Dissolved organic carbon for upland acidic and acid sensitive catchments in mid-Wales

The distribution of dissolved organic carbon (DOC) in rainfall, cloud water, spruce throughfall and stemflow, stream runoff and groundwater is described for the Plynlimon catchments in mid-Wales for up to 20 years of record. Rainfall concentrations average at 0.69 mg-C l−1 while equivalent values in cloud water are over three times higher. Spruce throughfall and stemflow averages are over an order of magnitude higher than rainfall. DOC averages are around 1.5 mg-C l−1 for most streams draining moorland and forested areas but much higher values occur for streams draining from organic-rich gley soils in the forest. Groundwater concentrations typically average about 0.5 mg-C l−1. For forested sites where felling has taken place, this has typically resulted in little change for either the streams or the groundwater, but larger changes can occur for a small stream drainage area with gley soils. For rainfall, DOC concentrations increased from 1983 to 1993 and then declined thereafter to 2003. For the streams draining forest there has been a fairly uniform rise in DOC concentrations over time throughout the period of typically about 0.055 mg-C l−1 year−1. However, when the influence of season, air temperature and flow was filtered out of the 'raw' time series, there appeared a residual trend for 1983-1993 and a moderate levelling off from 1983 onwards the streams draining forest. One moorland stream showed a lower gradient barely significant from zero, but this may be partially because data was only available from 1990. The variation in gradient across the sites for the full monitoring period of each of the streams was 0.019-0.146 mg-C l−1 year−1 for the raw and 0-0.113 mg-C l−1 year−1 for the residual time series.