GEM model for soilless cultures: review of process descriptions and suggestions for improvement

Samenvatting

The GEM model developed for soilless cultures consists of different submodels (A) for applications to crops grown on mats by drip irrigation, (B) for spray applications to crops grown on such mats, and (C) is for spray applications to crops grown in pots in an ebb/flood system (GEM-A, GEM-B, and GEM-C). The descriptions of the processes for pesticide behaviour in these submodels were reviewed, considering also their consistency with measurements available in the literature. For GEM-A it is recommended to include sorption to the mats, the foil surrounding the mats and the irrigation pipes and to include partitioning between the water in the mats and the plant roots. For GEM-B it is recommended to include direct contamination of the substrate mats and the troughs resulting from spray and Low Volume Mister (LVM) applications. For GEM-B and GEM-C it is recommended (i) to revise the procedures for calculating the initial concentrations in the air and the condensation water, (ii) to include deposition onto the roof by spray and LVM applications, (iii) to revise the procedure for calculating the volatilisation rates from the plant surfaces. For GEM-C it is recommended (i) to omit the sorption equilibration between the bottom 10 cm of the soil in the pots and the water on the ebb/flood tables, (ii) to revise the procedure for the flux in the gas phase between the greenhouse air and the top layer of the soil in the pots, and (iii) to use a crop-specific value for the fraction of the surface area covered by the pots.