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Riparian buffer zones are considered to be one of the most beneficial best management practices (BMPs) to improve surface water quality. In many cases they are assumed to be responsible for large reductions in sediment and agricultural chemical losses from croplands to water bodies. Filtration, deposition, infiltration, adsorption, absorption, decomposition, and volatilization are thought to be the biophysical processes governing the reduction of agricultural contaminants transported by surface runoff. Vegetation at the downstream edge of disturbed areas may effectively reduce runoff volume and velocity; initially because of increasing hydraulic roughness, and subsequently by enhancing water infiltration. Decreasing flow volume and velocity lead to sediment deposition in the vegetative filter (VF) as a result of reducing the transport capacity of runoff. Dimensioning and positioning these structural BMPs represents a "real life" challenge for soil conservation engineers, managers, planners and policy-makers. Different factors, such as trapping efficiency, implementation, management, opportunity (resulting from cropland surface reduction) costs, and government policies and regulations need to be weighed to meet this challenge. The trapping efficiency depends on many parameters, including: (i) VF characteristics such as width and slope, vegetation height, vegetation density, and species composition; (ii) flow characteristics such as runoff velocity, discharge volume, water height; and (iii) sediment characteristics such as particle size, aggregation, and concentration. Government policies and regulations may include dimension and location of VFs and/or a cropland percentage that needs to be converted into VF areas. The main objective of this paper is to describe the development of a vegetative filter dimensioning model (VFDM) to determine the optimal dimensions of riparian vegetated filter strips (RVFSs) in agricultural watersheds. VFDM calculates the optimal filter strip width with respect to vegetation, topographical, hydrological and sedimentological characteristics. A virtual watershed with six different hillslope types and a main channel has been used in model testing (Figure a). Future work will involve linking VFDM to an economics model in order to evaluate the effectiveness of RVFSs with respect to both water quality and economic objectives.