Drought has certainly been in the news recently and the beauty of the digital age means that we can monitor this in real time – note the spike on Monday 12th March when the UK government announced the imposition of a hosepipe ban:

Figure 1 Drought in the news.

We can even visualise it on an interactive map such the proto-type Walker Drought Watch (comments on this are most welcome!).

Drought is amongst the most deadly and costly of natural hazards. Systematic collection of data relating to natural disasters began around 1970 (Guha-Sapir 2004) and since then recorded droughts have affected the lives of nearly 2 billion people and killed over 600,000. In the European context, modern water supply infrastructure has all but eliminated direct mortalities yet the societal impacts of water scarcity remain and cannot be overstated. For example, the European Commission (European Commission 2007) estimate the direct costs of drought within the European Union to be €3 billion per year. This compares annual losses in Europe from windstorms (€2 billion per year) and flooding (€4 billion per year) at 2010 prices. A thorough understanding of the peril is essential for mitigating against the risk as it stands and for preparedness in the face of climate change.

Yet drought remains a nebulous concept and a universal definition has proved to be elusive. The Oxford English Dictionary defines drought as:

  1. The condition or quality of being dry; dryness, aridity, lack of moisture.
  2. Dryness of the weather or climate; lack of rain.

Unfortunately, the conflation of dryness with aridity and weather with climate serves more to confuse than illuminate. The WMO international meteorological vocabulary provides a

  1. Prolonged absence or marked deficiency of precipitation.
  2. Period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance.

However, to focus solely on precipitation is to neglect the importance of evaporation and transpiration as moisture sinks which reduce the amount of water available for use. The definition also ignores the importance of lateral inflows (stream and ground water flows) into a region that can serve as important water sources in addition to the local precipitation. Further, the definition makes no reference to the timing of the precipitation deficits, a factor which is crucial in the determination of many drought impacts. Sheffield & Wood (2011) succeed in defining drought both accurately and succinctly as ‘a deficit of water relative to normal conditions’.

Striving for a quantitative definition, many attempts have been made to describe drought numerically through the development of drought indices. The difficulty, and importance, of defining drought objectively is manifest in the large number of indices that have been proposed for use in drought monitoring (well over 100 and counting in the peer reviewed literature). Particular indices have typically been developed on a ad hoc basis to emphasise some particular drought impact, be it meteorological, hydrological, agricultural or socio-economic (to borrow the classification of Wilhite & Glantz 1985). Unfortunately, rather than clarify the definition, the plethora of indices creates further confusion and brings into question the very feasibility of defining drought in a quantitative fashion outside of specific impacts.

Drought occurrence depends on the interaction between the source of the available water and its intended use. This leads to different perceptions of the importance of a given drought for different segments of society. The meteorologist, who views drought as below normal precipitation in a region, might consider a run of 10 dry days to be significant. The arable farmer, who depends on adequate soil moisture for crops during the growing season, will be interested in monthly rainfall totals. Whereas, the water supply company may be interested in aquifer levels that take months or even years to recharge. Location also matters e.g. consider the impact of a summer dry spell of 30 days over London to the same over Tripoli, as does spatial extent.

The majority of notable high precipitation events are characterised by highly localised, short lived, heavy down bursts. The same is not true of the most notable drought events. These typically last for several months or even years and span thousands of square kilometres. Thus, drought characterisation is an intrinsically spatio-temporal problem. Lloyd-Hughes (2012) suggests that the space-time structure of the precipitation deficits is well suited to the characterisation the phenomenon.

The drought now affecting the UK provides an interesting example of the space time evolution of a large scale European drought. Figure 2 (a) is an isometric projection of the event. The view is from the southwest looking backward in time from March 2012 to January 2011. Whilst the image serves the pedagogical purpose of illustrating the 3 dimensional coherency of the event, it does little to reveal the spatio-temporal characteristics of the drought. Advances in web technology such as webGL might soon facilitate the inclusion of interactive displays of this sort of data. Until then, panel (b) provides a Hovmöller type plot of the number of voxels in the drought volume counted north-south through time. This view is made up of pixels which are shaded according to the number of cells within the event volume counted along each meridional band. The coloured elements represent the shadow, or `footprint’, of the event as projected onto the back plane of the bounding box shown in panel (a). The depth of colour represents the integrated thickness of the event in this direction. Panel (c) provides a similar view but counts cells east-west through time. Finally, panel (d) maps cell counts by location to represent the drought severity and maximum spatial extent. Whilst this remains an imperfect representation of the full 3-dimensional event structure, the combined views capture the essence of the event. Importantly, we see the core of the drought centred over mid-western Europe and a zonal expansion into Central and Eastern Europe.

Figure 2 A spatio-temporal representation of the 2012 European drought.

No discussion of the present drought would be complete without reference to the last comparable event which occurred in 1976. Using a standardised measure of the 12-month running total of precipitation (SPI12) we can directly compare the two events.

Figure 3 Comparison of the 1976 and 2012 European droughts.

We can immediately see that the events are of a very different character. The key difference is that in 1976 viewed ‘as if’ today the moisture deficits are orientated south-West – north-East as opposed to the more zonal structure seen today. Looking to the UK, the present drought is largely restricted to the England and Wales rather than the whole of the country and is seen in parts to be even more severe than the 1976 event. A SPI value of less than -2 equates to a return period of approximately 50 years and we need look back to 1921 to find a UK drought of similar severity.

Ascribing the cause of a drought is difficult. Meteorologists typically explain drought in terms of atmospheric circulation patterns that favour the suppression of precipitation (cue much discussion about blocking). However, to paraphrase Hounam (1975), such explanations merely describe the meteorological motions and processes rather than identify the fundamental dynamic and thermodynamic forces which caused the abnormal patterns. Thankfully, the digital age helps again, this time in the guise of computers powerful enough to run weather resolving climate models that are capable of simulating droughts with highly realistic spatio-temporal characteristics. It is likely to be through the analysis of this modelled world that we will gain deep insights into the
origins of drought.

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