LAS CONTRADICCIONES DE CLASE EN ROMA

Drought reconstruction and event identification is undertaken using available data sources in a region. In an assessment of UK drought severity, Mawdsley et al. (1994) emphasised that there are no national guidelines in the UK for determining what the most appropriate data sources are for drought assessment. They suggested that there are two types of drought data: environmental and water resource, which may be either qualitative or quantitative.

2.7.1 Pre-instrumental period

The UK has a rich history of observing and measuring the weather. The earliest observations of weather commonly coincided with astronomical observations, and were often an after-thought, appearing in the margins of astronomical tables. Observing the weather gained prominence during the medieval period and became increasingly organised: both in terms of how frequently observations were made and how they were recorded. The most common sources of these early weather observations are personal weather diaries and journals, kept by scholars, scientists and other professionals recording predominantly in isolation and who viewed such activities as a prestigious undertaking. Kington (1997) details one of the earliest examples of these sources, which is the journal of William Merle, rector of Driby, Lincolnshire. He kept a systematic record of weather in his journal entitled “Temperies aeris Oxoniae pro septennio”, from 1337 to 1344.

2.7.2 Instrumental period

Meteorological data

Instrumental meteorological measurement was made possible following the invention of the first meteorological instruments in Europe during the late-sixteenth and seventeenth centuries. Sir Christopher Wren designed and presented one of the earliest recording rain gauges in the UK in 1662. Shortly after, Richard Towneley began a period of regular systematic rainfall measurement at Towneley Hall, near Burnley in north-west England in 1677, which he continued until 1704 (Lewis 1977). Realising the benefits of quantifiable data and the potential in making observations at numerous locations, encouraged the establishment of a network of meteorological stations throughout Europe and beyond during the 1700s (Brazdil et al. 2005). The spatial distribution of early instruments and recording was confined to scientific societies, and private observers such as; physicians, clerics, and land owners, recording predominantly in isolation, but communicating through

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societies, with the Royal Society being the focal point in the UK (Kington 1997). These instrumental measurements are an important source of early data; which, helped to calibrate meteorological records from around the world and standardise units of measurement to ensure the production of reliable records.

Proliferation of meteorological stations, and subsequently the network, was greatly facilitated by advancements in communication technologies, such as the invention of the electric telegraph, during the first half of the nineteenth century. On this basis, the UK has reliable instrumental records available for the last two to three hundred years (1700-1800), increasing in frequency towards the present. It is also upon this foundation that long homogenous regional meteorological series, such as; the Central England Temperature series (CET: 1659-present, Manley 1974, Parker et al. 1992) and the England and Wales precipitation series were constructed (1766-present, Wigley and Jones 1987, Gregory et al. 1991, Jones and Conway 1997, Alexander and Jones 2001).

The pioneering meteorologist G. J. Symons was fundamental in the development of a formal meteorological recording network in the UK and the standardisation of observing practices. In 1860, Symons published his first pamphlet containing rainfall records from 168 gauging stations across Britain. In 1863, he was able to publish monthly rainfall totals from 900 gauging stations, the earliest extending back to 1677 (Towneley’s measurements). As the number of gauging stations across Britain grew, Symons’s published annual summaries of rainfall records. A reprint of the 1860 totals became the first volume in the series called

British Rainfall (Pedgley 2002). The accuracy of these early rainfall records are perceived as fairly high as the business of meteorological recording was seen as a prestigious undertaking amongst British society. Furthermore, Symons recognised the importance in standardising instruments and measurement techniques. At an early stage he strove to develop a network of rainfall gauges with uniformity in measurement and recording and personally inspected many of the sites contributing to British Rainfall.

The UK Meteorological Office’s MIDAS Land Surface Station Data set contains daily and hourly rainfall measurement from 1853 to present. Sub-hourly rainfall measurements are also available from 1853 to 2005 for a small number of stations. Measurements are derived from a network of over 50,000 weather stations across the UK and are available via BADC website.

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Instrumental hydrological measurement was limited in the UK until the twentieth century, when the first systematic network of water monitoring was established. Some records are available prior to this with one of the earliest being held at York since 1877. Others exist for the Thames at Teddington and the Lee at Feildes Weir, which also have records beginning in the late nineteenth century (Bayliss and Reed 2001; Macdonald 2004). Rapid extension of the river gauging network took place during the 1960s, after the passing of the Water Resources Act 1963 (Bayliss and Reed 2001), and Marsh and Hannaford (2008) state there are now approximately 1500 river gauging stations in Britain. Bayliss (1999) identifies that in a European context the UK has a dense network of river gauges; but from a historical perspective, the length of the formal network is short, with few sites providing a record of greater than 50 years (Macdonald 2004).

The instrumental gauged network produces a large hydrological data set, maintained by the National River Flow Archive (NRFA, CEH), and regularly published in the UK Hydrometric Register. Currently, the gauging network is maintained by the Environment Agency (England and Wales) and the Scottish Environment Protection Agency (Scotland). The data set can be regarded as highly accurate due to the modern hydrometric instrumentation and data processing systems used; however, extremes of river flow (high and low flows during floods and droughts) are said to be less precisely measured (Marsh et al. 2005). This is a result of the substantial effect of geological and land use characteristics of individual catchments and the impact of water use patterns upstream of the gauging stations. The influence of abstraction, for example, would result in an anthropogenically derived diminution in flow. Nevertheless, reliable datasets can be compiled for a variety of spatial scales; national, regional and catchment.

Additional shortcomings of the formal gauged network are readily identified within hydrological scientific research, particularly within the areas of flood frequency analysis and historical climatology (Macdonald et al. 2006; Barriendos et al. 2003; Brazdil et al. 2006). Some general limitations include: an uneven distribution of gauging stations within Britain with the majority centred around urban areas (Macdonald, 2004), as a result rural areas are under-represented within the hydrological history of Britain. In addition, McEwen (1987) highlights that there are fewer gauges and shorter records for Scotland than their English counterparts, which is a function of inaccessibility and isolation.

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Within the UK there also exists the observation well network, which monitors groundwater levels across the country and is an important source of hydro-geological data. Data is archived by the British Geological Survey (BGS) in the National Well Record Archive (NWRA) and the National Groundwater Level Archive (NGLA). Marsh et al. (2007) identify that groundwater level time series (similarly to river flow time series) are shorter in length than temperature and precipitation series, with the majority beginning late in the twentieth century and with few extending back to the nineteenth century (CEH 2012). Accuracy of groundwater level data is high (to the nearest 10mm, CEH 2012), as a result of the modern instrumentation used and the uniformity in measurement from the commencement of recording. There is an uneven distribution of observation wells throughout the UK; the majority are located in England and Wales, with a limited number in Scotland.

2.8 SUMMARY

This chapter has provided a review of key drought concepts, methods, UK drought literature and data sources for UK meteorological drought reconstruction and analysis. The following chapter (3) provides a detailed description of the methodological approaches adopted and applied in this thesis.

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Chapter 3