Background
Hydrometric data are the foundations upon which water management is built. In the UK, as in most of the world, the lack of any long-term trend in most lengthy river flow and groundwater level series (at least, where the impact of man is limited) serves to underpin water management strategies and operational procedures designed to mitigate the problems caused by flooding or drought. The resilience of these strategies has been brought into question both by the magnitude of the departures from seasonally typical runoff and aquifer recharge patterns over the last decade, and their broad consistency with a number of favoured climate change scenarios. It is important therefore to place the recent hydrological volatility in as long a historical context as possible in order to help identify and quantify hydrological trends.
Although in global terms the UK maintains relatively dense networks of flow measurement stations (around 1450 contribute data to the NRFA) and observation wells and boreholes (around 5000), it is less well blessed in terms of the length of flow records. This is especially true of those datasets which have been systematically archived to allow general access and analysis. For gauging stations incorporated in the National River Flow Archive the average gauged daily flow record length is around 40 years. A substantial proportion commence in the 1960s, a period of intense network growth in much of the UK, and there are around 370 stations on the NRFA with record lengths over 50 years long.
Leaving aside the impact of possible climate change, it is very unlikely that the full range of hydrological variability has yet been captured. Accordingly, a programme was instigated in the mid-1990s to augment the National River Flow and National Groundwater Level Archives' holdings of river flow and groundwater level data with less formal historical time series held by a range of measuring authorities (many no longer extant), collated or derived by individual researchers or published in the literature. The systematic archiving of early hydrometric datasets provides security against the loss or destruction of the original data.
Appraisal of Long Hydrometric Series
Some lengthy historical data series are excellently documented and reflect close attention to the highest standards of hydrometric data acquisition appropriate to the period when the data were collected. For most older series, however, considerable curatorial skill is required to prepare the datasets for dissemination and further analysis. Validation of these early datasets is an ongoing task. In order to capitalise fully on important historical data series it is essential to critically review the likely data accuracy and appraise, at least qualitatively, temporal changes in artificial influences and their impact on the flow regimes and aquifer recharge patterns. Data precision and consistency can be a major problem with many early hydrometric records. Over the twentieth century instrumentation and data acquisition facilities improved but these improvements can themselves introduce inhomogeneities into the time series - which may be compounded by changes (sometimes undocumented) in the location of the monitoring station or methods of data processing employed. In addition, man's influence on river flow regimes and aquifer recharge patterns has become increasingly pervasive, over the last 50 years especially. The resulting changes to natural river flow regimes and groundwater level behaviour may be further affected by the less perceptible impacts of land use change; although these have been quantified in a number of important experimental catchments generally they defy easy quantification. It will be appreciated therefore that the recognition and interpretation of trends relies heavily on the availability of reference and spatial information to help distinguish the effects of climate variability from the impact of a range of other factors; seldom is it safe to allow the data series to speak for themselves.
Historical Datasets
Wendover Springs:
The first data series to be released via the Long Records Initiative is, appropriately, that for the Wendover Springs - the earliest extant flow monitoring site in the UK. Contemporary spring outflows are measured at a thin-plate weir operated by the Canal and River Trust. The utility of these flow data is greatly enhanced by the existence of an 1841-97 dataset for the Wendover Arm of the then Grand Junction Canal (which was fed by the Springs). This early data provides a unique insight into hydrological variability in Victorian times. Time series plots, reference material, and details of how to request the monthly data can be found here.
Other Hydrometric Records:
Further selected historical series will be posted as they are prepared. Following the format established for the Wendover Springs series each historical dataset will be supported by information concerning its source, and locational and reference material relating to the monitoring site/catchment. Where appropriate, statistical summaries of the data and appraisals of any real or apparent long term trends will also be given.
Terms and Conditions
The Long Hydrometric Records held by the NRFA are subject to the same Terms and Conditions as other NRFA datasets, the details of which can be found here.
Disclaimer
By their nature early hydrometric data can be of very variable quality. Where practical, quality control procedures have been applied to the long record datasets and appraisals of their hydrometric quality have been undertaken. However, neither NERC nor the organisations responsible for the acquisition of the long records can accept any liability for any loss or damage, cost or claims arising directly or indirectly from their use.
