Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 6 2008 10.5194/bg-5-1529-2008 http://www.biogeosciences.net/5/1529/2008/ http://www.biogeosciences.net/5/1529/2008/bg-5-1529-2008.html http://www.biogeosciences.net/5/1529/2008/bg-5-1529-2008.pdf 1529 1535 2008-11-11 Importance of long-term monitoring for detecting environmental change: lessons from a lowland river in south east England T. P. Burt t.p.burt@durham.ac.uk N. J. K. Howden F. Worrall M. J. Whelan Department of Geography, Durham University, Durham DH1 3LE, UK Department of Natural Resources, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK Department of Earth Sciences, Durham University, Durham DH1 3LE, UK Rising nitrate concentrations in rivers and groundwater in regions with intensive agriculture have been of concern for several decades. Long records of nitrate concentration are rare; in the UK, few pre-date 1974. Records from the River Stour from 1937 are analysed using moving-windows to explore the effects of window width on trend detection and process inference. Nitrate concentrations rose sharply from the late 1950s in response to widespread ploughing of grassland, but have fallen since the early 1980s, suggesting that widespread nitrogen control measures, which were introduced after the start of the downward trend, may not be the principal driver for recent change. Short windows (5 years) are too noisy; longer windows (10, 15 and 30 years) identify signals (i.e. trends) more reliably. Since system response times can be of the order of 20–30 years, management decisions made on shorter data sets (even up to 15 years) could be misleading. 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