Over the last decade, several extended field monitoring programs were set up in the Netherlands to study hydrological and surface water quality impacts of agricultural drainage activities on clayey soils. At the field scale, agricultural land use data and hydrological measurements were combined with analysis of drainage water quality data and corelated with loading of the surface water system with salts and nutrients. Water, salt, and nutrient balances of the soil and/or surface water system were set up to analyze the hydrological behavior of the systems studied and to quantify solute sources. Field observations were performed by field data analysis and numerical modeling, using FIW MultiSWAP, a 2D dual-porosity code, and the MODFLOW code. Drainage activities displayed a significant impact on hydrology by influencing subsurface drainage water flow paths and regionalscale groundwater seepage. Through solute balances it was shown that the salt and nutrient loading of the surface water were clearly affected by both agricultural and drainage activities. The conclusion was that that the drainage activities and its surface water quality aspects need to be studied in an integrated manner. Also, several measures need to be taken to significantly reduce the losses of salt and nutrients from the clay soils to the surface water by drainage. Changes in subsurface drainage system, surface water level management, and the field-scale nutrient management need to be integrated.
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