Ž . Geomorphology 33 2000 25–36 Sediment export by water from an agricultural catchment in the Loam Belt of central Belgium An Steegen a, ) , Gerard Govers a,b , Jeroen Nachtergaele a , Ingrid Takken a , Laurent Beuselinck a , Jean Poesen a,b a Laboratory for Experimental Geomorphology, K.U. LeuÕen, Redingenstraat 16, 3000 LeuÕen, Belgium b Fund for Scientific Research, Flanders, Belgium Received 10 January 1999; received in revised form 2 August 1999; accepted 12 August 1999 Abstract An agricultural drainage basin of 250 ha was selected in the Belgian Loam Belt to evaluate sediment export by water. Water discharge was continuously measured at the outlet of the catchment and suspended sediment samples were taken proportional with discharge during rainfall events. From these samples supended sediment concentration and grain-size distribution were determined. Within the catchment, regular surveys were conducted in order to obtain detailed data on vegetation cover, soil surface parameters and erosion and sedimentation features. Important variations in sediment yield were detected at various time-scales, ranging from a year, over a season to one event. These variations could be linked with processes occurring within the catchment. Due to the development of a vegetation cover, the suspended sediment concentrations were lower during the summer period than during the winter period for the same discharge. The evolution in both suspended sediment concentration and grain-size distribution during the winter period is explained by variations in the soil surface state of the fields as well as the development of a rill and gully network. The positive hysteresis observed within a single event is explained by taking into account the distance between the sediment source and the catchment outlet and not by sediment flushing. Our analysis shows that the role of the main factors controlling the geomorphic response of such a catchment can be identified only if measurements at the outlet are complemented by detailed data on the conditions within the catchment. Catchment model calibration and validation are therefore impossible if this information is lacking. q 2000 Elsevier Science B.V. All rights reserved. Keywords: sediment delivery; soil erosion; deposition; hysteresis; extreme event 1. Introduction It has long been known that the relationship be- tween water discharge and sediment concentration in ) Corresponding author. Tel.: q 32-16-32-64-37; fax: q 32-16- 32-64-00. Ž . E-mail address: an.steegen@geo.kuleuven.ac.be A. Steegen . rivers shows important variations at various time Ž . scales. Gregory and Walling 1973 noticed that the relationship between sediment concentration and dis- Ž . charge in the Rosebarn catchment UK was differ- ent for the summer and winter seasons. Van Dijk and Ž . Kwaad 1996 found that this relationship was differ- ent for the St Gillisstraat-2 catchment between and within the summer and winter seasons due to the 0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-555X 99 00108-7