MITO Y REALIDAD

The scPDSI has been used to construct continuous records of drought for seven sites distributed across northwest England from meteorological data for 1757-2013 (257 years). These extend both the temporal length and spatial coverage of previous quantitative drought analyses for northwest England (Cole and Marsh 2006). A number of previously unknown or poorly documented severe droughts from the mid-eighteenth and nineteenth centuries have been identified from the longest records (Carlisle and Manchester), including: 1762-1763, 1764-1767, 1774-1789, 1793-1795, 1803-1809, 1831-1835, 1842- 1850, 1853-1861 and 1864-1871, which represent a substantial extension to the existing benchmark drought history for northwest England. A number of other severe droughts have been identified that are in broad agreement with well documented regional and national droughts identified using different selection criteria (Cole and Marsh 2006, Marsh et al 2007), but are of longer duration because of the broad thresholds chosen for identifying events and include: 1933-1944, 1970-1979, and 1983-1985. Hydrological and historical documentary data sources have been used to briefly investigate the impacts of a well-known (1995/96) and less well-known (1864-1871) severe drought which corroborate the identification of severe/extreme drought conditions using the scPDSI.

6.6.1 Temporal variability

The extended drought series show that severe/extreme droughts are a recurrent feature of the long-term natural variability of northwest England climate. These events are evident in all four centuries represented in the scPDSI series and are not just a feature of the most recent, twentieth and twenty-first centuries; although distinctly more drought-rich (twentieth century) and drought-poor periods (nineteenth century) are shown for the Carlisle and Manchester scPDSI series (Figure 6.3).

The most severe droughts identified in all seven records occurred during the twentieth century, and in some cases were exceptional in terms of their severity (e.g. Carlisle 1995- 1999, -7.5); however, all other severe/extreme events on record were of a similar magnitude (Table 6.4). Therefore, there is little evidence to suggest an increasing trend in the magnitude of severe droughts in the region. Conversely, there is an increase in the frequency of severe/extreme drought conditions during the twentieth century, making it a more drought-rich period than the nineteenth century (Figure 6.3 and Table 6.4). This is similar to the temporal pattern of long-term drought variability in southeast England, which

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also shows a shift towards more frequent severe droughts since 1890 (Todd et al. 2013), and is also apparent in the wider European context (Briffa et al. 2009). Here, the onset towards more frequent severe drought conditions is observed during the 1880s for sites in the lowland part of the region (Barnacre, Longdendale and Manchester) and later during the 1930s at Carlisle, the most northerly of the seven sites. Persistent drought conditions were observed at all sites except Carlisle following the shift to the drought rich period, which coincides with the period of ‘Long Drought’ during 1890-1910, described as the most sustained drought conditions on record (1800-2006) for the eastern English lowlands by Marsh et al. (2007). The identification of sustained drought conditions in northwest England suggests this event was more widespread across the UK and a severe national scale meteorological event. Furthermore, Briffa et al. (1994) show that sustained drought conditions during the 1890s were spatially coherent across much of Europe suggesting that the ‘Long Drought’ was a large-scale, continental event.

The increase in severe drought frequency identified here may in part reflect the increasing temperature trend observed during the twentieth century. Temperature is a key variable in the calculation of potential evapotranspiration (PET), calculated as part of the water balance model of the scPDSI. Recent studies have suggested that the traditional method (Thornthwaite 1948) for calculating PET (used here) is over-sensitive to temperature change (Sheffield et al. 2012); however, comparisons of PET-estimates using different methods has found resultant scPDSI values to be very similar in terms of classifying extremely dry and wet months, and that the use of alternative methods e.g. Penman- Monteith types only slightly reduce drying trends as found in analyses of global drought (Dai 2011b; van der Schrier et al. 2011; van der Schrier et al. 2013). Prolonged rainfall deficiencies have been identified as the primary cause of drought, exacerbated by other meteorological elements (Brázdil et al. 2008; Briffa et al. 2009). Analysis of seasonal rainfall totals based on the England and Wales precipitation series for 1845-1995 (Jones and Conway 1997) identified an increasing trend in winter rainfall and a decreasing trend in summer rainfall, which is consistent with analyses of other long precipitation series available in the UK (e.g. Durham Observatory, (Burt and Horton 2007). Further analysis of this precipitation series by Jenkins et al. (2008) for 1961-2006, confirmed the occurrence of these observed trends for northwest England, but identified only a slight decrease in summer rainfall. Drier summers coupled with increasing temperatures during the twentieth century are likely to have contributed to the increasing frequency of severe drought conditions in the recent part of the record, which is concurrent with an analysis of wet and

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dry summers in Europe since 1750 (Briffa et al. 2009) and a summer only aridity index series for England and wales for 1776-2005 (Marsh et al. 2007).

Multi-year, decadal and multi-decadal droughts occurred at all sites, often containing shorter more severe drought periods. There are many examples of multi-year events throughout the 257 year record. In contrast, decadal and multi-decadal severe events are rare, occurring only three times at Carlisle in 257 years and four times at Manchester in 228 years. These longer duration droughts include numerous multi-season and multi-year periods of severe drought conditions, which suggest severe drought conditions have a tendency to cluster together in time resulting in protracted droughts.

