Changes in water colour between 1986 and 2006 in the headwaters of the River Nidd, Yorkshire, UK

This study compares the spatial and temporal variability of water colour for fifteen sub-catchments of the River Nidd, northeast England, in 1986 and 2006/2007. Between 1986 and 2006/2007, mean annual water colour increased in all the sub-catchments. However, there was considerable variation in the increase, which ranged from 22 to 155%. Statistical analysis revealed that the sub-catchments could be split into two 'types' based on water chemistry and therefore dominant source of runoff; type 1 where flow was dominated by runoff from peat and type 2 where a greater contribution of flow appears to originate from mineral soil horizons, as indicated by the higher silicon, base cation concentrations and pH values. Largest proportional increases in water colour were observed in the sub-catchments that had the smaller mean annual water colour values in 1986 which were, in general, the type 2 sub-catchments. The higher rate of water colour increase in the type 2 catchments, in comparison to the type 1 catchments, may be related to changes in adsorption of dissolved organic carbon (DOC) within the mineral horizons of the organo-mineral soils on the lower catchment slopes possibly as a result of changes in acid sulphur deposition.