Annual freshwater river mass loads from Great Britain, 1975-1994: estimation algorithm, database and monitoring network issues

The paper presents 20-year time series (1975-1994) of annual mass loads discharged from the aggregated freshwater-monitored area of Great Britain for suspended solids, total nitrogen, orthophosphate and zinc. Corresponding time series are also given for regional groups of catchments draining towards five sea-areas recognised for monitoring UK fluvial inputs to the North-East Atlantic. A mass load computational scheme that effectively merges the national river flow and water quality databases is used to derive the mass flow time series, and their utility is considered within the context of strategic information needs and the capabilities of current national monitoring programmes. While the aggregated annual suspended solids loads presented in the paper are often severe underestimates of the actual load, because infrequent sampling tends to miss high concentrations at high flows, the loads for total nitrogen and orthophosphate, which exhibit less variability in concentration than suspended solids, are of a more acceptable quality. Mass loads for zinc, and other determinands often recorded as 'less than a limit of detection' (< LOD), can be very uncertain. Bearing in mind the uncertainties involved, the time series presented are assessed for the presence or absence of trends. Not surprisingly, annual loads exhibit correlation with annual runoff. Except for zinc, however, there is no obvious evidence for temporal patterns in the time series that are not associated with variations in annual runoff. Zinc loadings have decreased as a result of improved environmental management in industrialized catchments. Discussion of the sources of uncertainty in regional mass flows is followed by some general recommendations related to capitalising most effectively on available mass flow time series. (c) 2004 Published by Elsevier B.V.