In Finland, the major part of diffuse phosphorus load to water ways is carried with soil material eroded from clayey and silt arable fields. According to several field studies, subsurface drain flow carries a large amount of sediment besides overland flow. Cesium 137 measurements indicated that most of the soil material transported through subsurface tile drains originates from the soil surface layer. The goal of the study was to create a process based model which could be used to simulate sediment transport via overland flow and subsurface drains at clayey, subsurface drained agricultural fields. The model was developed and tested with data from a clayey field in southern Finland. The developed modeling system is distributed and dynamic in nature and it is divided into surface and subsurface parts. The former includes 2-D overland flow and coupled erosion, and the latter 3-D subsurface flow and coupled transport models. Overland flow is modelled with a kinematic wave approximation of the St. Venant equations. A standard mathematical approach to erosion modeling was adopted with a sediment continuity equation and hydraulic erosion based on shear stress of the flow. Preferential and soil matrix flow and transport in subsurface domain were implemented with a dual-permeability approach. Both pore domains in dual-permeability flow and transport models were implemented with Richards and advection-dispersion equations respectively. The partial differential equations were solved implicitly with a finite volume method. The Sjökulla experimental field site in southern Finland comprises a field section of 3.3 ha within a larger area of arable land. The topography of the field is undulating with the maximum slope of almost 5%. The clay content of the soil varies from 38% to 90% increasing with depth. In the study years, small grain crops were grown in the field. The time series used in the modelling were from May-November 1996 and May-October 1998. They included hourly values of overland and subsurface drain runoff rates, groundwater level and meteorological data. The water quality data comprised total suspended solids (TSS) concentrations in subsurface and surface runoff samples. In addition extensive data on soil physical properties were available from a field section of about 1 ha within the experimental site. A novel part of the study was to simulate both overland and subsurface drain sediment loads simultaneously with linked process models. The sediment was allowed to move freely between field surface and soil matrix and macropores in the tillage layer. Under the tillage layer, sediment was allowed to move only in macropores. The model was calibrated with data from 1998 and validated with data from 1996. Runoff rates produced by the flow model where in good agreement with the measurement data for both calibration and validation years. Sediment loads during calibration year were quite similar to measured ones even though there were some differences in the dynamics. However the present model structure and/or parameterization cannot explain the very high sediment load from subsurface drains during the validation year. The effect of grid resolution was inspected with two relatively low resolution grids. Among other things the lowest resolution grid produced less drainage runoff and had a tendency to cut down drain runoff peaks.