The flux of suspended sediment from the UK 1974 to 2010

Abstract

• Flux of suspended sediment from the UK varied between 2199 k tonnes and 27,550 k tonnes/yr. • Export of suspended sediment was equivalent was between 9.6 and 119.8 tonnes/km 2 /yr. • Suspended sediment flux was highest from grazed, organic soils. • Fluvial export from the biosphere was 22.2 tonnes C/km 2 /yr and 9.1 tonnes N/km 2 /yr. The suspended sediment flux has been estimated for 270 catchments across Great Britain from 1974 to 2010: a total of 6026 catchment-years. The fluxes were corrected for inconsistencies in sampling frequencies between catchments and years and combined by a regional, area-weighted technique to give the national flux from 1974 onwards. Soil-cover, land-use and hydro-climatic variables were derived for 192 catchments for which annual average suspended sediment flux could be calculated between 2001 and 2010. The results show that: (i) Suspended sediment concentrations in UK rivers significantly declined from 1974 to 2010, but this does not cause a decline in suspended sediment fluxes: this suggests declines in concentration were mainly at low flows. (ii) Suspended sediment exports have a median of 22.2 tonnes/km 2 /yr with a 5th percentile = 5.4 tonnes/km 2 /yr and 95th percentile = 107.7 tonnes/km 2 /yr: giving a national flux between 2199 and 27,550 k tonnes/yr. It was not possible to estimate the turnover of suspended sediment through the watersheds but the results indicate that the total fluvial flux of carbon from the UK terrestrial biosphere is at least 22.2 tonnes C/km 2 /yr and 9.1 tonnes N/km 2 /yr for the total fluvial flux of nitrogen – more than twice the previous estimates.