Freshwater supplies on Terschelling

The island of Terschelling is entirely surrounded by seawater and it is concerned about its freshwater supply. Will there continue to be enough fresh water for all the local people and, if so, how can you structure the water system and land use to be water-resilient? Deltares is working with other organisations to study how the island can guard itself against the effects of climate change.

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Terschelling: view on a special field trial

Coastal areas and islands face salinisation and freshwater shortages, with consequences for users including agricultural producers and nature. On the island of Terschelling measuring, modelling, and field trials aim to seek future prospects.




The current water system of Terschelling













Climate change

Dutch summers are getting drier and warmer, making freshwater shortages more frequent. At the same time, winters are getting wetter, making it necessary to think about flood prevention.

Too much and/or too little water

With drier summers and wetter winters, measures must seek a balance between water retention and water discharge. This means adapting the water system.

Drinking water demand

Current drinking water resources are under pressure. In the future, Terschelling may have to be self-sufficient for drinking water. This requires having new water resources.

Sea level rise

Sea level in the Netherlands is expected to rise between 30 centimetres to 1.2 metres by 2100. Terschelling is surrounded by sea so this will have a major effect on the groundwater system.


Brackish and salty water occurs in the subsoil and can flow towards the surface via seepage. We expect this to occur in more places in the future.


As one of the Wadden Islands, Terschelling is a popular holiday spot. Holiday parks, campsites, and hotels generate income for the island. The beach and nature are always close by.


Governments ultimately take decisions on the layout of the landscape and water system. What can go where? Ultimately, various interests have to be balanced. Knowledge is the basis here.


Terschelling's dunes are a protected nature reserve, but its polders are also a high-quality agricultural nature landscape, especially for meadow birds.


Terschelling’s main agricultural activity is dairy farming, and by extensoin maintaining grassland. Agricultural nature management is an important aspect to this.

Water extraction

On Terschelling, a portion of the drinking water is extracted from the freshwater lens beneath the dunes. Additionally, drinking water is supplied via a pipeline from the mainland.


We want to retain as much (fresh) water as possible on the island. Sometimes there is too much water, resulting in (groundwater) flooding. At this location, a campsite stays dry by means of underdrainage, which controls the water level. We measure the overall effect on groundwater levels.


Water extraction in the dunes

If Terschelling's water supply needs to become independent of the mainland in the future, extra sources will be needed. Together with the water company, we are investigating the what if and how to achieve a win-win. Can we prevent flooding by using excess water in the dune edge for drinking water production?


Revenue models for farmers

As a future perspective, farmers on Terschelling must have a robust revenue model. Wageningen UR is investigating the possibilities within the current and future water system.


Monitoring biodiversity: meadow birds and insects

Van Hall (Larenstein University of Applied Sciences), together with the local Bird Watch and the Society for the Protection of Birds, measures the quantity of meadow birds and chicks. The quantity and size of insects are of significance as an important food source.


Puddle trials

Puddle sites are set up for waders and meadow birds, using a solar pump to create wet spots in a field. We monitor the effect on biodiversity (insects and birds) and the water system (groundwater levels and fresh and salt water in the subsoil).


Saline cultivation

If the soil becomes to saline, not all vegetables can be grown. The Salty Taste Foundation tests saline crops such as samphire and sea aster. These products are already being used in restaurants on Terschelling.


Groundwater model

Using computational models, we simulate the island's water system. Subsequently we predict how the water system will change in the future due to sea level rise and climate change.


Water level rise

The water level has been kept low at this location. Together with farmers, we are exploring whether the water system layout can be altered and water levels can be raised. This would mean fewer problems with drought and salt. We can thus retain fresh water for longer and keep salt out of the root zone.


Monitoring (ground) water levels

Throughout the polder we monitor water levels, groundwater levels, and water salinity. We gain more insight into the functioning of the water system, determine the effects of the field tests and are improving the island's groundwater model.


