainty in the LHS500. The Standardised Streamflow Index (SSI) was used to assess the model simulations' ability to simulate extreme events. The peaks and troughs of the SSI time series were well represented despite slight over- or underestimations of past drought event magnitudes, while the accumulated deficits of the drought events extracted from the SSI time series verified that the model simulations were overall very good at simulating drought events. This paper provides three key contributions: (1) a robust multi-objective model calibration framework for calibrating catchment models for use in both general and extreme hydrology; (2) model calibrations for the 303 UK catchments that could be used in further research, and operational applications such as hydrological forecasting; and (3) ∼125 years of spatially and temporally consistent reconstructed flow data that will allow comprehensive quantitative assessments of past UK drought events, as well as long-term analyses of hydrological variability that have not been previously possible, thus enabling water resource managers to better plan for extreme events and build more resilient systems for the future. [ABSTRACT FROM AUTHOR]

There are numerous anecdotal examples of drought terminations documented throughout the historical record on most continents. The end of a drought is the critical time during which water resource managers urgently require information on the replenishment of supplies. Yet this phase has been relatively neglected by the academic community, with much of the existing body of research on drought termination assessing the likelihood of droughts ending rather than its temporal profile. In particular, there has been little effort to characterise drought termination events themselves. This is partly explained by existing definitions of drought termination as a specific point in time when drought is considered to have finished, rather than a more holistic consideration based on approaches developed within biological sciences. There is also a lack of understanding about how drought termination propagates through the hydrological cycle. This paper specifically examines and reviews available research on drought termination, highlighting limitations associated with current definitions and offering suggestions for characterising the temporal stages of drought. An alternative definition of drought termination is proposed: a period between the maximum negative anomaly and a return to above-average conditions. Once this phase has been delineated, the duration, rate and seasonality of drought termination can be derived. The utility of these metrics is illustrated through a case study of the 2010-2012 drought in the UK, and the propagation of drought termination between river flows and groundwater levels. [ABSTRACT FROM AUTHOR]

Long-term planning of water resources has been a key requirement of the providers of water supplies, and this is extremely important nowadays. There are multiple competing demands on public water supplies due to increasing populations, climate change and the need to protect the environment. In the UK, public water supply is provided by around 25 water companies. The regulators require water companies to produce management plans on a five-year cycle, ensuring that the companies have given the necessary consideration of how to manage the supply-demand balance at present and future. It has recently been necessary to test water supply systems against the 'worst historical' drought event and consider those 'beyond historical' that might occur in future. However, this event varies by region. There is also no agreed methodology for determining this worst historical event, and each water company may use different datasets to reach an answer. Given this lack of consistency at a national scale, there is considerable interest in rationalising the approach to help answer questions relating to drought affecting multiple neighbouring water companies and around the feasibility of large-scale water transfers.This need is addressed by the UK Droughts & Water Scarcity Programme, which supports improved decision-making and communication in relation to droughts and water scarcity for a range of sectors. Capitalising on new improved national-scale data for extensive historical and future timeframes, consistently derived 'drought libraries' are developed. These drought libraries are sets of drought events across a range of durations and severities. They have been produced for a range of spatial domains, facilitating the exploration of questions around the spatial coherence of drought. Due to the extended historic period, the libraries incorporate information about droughts events back to the second half of the 19th century and provide the opportunity to characterise the recent events in their wider historical context. Moreover, the information about possible realisations of the future droughts, with climate model projections up to 2099, can inform stakeholders on the expected severity of future events. These can be used by the water industry for stress testing water supply systems and quantification of the probable future drought hazard.This study demonstrates the applicability of the drought libraries and utility to the water industry through a number of case study catchments in southern England. Based on the results, the severity of a specific historical drought event varies notably across different rainfall accumulation periods, showing that the reference events should be carefully selected for the various durations. Moreover, the future projections indicate that the drought hazard will be intensified in this part of the UK. Events of a certain severity in historical datasets are expected to increase in frequency in the future. The application of nationally consistent drought libraries in assessments of resilience to past, present and future droughts will maximise the benefit in managing the UK's national water resources, and begin to address some of the critical questions around how to reconcile the often competing demands from society, economy and environment. [ABSTRACT FROM AUTHOR]

Studies of the behaviour of hydrological state along intermittent rivers and through time are required to fully characterise their behaviour and assess their response to climatic and anthropogenic drivers. However, long term, spatially distributed datasets, and therefore opportunities to conduct such analyses, are rare. Monthly observations of flowing, ponded and dry states at multiple sites along ten Chalk rivers in the south east of England are used to demonstrate one method of characterisation (longitudinal permanence) and one of assessing response to a climatic driver (drought tracking).The behaviour of the rivers is characterised using long term permanence of hydrological state for each site, integrated temporally across all months in a ten-year period. Charted longitudinally, hydrological state permanence thus reveals spatial variability in intermittence, and by extension regime, along a river. Conversely, by spatially integrating all sites on a given river, the monthly composition of hydrological state allows comparison of the development and extent of drought events. Charts of flow, ponded and dry permanence reveal both broad shapes, as their relative permanences alter incrementally downstream, and localised augmentations from natural and artificial sources. Similarities and differences along and across the rivers are revealed. In the tracking of drought development, contraction of the network increases the proportion of dry state each month on groundwater-dominated rivers, whilst the proportion of ponding is more variable in time and space. The response of the rivers to recent dry winters is compared with that of drought events from the last two decades.The appraisal of a developing drought against previous dry years and long-term average conditions illustrates the importance of such datasets with good temporal resolution and extent in assessing the response of intermittent rivers to climatic and anthropogenic drivers. The equal importance of good spatial resolution is demonstrated in the characterisation of the rivers. The techniques demonstrate the potential for spatial zoning of intermittent river regime and subsequent assessment of the vulnerability of river zones to drought and artificial influences. [ABSTRACT FROM AUTHOR]

Drought can occur in any climate zone (including the UK), and studies aiming to characterise and understand hydrological drought should utilise appropriate hydrological data (rather than, for example, rainfall data which best characterises meteorological drought). Although the UK has a dense flow gauging network, the majority of the river flow records begin in the 1960s. Yet, we know from individual long river flow records, regional reconstructions and qualitative data that severe hydrological droughts occurred in the late 19th and early 20th century. Until recently, it has not been possible to identify and characterise these hydrological droughts at the national scale.Using a new dataset of daily river flow reconstructions dating back to 1891, it has been possible to identify, characterise and rank hydrological drought events nationally. Drought events were systematically identified using the Standardised Streamflow Index (using accumulation periods of 3 and 12 months) for 108 near-natural catchments across the UK from 1891 to 2015. Events were then characterised and ranked according to four characteristics: duration, accumulated deficit, mean deficit and maximum intensity. The results provide an unparalleled account of hydrological droughts in the UK, their characteristics according to different measures of severity and their relative ranking. This enables a comparison of recent events to those outside of the observed record, as well as shedding light on the poorly documented events of the late 19th and early 20th century. This extended understanding based on an objective identification protocol over ~125 years, provides vital information: informing the planning and management of future events; aiding the development of scenarios beyond those observed; and providing benchmarks against which current and future events can be compared. [ABSTRACT FROM AUTHOR]