How did the increasing variability of precipitation and evaporation over the past 50 years propagate to groundwater heads in the Netherlands?

In the Netherlands, precipitation minus reference evaporation (Makkink) is a good indicator for groundwater recharge. In the following, this difference is referred to as potential recharge. The climate change since 1940 has led to systematic changes in precipitation and in evaporation. However, the 30-year running average of the potential recharge does not show a continuous trend over this period, but the 30-year running standard deviation steadily increases for aggregation intervals shorter than a year. This suggests that the variability of the groundwater heads may have increased also.

Analysis of a set of long timeseries of groundwater head in the Netherlands does not show such an increase in variability. In order to determine whether this is due to the properties of the groundwater system or due to other (anthropogenic) influences, simulations of the groundwater heads have been carried out using timeseries modelling with a timeseries of the potential recharge and transfer functions covering the range of responses determined in the Dutch national Groundwater head viewer. The simulations show that an increase of the variability of the groundwater heads due to the increased variability of the potential recharge is only to be expected for short response times. However, comparison of the aggregated trends of the groundwater heads with the trends of the potential recharge indicates that there is a strong anthropogenic impact besides the influence of the climate (change). Therefore, long term assessment of (geo)hydrological systems has to include land-use changes, groundwater abstraction and other anthropogenic influences.