OR/15/007 Drought classification and conclusions

Durant M. 2015. Description of groundwater droughts in the UK: 1890 to 2015. British Geological Survey Internal Report, OR/15/007.

Literature

The literature and data available for each drought varied greatly. In general, post-1976 there was improved reporting, with Marsh et al. (1994, 2007, 2013), Marsh (1995, 2004, 2007), Marsh & Turton (1996), Rodda & Marsh (2011) and Kendon et al. (2013), all covering major droughts since then with a detailed summary. Prior to 1976, reporting, particularly with respect to groundwater, is largely diminished, and decreases with age. In addition, the more extreme the drought and the greater the public awareness of the drought, the more information there is for those periods (e.g. 1976, 2003–2006, 1890-1910). In the case of older droughts, British Rainfall publications, alongside anecdotal evidence, provides the majority of information which then had to be related to the limited borehole data of the period. Generally, the greater the aridity and surface water impacts associated with a drought, the more it appears in the literature. As this report focusses on groundwater, there are several episodes, such as the 1913-14 and 1964-65 which are widely underreported. In an attempt to reverse this discourse, an investigation into the widely reported drought of 1959 that had severe surface water impacts but little impact on groundwater is also included (see Appendix 1). The public perception, and also to some extent the academic discourse, is highly linked to dry, hot summers and not dry winters which tend to propagate groundwater droughts. As a result, the droughts with marginal surface water impacts but large groundwater impacts tend to be underreported. This is demonstrated in the 1964-65 drought, which recorded the 3rd driest year since 1887 and ranks as the 2nd most intense northern drought but is almost completely unreported in the literature.

The review of the literature also highlighted discrepancies between the perceived drought and the actual state of groundwater resources, most notably in 2003-06 and between 1988 and 1998. The literature reports the 2003-06 drought as two separate events, however an assessment of the SGI and groundwater data indicate that these two events were highly linked. This could be a reflection of the tendency to focus mainly on surface water impacts. For the period from July 1988 to January 1998, Dalton Holme records just 9 months where the SGI is a positive value (Bloomfield & Marchant, 2013). This reflects a very different situation to the one given in the literature. When considering only the Dalton Holme SGI time series, one might be tempted to classify the 1988-1998 period as one long drought episode. However, the Dalton Holme borehole gives an incredibly localised view. Whether this is representative of very specific local geology, the borehole itself, the meteorology of this eastern and sheltered part of the country, or all three, is unclear.

Drought classification

There have been many attempts to classify drought in the UK, however they are mainly related to weather type (Lloyd-Hughes & Saunders, 2002; Wedgbrow et al. 2002; Hannaford et al. 2011; Parry et al. 2010; Fleig et al. 2011; Kingston et al. 2013) or hydrological and groundwater processes on a catchment scale (Peters, 2003; Peters et al. 2003, 2005, 2006; Tallaksen et al. 2006, 2009; van Lanen et al. 2013). There is as yet no definitive groundwater drought classification system for the UK.

From a review of the literature, no specific drought classification can be derived. There are many character traits of drought periods in the UK that are well documented including the exaggeration of the Northwest/Southeast rainfall gradient, the propagation of drought over successive winters, and summer aridity. All major drought periods exhibit at least one of these traits. However, this is not a classification system, as they are qualities that are intrinsic to drought in the UK and they often occur independently of one another and in no discernible pattern. They are all associated with anticyclonic weather which is the main driver for drought in the British Isles.

There are some general patterns in the findings of this report, demonstrating that groundwater droughts that display limited surface water impacts tend to be underreported, despite sometimes being associated with large rainfall deficiencies. These droughts tend to be focussed in the North East, rather than the South and over a single winter. This could be a reflection of the use of groundwater resources within these areas, along with higher demand in the South of England.

In the absence of a definitive classification system, a matrix has been compiled with parameters important to groundwater drought. These have been selected due to their inherent simplicity and objectivity when compared with other parameters. Although not applicable to all droughts, the SGI in the location where the drought was most intense has also been included for ease of comparison of drought severity.

Table 3 Summary of the nine groundwater drought events.

	Multi‐winter	Surface water impacts reported	Region	Cumulative SGI
1890–1910	ü	ü	National (S + E)	N/A
1913–1914	û	û	NE	0.17 (S) / ‐1.11 (N)
1933–1935	û	ü	S + SE	‐1.36 (S) / ‐0.41 (N)
1964–1965	û	û	NE	‐0.24 (S) / ‐1.30 (N)
1975–1976	û	ü	National (S + E)	‐1.81 (S) / ‐1.23
1988–1993	ü	ü	SE + E	‐0.71 (S) /‐1.53 (N)
1995–1998	ü	ü	N/SE	‐1.25 (S) / ‐1.23 (N)
2003–2006	ü	ü	National/SE	‐1.14 (S) / ‐0.40 (N)
2010–2012	ü	ü	NW/SE	N/A

Note: in the ‘Region’ column the brackets denote national drought impacts, focussed in the bracketed region and a solidus indicates a change in drought focus from one region to another. In the ‘SGI’ column (N) denotes Dalton Holme record and (S) Chilgrove House record.