Impacts of drainage activities on clay soils on hydrology and surface water quality

Several extended field-monitoring programmes were set up in the Netherlands to study hydrological and surface water quality impacts of agricultural drainage activities for clay soils. At the field scale, agricultural land use data and hydrological measurements were combined with analysis of drainage water quality data, and related to the 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 analyse the hydrological behaviour of the systems studied and to quantify solute sources. Field observations were performed by field data analysis and numerical modelling, using the 1D F1W-MultiSWAP code, a 2D dual-porosity code, and the MODFLOW code. Drainage activities showed a significant impact on hydrology by influencing subsurface drainage water flow paths and regional-scale 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 the drainage activities and their surface water quality aspects need to be studied in an integrated way. Changes in subsurface drainage system redesign, surface water level management, and the field-scale nutrient management need to be integrated.