Quantitative assessment of best management practices for soil and water conservation: A case study from the Tarquinia plain

Soil erosion and nutrient pollution are critical global environmental challenges, particularly in agricultural areas. Implementing sustainable soil and water conservation measures, known as Best Management Practices (BMPs), is an effective approach to mitigating soil loss and nutrient contamination. This study presents an integrated approach to support stakeholder-oriented BMP selection, modelling, and assessment to identify the most effective measures for controlling off-site impacts of soil erosion and nutrient pollution. The Soil and Water Assessment Tool (SWAT) was applied to develop a hydrological model, identify sub-basins with high soil loss and nutrient yield, and simulate different BMPs such as terracing, contour farming, no-tillage, residue management, and their combinations, to evaluate their effectiveness in reducing river sediment and nutrient loads. The study focuses on the Tarquinia plain, Italy, a highly vulnerable agricultural region. Results show that: (1) combined BMPs are most effective, reducing sediment load by up to 33.9 % (from 2627.05 to 1736.72 t y⁻¹); (2) among individual BMPs, terracing performs best, reducing sediment by 22 % (to 2049.24 t y⁻¹); (3) combined BMPs also reduce nutrients, lowering total nitrogen (TN) by 27 % (from 14,018.01 to 10,236.62 kg y⁻¹) and total phosphorus (TP) by 27.5 % (from 108,464.81 to 78,642.18 kg y⁻¹); (4) terracing alone reduces TN and TP by up to 26.2 % and 22.7 %, respectively. The findings enhance understanding of on-site and off-site BMP impacts and provide a replicable methodological framework for other watersheds. The study bridges knowledge gaps by quantifying BMP effects on sediment and nutrient reduction while actively involving stakeholders in co-design and evaluation. By integrating hydrological modelling with participatory approaches, BMP selection and assessment become scientifically sound and socially relevant.