Existing groundwater table (GWT) class maps, available at full coverage for the Netherlands at 1:50,000 scale, no longer satisfy user demands. Groundwater levels have changed due to strong human impact, so the maps are partially outdated. Furthermore, a more dynamic description of groundwater table dynamics representative for the current climate is needed. A mapping method to obtain a large set of parameters describing groundwater table dynamics was developed. The method uses time series analysis and well-timed phreatic head measurements to obtain a data set at point support. This point data set is correlated to groups of exhaustive high-resolution ancillary data by stratified multiple linear regression. Finally, simple kriging is applied to interpolate the residuals of the regression model. The method was applied in a 1,790,000 ha area and its performance was measured in 10,000 and 179,000 ha test areas. The relation between higher sampling density, mapping cost and map quality was explored. Validation results show that reasonable to good quality maps of various aspects of groundwater dynamics can be obtained by this method, at much lower cost than traditional survey-based mapping methods. The method includes the quantification of uncertainty at the actual sampling density and allows the a priori estimation of uncertainty at other sampling densities. Future research aims at identification of the effect of sources of error in ancillary data and how to diminish these. D

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GeoPEARL_NL is used as a higher tier instrument in the leaching assessment of plant protection products in the Netherlands. Because the soil organic matter contents in arable soils in the current version were too high, a new soil organic matter for the Netherlands was needed. The new 3D organic matter map has been generated in two steps; first a trend model was developed based on data at 1210 locations selected using a well-defined stratified random sampling scheme followed by interpolation of the residuals using about 770 000 determinations of organic matter content from the Dutch Soil Information System (BIS-Nederland). In general, the predicted soil organic matter contents in the top layer of arable soils correspond well with those measured. Because preliminary calculations showed higher leaching concentrations using the new GeoPEARL version, the consequences of this new version in combination with the Dutch Decision Tree for leaching to groundwater was investigated. The results of the computations revealed that the PEC90 calculated using GeoPEARL in tier 2 is higher than the PEC80 calculated using FOCUSPEARL in many cases. This inconsistency between the first and second tiers could be remedied by the introduction of a calibration factor.Calibration factors of 5 and 10 are necessary to ensure consistency between these tiers for spring and autumn applications, respectively. Suggestions are given for improvement and justification of the calibration factors in the Decision Tree. It is recommended to systematically compare the predicted leaching concentration in groundwater abstraction areas with that in the Dutch agricultural area as a whole. To improve the reliability of model predictions for these smaller areas, development a more flexible schematisation is needed.Keywords: decision tree, GeoPEARL, groundwater, leaching, national authorisation, plant protection product, pesticide, soil organic matter • Acquisition, duplication and transmission of this publication is permitted with clear acknowledgement of the source.• Acquisition, duplication and transmission is not permitted for commercial purposes and/or monetary gain.• Acquisition, duplication and transmission is not permitted of any parts of this publication for which the copyrights clearly rest with other parties and/or are reserved. Analysis of the data showed that the organic matter content in the top 0.3 m of arable soil profiles in GeoPEARL was 1.5. to 2 times higher than that measured in these soils. This difference was introduced in the schematisation at the time of the development of the schematisation, because the aim was to estimate the average organic matter content in a grid cell, which results from a mixture of grassland and arable soil profiles. The consequence of the error in the organic matter content of arable soils is that the leaching concentrations to groundwater are underestimated. To remedy this, the Working Group on the Dutch Decision Tree for Leaching to Groundwater started to work on the improvement of the soil schematisation for G...