The Wendover Springs record: an insight into the past and a benchmark for the future

Abstract

The chalk of the English lowlands is the most important of the British aquifers. It is the principal source of water supply in much of southern and eastern England, as well as sustaining a network of chalk streams that, ecologically, are of international importance. Replenishment of the aquifer normally takes place during the winter months when evapotranspiration is low and the soil moisture deficit is negligible. In south-eastern England the margin between annual rainfall and evaporation losses is typically small and inter-annual variability in recharge totals can be very large. It is estimated that the average annual effective rainfall in the Chilterns is around 200 mm (Anon. 2003) and this modest figure – about 30% of average rainfall – is sensitive to changes in climatic conditions. Recent scenarios produced by the UK Climate Impacts Programme (Hulme et al. 2002) suggest that this sensitivity may increase if temperatures continue to rise and rainfall patterns change. Given the importance of chalk aquifers, in the context of both biodiversity and water supply, it follows that there is considerable interest in the possible impacts of climate change. Climate change scenarios are produced using increasingly sophisticated models but, given the expenditure required to address the predicted effects of climate change on water resources, policy-makers and managers will also look to the scientific community to demonstrate that there is evidence of change in observational records. The UK is blessed with extensive climatological data, with a number of temperature and rainfall records exceeding 250 years. However, with respect to the measurement of river flow, records are relatively short – the average record length held by the National River Flow Archive is less than 23 years and only 11 records exceed 70 years in length. The case for using long records is a strong one. For example, following a study of trends in UK floods, Robson (2002) concluded that a 40-year record (which exceeds that typically available) was insufficiently long to distinguish between the possible effects of climate change and natural short-term variability. Long records provide the greatest opportunity for trend identification, but a facet of their longevity is that the record is more likely to have been influenced by factors unconnected to possible changes in the climate – for example, inconsistent measurement, increasing abstraction and progressive catchment urbanisation. Consequently, it is important to establish the homogeneity of the time-series, particularly so in the case of informal records, where the data are unlikely to have been subject to the same level of scrutiny as those taken from the modern gauging network. Although there is a paucity of long records of observations that have been formally measured, reviewed and archived, there is considerable material giving contemporary accounts of historical floods and droughts awaiting discovery. The British Hydrological Society’s “Chronology of British hydrological events” (www.dundee/ geography/cbhe) is a significant step forward in locating and collating this information for public use (to date there are over 7000 entries). In addition to descriptive accounts, the river gauging authorities (principally the environment agencies) and others in the private sector (e.g. water companies) may have informal level or flow records that can usefully extend or supplement formally gauged records. This paper outlines the assembly of a unique record of flow from the Chiltern springs at Wendover, Buckinghamshire, and describes the flow patterns evident in a series of over 90 years. Globally there are few records that describe flow variability in the latter part of the nineteenth century as completely as the Wendover series and an exceptional drought episode that occurred during that era, described here, is not well documented elsewhere. The paper concludes by discussing recent events in the context of this long historical record.