Systematic application of United Kingdom river flow and quality databases for estimating annual river mass loads (1975–1994)

Abstract This article describes the first effective merger of the Harmonised Monitoring Scheme (HMS) database of river quality and the National River Flow Archive (NRFA) for systematic estimation of UK river mass loads. Benefits and pitfalls of applying these national river flow and quality datasets for calculating river mass loads are described and demonstrated. The benefits of using the HMS database and the NRFA as a combined super-database include a synergy of information in terms of improved precision in river mass load estimates. Special software is introduced for the systematic c alculation o f r iver mass a ggregated l oads (CORAL) for rivers grouped by coastal zones around the UK. Such an approach necessarily establishes an audit trail of the data sources and estimation method(s) employed, facilitating more meaningful comparisons of river mass load estimates. Possible pitfalls include the application of a `blanket' estimation methodology — irrespective of the hydrological regime at the site in question or the dynamic flow–concentration behaviour of the determinand of interest — where a suite of site/determinand-specific estimation algorithms would make better use of the available data. Other issues discussed include the need to make allowances, when interpreting time series of river mass loads, for any temporal variations in: (1) monitoring networks; (2) quality of flow measurements; (3) sampling regimes; and (4) methods of laboratory chemical analysis. A focus of interest is the quality of data and information which the United Kingdom, along with several other north-western European countries, provides to the Oslo and Paris Commission (OSPAR) for monitoring the contaminant loading of the North Sea by rivers and by dumping and incineration at sea (e.g. from ships). Time series of annual mass loads from the aggregated area covered by the HMS network of catchments in Great Britain, from 1975 to 1994, are presented for 12 determinands, with comments on their reliability for detecting trends. For 10 determinands, annual estimates are compared with previous, independently-derived, estimates for selected years which have been submitted to OSPAR. Future monitoring and database requirements to meet the need for reliable time series of river mass loads are discussed.