Flood blocks

In two areas, the ditch level around several plots is raised with weirs and pumps. By retaining fresh water for longer, we try to prevent salinisation and at the same time are create favourable conditions for nature. We monitor this via a measuring network on both a polder and plot scale.


Grass yield

Having grass plots is important for feeding the cows on Terschelling. Together with the alterations to the water system, we want to maintain or even improve the grass yield. We gain insight by cooperating and measuring with farmers.


Grass trials

A grass breeder tests which types of grass grow best under dry and saline conditions. They do this by testing different strips per species. We take measurements to determine how dry and salty the subsoil is.


How do you remain as independent from the mainland as possible? This is a question that has occupied the people of Terschelling for centuries. According to Durk Holwerda of the province of Fryslân – himself a Frisian – this is part of the islanders’ nature. ‘They want to be self-sufficient and not be dependent on the mainland. Their isolated location means that the islanders want to manage their own affairs.’

But the island is still not entirely self-sufficient and it depends on a drinking water pipeline from the mainland. Although Terschelling does have its own freshwater lens below the dunes, climate change is threatening the island’s freshwater supply. The freshwater lens floats on salt water and, as sea levels rise, the salt water follows, with all the inevitable impact on residents, agriculture and tourism. Dry summers lead to shortages of fresh water, farmers suffer from wet, brackish fields and campsites are flooded more often.

Durk Holwerda

Area coordinator for the Frisian Wadden Islands

And so the province of Fryslân decided to launch a major research project to see how the island can guard against the effects of climate change. Holwerda: ‘The provincial authority wanted to draw up an integrated water plan. This fitted in nicely with the project of Deltares, Van Hall Larenstein and WUR, leading to a win-win situation. The preparations were tense because this is quite a large project for a long period of time: four to five years. On the one hand, we had to secure financing, a phase in which people involved at the outset can drop out, and that required a lot of time, energy and stamina from everyone. But we managed to get everyone involved together and to make sound financing arrangements.’

Living Lab

Deltares, Van Hall Larenstein and WUR are contributing the knowledge, including the Delta Technology top sector, and AgriFood, the Wadden Fund and the Provincial Authority of Fryslân are providing the funding in collaboration with island partners. The approach used for the project is unusual. Virtually all the organisations involved with water on the island will play an important role. Holwerda: ‘Normally, we as the provincial authority write down what we want to achieve and ask the contractors to work out the details. That wasn’t the case this time round. We are ourselves part of a bigger project with broader support, the Living Lab, in which we are looking together at problems and solutions on Terschelling associated with climate change. In addition to the knowledge institutes and government authorities, campsite owners, farmers and bird spotters have seats at the table. Theory, practice and sensitivities meet here. We are moving out of our comfort zone and looking for solutions together. It’s a more playful and independent approach, and it is working well.’

Testing ground for islands

Mindert de Vries is one of the initiators of the freshwater project on Terschelling. He works for Deltares as an integrated coastal management specialist and as a senior researcher at Van Hall Larenstein.

‘I launched this project four years ago after a series of LTO meetings where I spoke about how climate change...

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Insects as an indicator

To the south of Midsland near the Wadden dike on Terschelling, Luuk Bruinier, a student at Van Hall Larenstein University of Applied Sciences, is studying the island’s insects. They are a good indicator for the habitat...

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Climate model

Field trials are currently taking place, and groundwater levels and salinisation are being monitored. ‘One of the province’s goals is to get schoolchildren and students involved with the Wadden area. Van Hall Larenstein students are conducting research on the island.’ Deltares has now developed a climate model that provides a picture of the effects of climate change. Terschelling is acting as a kind of testing ground in this regard because the results will also be of interest to the other islands. ‘Vlieland and Schiermonnikoog are still entirely self-sufficient in terms of water and they want to keep it that way. They face similar problems and want to draw on Terschelling’s experience for an integrated water system that is future-resilient.’

Vince Kaandorp

Deltares groundwater expert