Smart agricultural water management

First winter water goes into the subsurface at Hoeve Lotmeer

The Hoeve Lotmeer farm in Anna Paulowna has a circular water system. The fresh drainage water from the fields now no longer flows away into the ditches; it is stored in the subsurface to be used again later for irrigation. This makes the farm self-sufficient in terms of water and less vulnerable during dry periods.

“It was actually quite tense,” explains Deltares project leader Vince Kaandorp. “We are building an Aquifer Storage and Recovery system here. In other words: storing water in the subsurface for later use. It’s definitely a relatively new concept in arable farming and there were no protocols in place. We first had to look carefully at the depth where the local subsurface is suitable, how to pre-treat the water and what is feasible in terms of quantities of water storage. At a depth of 23 meters, we found a suitable layer of coarse sand. We are very proud to be storing the first water there at a rate of 8 m³ an hour.”

Agricultural alliance 'Zoetwaterboeren'

The circular water system is the result of an alliance between Vertify, Deltares, WUR, Pipelife and arable farmer Klaas Schenk in the ‘Zoetwaterboeren’ project. The complete system consists of adjustable drainage on the fields where fresh drainage water is ‘harvested’, a surface storage facility of 1,000 m³, and a treatment and infiltration system for the underground storage.

Kaandorp: “The unpredictable rainfall is a challenge. By buffering peak rainfall in the fields and in surface storage facilities, we expect to be able to store most of the water in the subsurface during the winter. We will be conducting tests this winter to see whether that works.”

In recent months, Deltares researcher Niels Mulder has been working on the technical details and online monitoring of the treatment and infiltration system: “The process from design to implementation wasn’t just a question of phoning a supplier. This is an innovative system and, by building and testing it, we are learning a lot about practical application. The online monitoring system means that we can now follow the water storage and the effect on the groundwater system in real time”.

Farming at four metres below sea level

Hoeve Lotmeer is located four metres below sea level in a polder consisting of sea clay. The future of food production in these areas is uncertain; increasingly prolonged dry periods mean that brackish seepage is migrating up into the root zone and that less and less fresh water is available for irrigation. Another challenge is the impact of arable farming on water quality in the local area and compliance with the goals of the Water Framework Directive. Collecting and storing drainage water means that fewer nutrients and other substances enter the receiving water system, in this case the Amstel Lake.

Arable farmer Klaas Schenk is now ready for new dry periods with the circular water system and, looking back on the preparations and construction, he is satisfied: “This circular water system is a win-win: it eliminates the need for external fresh water supplies while minimising the impact of the farm on the local area. In this way, we are trying to provide a perspective for the arable farming of the future, where climate change and water quality are major challenges”.

Combining woodchips with iron sand

The Hoeve Lotmeer farm in Anna Paulowna, Noord-Holland, has had the world’s first combined woodchip-iron sand filter since late 2024. Initial monitoring results show that this filter removes virtually all the nitrate and phosphate from the drainage water passing through it. And that is good news.

The treatment of drainage water is an interesting measure for intensive crops where, even when nutrient utilisation is maximised, there are still losses to surface water. Connecting a newly constructed drainage system to a buried filter filled with woodchips and iron sand minimises the burden on the receiving surface water. The woodchips in a filter of this kind stimulate nitrate breakdown by the bacteria that occur naturally in groundwater. Iron sand, a residual product from drinking water preparation, is known to effectively capture and sequester phosphate from drainage water.

Deltares expert Stefan Jansen is happy with the initial trials: “Preliminary research showed that both nitrate and phosphate were flowing into the surface water through the drains on this farm. That gave us the idea of combining woodchips and iron sand.”

Additional benefits of the woodchips and iron sand filter

Combining woodchips with iron sand delivers additional benefits. Jansen: “In other tests with woodchip filters, we saw that biological activity was stimulated so much that sulphate was broken down as well as nitrate. The process led to the formation of harmful sulphide (the gas that smells of rotten eggs). The buffering effect of the iron sand mitigates this side-effect considerably. We also saw phosphate being released from woodchips during the very first flushing in previous trials. That phosphate is sequestered immediately in a woodchip-iron sand filter”.

The first tests with the woodchip-iron sand filter were in January and February 2025. For more than a month, virtually all the leaching nitrate and phosphate was removed from the drainage water passing through the filter. Then the drainage flow stopped early in the relatively dry spring. The filter is ready to treat the new drainage water in the winter of 2025 and 2026.

Future applications

Once the drainage flow resumes, researchers will continue to monitor the woodchip-iron sand filter to learn more about how it performs in the long term.

Jansen: ”We are learning more and more about how and where we can best use these methods in the Netherlands. For the cultivation of flower bulbs, potatoes, onions and many types of vegetables, for example, it is difficult to prevent nitrate leaching completely. A woodchip-iron sand filter is a low-cost, low-tech measure here. The filter uses little space and it protects the receiving surface water, helping to meet the goals of the Water Framework Directive.”