Data driven assessment of quantitative status of groundwater in the Netherlands

In the Netherlands, there is no national groundwater monitoring network. The national government has delegated groundwater quantity management to the regional authorities, which perform groundwater head monitoring for this purpose. The regional authorities upload their groundwater head measurements to a national repository that is maintained by TNO Geological Survey of the Netherlands (TNO-GSN).

As part of the Geological Survey task,  TNO-GSN makes available information based on these groundwater head measurements through an online Groundwater head viewer (https://www.grondwatertools.nl/gwsinbeeld). Currently, we are working on an extension of this viewer to show the status of the heads for the entire country. The aim is to give a data driven assessment which is independent of physics based distributed three dimensional groundwater modelling. The main reason is that the use of such modelling depends on successful calibration of a national groundwater model and on inclusion of all relevant processes and changes in boundary conditions (land use, pumping, water management, . . .). A second reason is that an independent assessments helps to improve the existing national model (https://nhi.nu) and better understand the changes in the Dutch groundwater system. For the assessment a limited number of monitoring wells are selected, so that the status can be shown qualitatively by colouring the dots. The main challenge is make a representative selection for:

• Yearly change of groundwater volume.
• Drought or wetness.
• Regional differences.
• The 3 dimensional character of the groundwater system: not only phreatic but also deeper aquifers.
• Assessment of large scale trends caused by e.g. climate change, urbanisation.
• Effectiveness of large scale governance measures (‘water en bodem sturend’).

Monitoring wells in the national database are selected based on the following criteria:

• Being used currently;
• The longer the measurement timeseries the better;
• Preferably multilevel;
• Spatial spread and variation in land use;
• Covering national variation in precipitation and reference evaporation;
• Covering range of response times of transfer-noise models with precipitation and reference evaporation as explaining variables;
• Preferably equipped with telemetry and transmitting data to the national database daily.

The current groundwater head for each piezometer can be characterized in various ways, with and without seasonal correction, such as the percentile of all measurements or all measurements in the same month, or a percentile in the regime curve generated with a 30-year simulation of a timeseries model with precipitation and evaporation.

The resulting selection will be a useful extension of the trends, vertical head differences, dynamics already available on the Groundwater head viewer for national operational water management, groundwater governance and outreach.