A systematic assessment of drought termination in the United Kingdom

Abstract

Abstract. Drought termination can be associated with dramatic transitions from drought to flooding. Greater attention may be given to these newsworthy and memorable events, but drought terminations that proceed gradually also pose challenges for water resource managers. This paper defines drought termination as a distinctive phase of the event. Using observed river flow records for 52 UK catchments, a more systematic and objective approach for detecting drought terminations is demonstrated. The parameters of the approach are informed by a sensitivity analysis that ensures a focus on terminations of multi-season to multi-year droughts. The resulting inventory of 467 drought terminations provides an unprecedented historical perspective on this phenomenon in the UK. Nationally and regionally coherent drought termination events are identifiable, although their characteristics vary both between and within major episodes. Contrasting drought termination events in 1995-1998 and 2009-2012 are examined in greater depth. The data are also used to assess potential linkages between metrics of drought termination and catchment properties. The duration of drought termination is moderately negatively correlated with elevation (r s = −0.47) and catchment average rainfall (r s = −0.42), suggesting that wetter catchments in upland areas of the UK tend to experience shorter drought terminations. More urbanized catchments tend to have gradual drought terminations (contrary to expectations of flashy hydrological response in such areas), although this may also reflect the type of catchments typical of lowland England. Significant correlations are found between the duration of the drought development phase and both the duration (r s = −0.29) and rate (r s = 0.28) of drought termination. This suggests that prolonged drought development phases tend to be followed by shorter and more abrupt drought terminations. The inventory helps to place individual events within a long-term context. The drought termination phase in 2009-2012 was, at the time, regarded as exceptional in terms of magnitude and spatial footprint, but the Thames river flow record identifies several comparable events before 1930. The chronology could, in due course, provide a basis for exploring the complex drivers, long-term variability, and impacts of drought termination events.

Lags in hydrologic recovery following an extreme drought: Assessing the roles of climate and catchment characteristics

“…The effects of elevation and latitude are not purely catchment controls, because their impacts on Lag_Q and/or Lag_Q B are also possibly related to mean climate conditions, i.e., temperature and radiation. This is consistent with Parry et al [2016], who showed that catchment mean elevation significantly controls the drought termination duration in UK. On the other hand, the effects of temperature and radiation are also partly included in E 0 .…”

“…Using a power function of P rr alone could account for 72% 74% of the observed spatial variance in both lag terms ( Figure 6), suggesting that changes in precipitation dominantly controls the variability of surface hydrological behaviors in this semi-arid environment [Budyko, 1974]. Parry et al [2016] also reported a significant role of catchment rainfall amount in the control of drought termination duration (note here drought termination Water Resources Research 10.1002/2017WR020683 duration is slightly different from the time lags in the hydrological drought recovery) and drought termination rate across UK catchments. The valley bottom flatness (VBF) was found to be the secondary factor that partly controls the time lag in the recovery of streamflow and base flow (Tables 2 and 3).…”

“…It is worthwhile noting that besides the time lags analyzed herein, there are many other drought recovery characteristics reported in the literature (e.g., drought recovery rate and drought recovery duration) [Iniguez et al, 2016;Parry et al, 2016]. However, as these drought recovery characteristics are all controlled by similar factors (primarily by external water input after drought [Iniguez et al, 2016;Parry et al, 2016]), we would expect a similar spatial pattern of these recovery characteristics after the Millennium drought across catchments, i.e., a shorter time lag would correspond to a faster recovery rate and shorter recovery duration. Nevertheless, a comprehensive understanding of different drought recovery aspects remains a challenge for our further studies.…”

Benchmarking Ensemble Streamflow Prediction skill in the UK

“…Although it is encouraging that GR4J storage parameter values (X1 and X3) appear to show some physical realism, a note of caution is needed as GR4J is not a physics-based hydrological model, nor is it guaranteed that these results are directly transferable to any lumped catchment hydrological model. It has also been noted that the BFI is influenced by many other factors such as lake and snow storage (Parry et al, 2016), therefore a more 25 detailed examination of the physical hydrogeological controls on catchment BFI, such as in Bloomfield et al (2009) for the Thames, is needed at a national scale.…”

Demonstrating the utility of a drought termination framework: prospects for groundwater level recovery in England and Wales in 2018 or beyond

“…Vicente-Serrano et al 2012, Tijdeman et al 2018) and the sensitivity of the framework metrics to ill-fitted extreme values. The drought termination framework was applied here using the parameters established by Parry et al (2016b). The framework sub-divides identified droughts into development and termination phases either side of the maximum negative anomaly (drought minimum; DM).…”