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.