Sediment export by water from an agricultural catchment in the Loam Belt of central Belgium

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. (C) 2000 Elsevier Science B.V. All rights reserved.