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National Hydrological Monitoring Programme
What is a drought?
Although, across much of the world, drought can constitute one of the worst of natural disasters - posing a real and continuing threat to lives and livelihoods - there is no generally accepted definition of exactly what a drought is. This is a reflection of the multi-faceted nature of droughts and their wide range of impacts.
The concept of a drought is generally well recognised by the public at large but translating this intuitive understanding into a rigorous and objective procedure for identifying droughts and monitoring changes in their severity, is far from straightforward. This results from the difficulties in quantifying a phenomenon which varies in its duration, intensity and impact both regionally and locally.
Drought has been defined in various ways, often with a particular target audience in mind but, generally, it is possible to distinguish between meteorological droughts, hydrological droughts and agricultural droughts, with the causation shifting from rainfall deficiencies through runoff deficiencies to the availability of water for crops in the growing season. Furthermore, it may be argued that a drought could most usefully be characterised by its impact on the community; this introduces the need to monitor economic, social and environmental stress. Such appraisals can normally only be fully assessed after a drought has terminated.
Most drought criteria employ a measure of the absence of rainfall, but some incorporate mean temperature, soil water and crop parameters, evaporation and other climatic indices. For many applications river flow, which integrates the wide range of hydrometeorological processes operating throughout a catchment, is the most appropriate drought index. Accumulated runoff deficiencies or n-day minimum flows find very wide application in assessments of drought severity but their interpretation needs to be undertaken with care due to the impact of artificial influences, on low flow especially, in many UK rivers.
A prolonged hot dry spell may well be viewed in a more sanguine manner by the holiday maker than the farmer or the water supply manager but, generally, the status of water resources (embracing both reservoir stocks and groundwater storage) assumes a particular importance during periods of drought stress. Small agricultural reservoirs may be sensitive to rainfall totals over a few months whilst major gravity-fed reservoirs are vulnerable to rainfall deficiencies over longer timeframes (with limited rainfall during the winter half-year being of particular importance), and in some slow-responding aquifers, groundwater levels will reflect rainfall patterns over several successive winters.
It will be evident from the above that different manifestations of drought stress are often out-of-phase and the range of impacts may vary both through time and from region to region. Severe groundwater droughts may continue through summers with well above average rainfall (as happened in 1997) whilst exceptional spring and summer rainfall deficiencies can coincide with a healthy water resources outlook (as happened in southern England in 2003). In its final phase, the extreme drought of 1975/76 exhibited substantial impacts not only on water supplies but across a broad range of the country’s economic, social and leisure activities. For the generality of droughts, impacts are less pervasive but it remains important to select drought severity indices which capture most effectively the particular mix of impacts associated with an individual rainfall deficiency.
The National Hydrological Monitoring Programme has produced a range of publications which examine the science of droughts in the UK and in Europe.
For an analysis of major historical droughts since 1800 in England and Wales, which also considers the impacts of these events, see this paper in the journal Weather (free access).
For a recent characterisation of historical droughts on a European scale, using indicators of both rainfall and river flows, see this reference (requires subscription). A poster summarising this work is also available to download for free here.
The NHMP has also produced reports which document the severity of drought events of the last 40 years, in terms of rainfall, river flows and groundwater levels. These reports also provide an overview of the impacts of these events. These reports can be downloaded from the NHMP reports page. Reports can be downloaded for the following drought events: 1975 - 76; 1984; 1988 - 1992; 2003; 2004 - 2006.