Decomposition of river water nitrate time-series — comparing agricultural and urban signals

This article explores the combined effects of agricultural and urban influences on the time-series of the nitrate concentration in river water. The temporal and spatial variation of the river water signal for nitrate are examined using time-series analysis. The catchment chosen for this study was the River Wansbeck in south-east Northumberland where a distinct agricultural to urban gradient exists down the catchment. The time-series were obtained from the network of Environment Agency monitoring stations on the river. Twelve such stations provided monthly data stretching, in one case, as far back as 1973. The time-series from parts of the catchment dominated by agricultural land use showed a clear annual cycle which was lost downstream as larger settlements begin to appear on the banks of the river. The time-series, taken at the tidal limit, showed an asymmetric annual cycle that was not seen higher up the catchment, a pattern which could not be linked directly to the influence of sewage works discharging into the lower sections of the river. This study was not able to account adequately for the variation in the nitrate concentration of the river as it enters its estuary. Two new approaches are suggested to solve this problem.