Simon Parry

How Well Do Large-Scale Models Reproduce Regional Hydrological Extremes in Europe?

AbstractThis paper presents a new methodology for assessing the ability of gridded hydrological models to reproduce large-scale hydrological high and low flow events (as a proxy for hydrological extremes) as described by catalogues of historical droughts [using the regional deficiency index (RDI)] and high flows [regional flood index (RFI)] previously derived from river flow measurements across Europe. Using the same methods, total runoff simulated by three global hydrological models from the Water Model Intercomparison Project (WaterMIP) [Joint U.K. Land Environment Simulator (JULES), Water Global Assessment and Prognosis (WaterGAP), and Max Planck Institute Hydrological Model (MPI-HM)] run with the same meteorological input (watch forcing data) at the same spatial 0.5° grid was used to calculate simulated RDI and RFI for the period 1963–2001 in the same European regions, directly comparable with the observed catalogues. Observed and simulated RDI and RFI time series were compared using three performance...

Persistence of Hydrometeorological Droughts in the United Kingdom: A Regional Analysis of Multi-Season Rainfall and River Flow Anomalies

This paper investigates the spatial and temporal properties of persistent meteorological and hydrological droughts in the UK at national to sub-regional scales. Using 1961–1990 as the reference period, it is shown that the longest observed run of below average rainfall since the 1870s persisted for four years in northern England and parts of Scotland during 1892–1896. The longest observed run of below average discharge since the 1950s/1960s was found for some groundwater fed rivers in the English lowlands and lasted up to 5.5 years during 1988–1993. Distributions of dry-spell lengths were represented by a Markov model fit to each rainfall and discharge record. This model provides a good fit to observed geometric distributions of spell lengths and provides credible runs of below average river flows lasting up to a decade in some vulnerable catchments in southern England. Droughts of this persistence may not yet have occurred within the instrumented record but could have profound water management implications for the region. Predicted 100-year drought durations for catchments in northern England may not be as long but could have serious ramifications for surface water supplies. These findings point to a risk of irreversible drought impacts on aquatic communities that are simultaneously stressed by unsustainable abstractions, poor water quality and/or habitat modifications.

Hydrological Outlook UK: an operational streamflow and groundwater level forecasting system at monthly to seasonal time scales

ABSTRACT This paper describes the development of the first operational seasonal hydrological forecasting service for the UK, the Hydrological Outlook UK (HOUK). Since June 2013, this service has delivered monthly forecasts of streamflow and groundwater levels, with an emphasis on forecasting hydrological conditions over the next three months, accompanied by outlooks over longer time horizons. This system is based on three complementary approaches combined to produce the outlooks: (i) national-scale modelling of streamflow and groundwater levels based on dynamic seasonal rainfall forecasts, (ii) catchment-scale modelling where streamflow and groundwater level models are driven by historical meteorological forcings (i.e. the Ensemble Streamflow Prediction, ESP, approach), and (iii) a catchment-scale statistical method based on persistence and historical analogues. This paper provides the background to the Hydrological Outlook, describes the various component methods in detail and then considers the impact and usefulness of the product. As an example of a multi-method, operational seasonal hydrological forecasting system, it is hoped that this overview provides useful information and context for other forecasting initiatives around the world.

Drought termination Concept and characterisation

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.