Soilless cultivation suggests a closed system of water flows, the principal of which are (drip) irrigation evaporation and - in more high-tech systems -condensation water. However, in practice growers discharge water during the process of filter cleaning and also actively discharge water due to high levels of sodium or contamination with chemical or biological components. On average in the Dutch greenhouse situation 2-5% of the annual irrigated water is discharged annually. These discharges lead to pollution of surface water with nutrients as well as (residues of) plant protection products (PPPs). This awareness led in 2008 to the start of a working group that aimed to develop a risk evaluation tool for pesticide authorisation in Europe. The evaluation tool consists of a modelled approach for determining expected concentrations in surface water based on a reference scenario per crop i.e., a description of an actual situation including the technical layout of the glasshouse, the climatological year and the receiving ditch For two currently registered PPPs that are used in spray applications, the annual emission to surface water was calculated using different scenarios. As scenarios, substrate-based rose production and pot plant production were selected in combination with water sources of variable sodium concentrations that commonly occur and are used in The Netherlands. Sodium was found to be the only quantifiable argument for growers for active discharge, and therefore it was used as a leading principle for discharge in the water flow model. Water discharge was found to range from 80 to 700 m3 ha 1 year-1, leading to nitrogen emissions of 17 to 149 kg N ha∗ year-1. The emission of PPPs in these scenarios amounted to 0.02-3% of the total active substance that was applied.