SEDIMENT AND PHOSPHORUS EXPORT FROM A LOWLAND CATCHMENT - QUANTIFICATION OF SOURCES

Storm event and annual export of suspended sediment (SS) and particula te phosphorus (PP) was measured during three hydrological years (June 1993 to May 1996) in Gelbaek stream, a Danish lowland stream draining a 11.6 km(2) arable catchment area. The contribution of subsurface dra inage water, surface runoff and stream bank and bed erosion to catchme nt SS and PP losses was estimated using three different strategies: 1) Simultaneous and comparative monitoring of subsurface water. 2) A mas s-balance and budget approach dividing the Gelbaek catchment into two subcatchments. 3) Application of the fingerprinting technique to singl e storm events. Subsurface drainage water proved to be a significant S S and PP source. Subsurface drainage water from half of the catchment area accounted for 9.8-15% of the total annual SS loss from the Gelbae k catchment and 9.6-18.2% of the annual PP loss. The mass-balance and budget approach showed stream bank and bed erosion to be the major sou rce of SS and PP in this channelized and highly managed lowland stream . These findings were consistent with the fact that the annual loss of SS and PP from an upper culverted stream sub-catchment was significan tly lower than that estimated from a mass-balance for a lower sub-catc hment with an open stream channel. Comparison of the tracer content (e .g. Cs-137) of SS collected during four storm events with that of tops oil and subsoil using a simple mixing model revealed subsoil to be a m ajor source of SS.