The Geological Survey of the Netherlands (TNO-GSN) maintains a public national database of groundwater head observations (http://www.dinoloket.nl). These measurements contain valuable information about the groundwater systems. All groundwater head time series have been simulated using transfer-noise modelling with precipitation and evaporation as explanatory variables. The impulse response function (IRF) for precipitation has been used as a system signature. The individual time series models are available online with interactive graphics (https://www.grondwatertools.nl/grondwatertools-viewer).
The total response (area underneath the IRF) and the average response time have been mapped for the major aquifers. Thereby, the lateral consistency has been checked. Also, the vertical consistency has been checked for multi-piezometer wells. The application of model evaluation criteria and the consistency checks on the modelled precipitation IRFs allowed us to select piezometers with reliable time series models.
This selection offers a far more homogeneous dataset of piezometric head informations than the original measurements. This means it can provide more reliable insight into the groundwater systems. We used the selection to create maps showing various aspects:
1. Response time of precipitation response
2. Yearly fluctuation - regime curve
3. Long term trends
The average response time in the precipitation IRF is a major characteristic of the groundwater system providing insight in the boundary conditions and hydrogeological situation. Also, the response time is important for monitoring and management activities.
The yearly fluctuation of the groundwater head has practical implications for land use and utilization of the subsurface. This fluctuation can be visualized for a piezometer using the regime curve, showing the average fluctuation over the year. Stochastic simulations using the time series models provide reliability bands around the regime curve, so that it can be put into perspective together with the deviations that may be expected. The yearly fluctuation has been mapped per major aquifer.
The long term trends due to anthropogenic influences are a major input for water (resources) management. The reliable time series models separate the influence of precipitation and evaporation from the other influences on the groundwater heads. The latter show up in the unexplained part (residuals) of the models. We assumed these are mostly anthropogenic influences. We applied a trend analysis to the resisuals and mapped the resulting trends per major aquifer to show the occurrence of trends in the groundwater systems in three dimensions.
Based on head measurements from the Dutch national database with subsurface information, we provided insight in the groundwater dynamics of the Netherlands by means of maps covering various aspects. This is useful information for groundwater management and subsurface planning.