| Summary: | A set of 10 representative multi-decade long time series of piezometric levels of phreatic aquifers were selected in different parts of Low Belgium to investigate correlations between groundwater levels, precipitation rates and the North Atlantic Oscillation (NAO) index. Correlations between piezometric levels and precipitation rates are always high and significant at the 5% level for all series. A direct relation between groundwater levels and the NAO is only possible if there is a correlation with aquifer recharge, which in north Belgium is limited to the winter period. Correlation between monthly precipitation and monthly NAO indices are highly temporal, with two distinct periods during which the correlation is significant at the 5% level. In summer (July and August) there is a negative correlation, in winter time (December and January) a positive one. As in summer aquifer recharge is negligible, only the winter NAO index can have an impact on groundwater levels. Of the ten investigated series, only two have a significant correlation at the 5% level, between piezometric levels and the December-January NAO values. Both wells lie in the same region, in the upper part of the lithologically heterogeneous Campine Complex. The local hydrogeological conditions here seem to increase sensitivity to winter NAO modus.
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