Long-term monitoring of river water nitrate: how much data do we need?

Long records of river water quality are invaluable for helping to understand the biogeochemistry of hydrological systems. They allow relationships to be established between changes in water quality (including seasonal cycles, episodic responses and long-term trends) and potential drivers, such as climatic forcing or human activity; they can act as a stimulus for process-oriented experimental research; they can be used to help to make predictions about future temporal and spatial patterns; and they can help to guide management options to mitigate water pollution. In this paper we present the case in favour of maintaining some long records of river water nitrate concentration at "benchmark" sites, in terms of enhancing process understanding and identifying system lags. Many long-term time series of nitrate concentration data share similar features including a pronounced seasonality characterised by a clear winter maximum, an upward trend in the post-war period followed by a levelling off, or even a decline in the last 20 years, and unusually high concentrations following drought years. Concentrations in any one year are often dependent on conditions in previous years; relationships can be established between concentrations and hydrological drivers (such as rainfall) with different lag periods which can yield information about supply or transport limitations to nitrate transfers. The interpretation of any record is dependent on its length: short records have a high potential for misinterpretation. Often, the value of long records only becomes apparent when analysed in retrospect, perhaps yielding insight into processes and phenomena for which the data collection programme was not originally designed. We, therefore, urge monitoring agencies to devise a strategy for maintaining long records--at least for a few benchmark stations.