6.6.2 Spatial variability

In general, severe droughts show regional coherence in northwest England, with droughts for similar time periods appearing on record at multiple sites. Analysis of severe drought characteristics (e.g. duration and minimum event severity) during 1911-2013, however, shows there is variability in drought behaviour between sites, resulting in intra-regional variability and highly variable correlation coefficient values. Other regional scale studies of UK drought variability have also shown that drought characteristics vary across relatively small spatial scales resulting in inter-site variation in drought behaviour (Phillips and McGregor 1998; Fowler and Kilsby 2002; Lennard et al. 2014). Based on correlation analysis and patterns identified in coefficient values, distinct drought climatology sub-regions were depicted in these studies. Similar analysis was undertaken here, but as a result of much more highly variable coefficient values achieved between sites, it was not possible to sub- divide the region into smaller areas of coherent drought behaviour. Distance between sites and localised variation in rainfall were identified as important for determining the regional coherence of droughts. Rahiz and New (2012) investigated the spatial coherence of meteorological drought for the whole of the UK since 1914 and found that spatial variability in precipitation partly explains intra-regional differences in the coherence of drought. The diverse topography and highly variable maritime climate of northwest England, in comparison to other regions of the UK (i.e. southeast and northeast England) partly explains the observed greater variation in drought behaviour and reduced coherence in northwest England.

Rahiz and New (2012) also attributed variation in the regional coherence of droughts to different meteorological drought durations and the timing of their occurrence. Using the

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rainfall-based drought severity index (DSI), they found a greater coherence for shorter (3 month) duration wet season droughts than for longer duration (6 months) dry season events, and that long-duration severe meteorological droughts lacked a tendency to co- vary in northern and western regions of the UK; although, this observation is partly a function of the greater statistical rarity of longer-duration severe events. Developing a denser network of PDSI time series and subsequently undertaking cluster analysis, adopting a gridded approach, or analysing different drought severities and durations following Phillips and McGregor (1998), Fowler and Kilsby (2002) and Rahiz and New (2012) for example, might further improve understanding of the regional coherence of drought in the climatically diverse northwest region of England.

6.6.3 Implications for water resources management

This study has shown that severe droughts are a recurrent feature of the climate of northwest England, as they are in other regions of the UK (Marsh 2007). With the extension of existing drought histories, a number of additional severe events during the late eighteenth and first half of the nineteenth centuries have been identified that were previously unknown or poorly documented. These are useful for contextualising and evaluating recent severe droughts used as benchmark events in contemporary water resources and drought planning (e.g. 1995-97), which has revealed that the most severe events during the last 257 years occurred during the twentieth century at all sites presented here. This implies that current water resources planning using twentieth century events as the worst historic examples, are based on reliable yield estimates. It is important to recognise that decisions made on selecting droughts for water resources planning are often made using an evidence base broader than just meteorological, but the long-term perspective of these data outperforms other hydrological and socio-economic evidence in terms of duration, which confers confidence in the current regional water resources plans based on data for the last 100 years at best.

The concurrent timing of the most severe drought periods within both meteorological and hydrological datasets evidenced within this chapter increases confidence in the drought events identified using the scPDSI, and demonstrates its usefulness as a tool for identifying historic periods containing clusters of severe droughts. The hydrological evidence of drought also highlights the limited capability of the scPDSI for identifying other types of drought than meteorological (i.e. hydrological, water resource), as the onset and termination of events depicted using the scPDSI in comparison to the hydrological data

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differs considerably (depending on which scPDSI drought thresholds employed), which may be attributable to the way factors such as; available water holding capacity, are calculated in the scPDSI, that varies considerably across northwest England (Hannaford and Marsh 2008).

Analysing long-term drought variability based solely on the magnitude of their severity is one important aspect of understanding droughts; however, patterns of other drought characteristics, such as frequency and duration, can have implications for water resources management. An increase in the frequency of severe drought conditions has been identified here during the twentieth century, which is in part attributable to global climate warming. There is evidence for enhanced seasonality of rainfall in the northwest region, with wetter winters and slightly drier summers (Jenkins et al. 2008). Coupled with increasing average temperatures and a rising population, this may lead to greater demands for water, during increasingly drier summer seasons. Increasing winter rainfall over the last 50 years has contributed to an increasing trend in runoff, particularly during the late 1960s to the early 1990s, which is favourable from a water resources perspective (Marsh and Dixon 2012). Negative impacts from climate changes on water resources depend upon the amount of water stored by reservoirs and groundwater. The majority of water supply in the northwest region is derived from surface waters, which are replenished over winter. As such, increasing trends in rainfall and runoff during winter are potentially quite advantageous from a water resources perspective, however; the benefits may not be realised if there is not enough available storage capacity to capitalise on the enhanced delivery of water to reservoir and groundwater stores. This may present challenges for the existing maximum storage capacity of resources to be able to provide a continuous supply during seasons were demands for water may be increasing. Any increase in the frequency of severe drought conditions, likewise with the duration of droughts, may necessitate more sustained water use restrictions to reduce demand and mitigate negative impacts.

Severe droughts are broadly regionally coherent in northwest England, but considerable intra-regional variability in drought characteristics and behaviour is also evident. Variability in drought severity and duration, and the termination of events within the region may result in differences in the recovery times of reservoirs for example. This would necessitate a varied approach to water resources management, adopting a range of demand management and/or supply enhancement measures; such as, relying on those reservoirs where levels have recovered from drought conditions terminating sooner to augment

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supplies in areas experiencing shortages, and rehabilitating additional sources that have not been used for various cost and water quality issues. These measures would help to maintain adequate supplies to meet demands and mitigate widespread negative impacts during significant events. During and following the significant drought of the mid-1990s (1995-96 the water authorities perspective) a major bi-directional pipeline was constructed between Liverpool and Manchester to transfer water more readily between the west and east of the region (DWRMP 2013). A large proportion of the water supply network in northwest England is integrated providing improved capabilities to manage the supply- demand balance and reduce the risks associated with drought. Nevertheless, the high intra- regional variability in the coherence of drought behaviour underlines the need for effective management of resources and highlights the importance of a dense monitoring network of not only water resources, but rainfall, temperature and soil moisture conditions to provide essential information for drought monitoring and mitigation.