SUSPENDED SEDIMENT AND PARTICULATE PHOSPHORUS TRANSPORT AND DELIVERY PATHWAYS IN AN ARABLE CATCHMENT, GELBAEK STREAM, DENMARK

Reliable quantification of suspended sediment (SS) and particulate pho sphorus (PP) transport, and identification of the various delivery pat hways at the catchment level, is an important and necessary aid to app ropriate catchment management. In this study we measured storm event, seasonal and annual losses of SS and PP from a Danish arable catchment , Gelbaek Stream, using a multisampling strategy. SS losses for the st udy years May 1993-April 1994 and May 1994-April 1995 ranged from 71 t o 88 kg ha(-1), while PP losses ranged from 0.32 to 0.36 kg P ha(-1). In both cases losses mainly occurred during infrequent storm events. I n comparison with intensive storm sampling, infrequent (fortnightly) s ampling underestimated annual transport during the two study years by -24 and -331%, respectively, for SS, and by -86 and -151%, respectivel y, for PP. Reliable estimation of the transport of sediment and sedime nt-associated nutrients and other substances thus necessitates the use of an intensive monitoring approach. Turbidimeters proved to be a goo d substitute for direct measurement of SS, especially during storm eve nts, although careful calibration is needed at the seasonal and storm event levels. Experience shows that in artificially drained and geolog ically complex catchments such as Gelbaek, simultaneous comparative mo nitoring of different sources (e.g. subsurface drainage water) is an i mportant means of reliably discriminating between the various diffuse sources of sediment and phosphorus. Subsurface drainage water was foun d to account for 11-15% of the annual SS export from the catchment; th e corresponding figure for PP being 11-18%. Surface runoff was only a source of SS and PP during the first study year, when it accounted for 19% of SS and 7% of PP catchment export. Stream bank/bed erosion must therefore have been the major diffuse source of SS and PP in both stu dy years. The study also revealed that analysis of the trace element c ontent (e.g. Cs-137, (210)pb) of the SS transported in subsurface drai nage water and stream water during storm events is a useful means of d iscriminating between diffuse losses of SS delivered from topsoil and subsoil compartments. (C) 1997 by John Wiley & Sons, Ltd.