Helping growers to improve their own freshwater situation

The DrainStore field trial is located at the Meulwaeter biodynamic farm in Kruiningen. There is a freshwater lens at a depth of 20 metres that could be used to tide the farm over during droughts. For the farm in Zeeland, this is a valuable opportunity that merits investigation: the province is struggling with a difficult freshwater situation. Supplies of fresh water are scarce or non-existent and so growers depend on rainfall. After the dry summer of 2018, Meulwaeter decided to make a change. The farm wanted to know to what extent the underground freshwater lens could be used to improve the freshwater supply and make it more sustainable. This practical question led to DrainStore, which was conducted in collaboration with Meulwaeter, the provincial authority of Zeeland and the Scheldestromen water authority.

Indicative illustration showing the subsurface structure, and the distribution of fresh and saline groundwater, based on the CVES profile A-A’, probe tests (Figures 3-5 and 3-7 in report KWR 2019.071) and well boring records. The wells are not located exactly on the CVES profile A-A’ but they have been positioned at the closest location on this profile for indicative purposes.

Drainage and sub-irrigation

KWR first conducted a theoretical exploration and came up with the recommendation to apply a twin system: a level-controlled drainage system in combination with an underground water storage system with vertical infiltration and extraction wells. In practice, the idea amounted to using the drainage system to collect excess rainwater during the winter on the farm’s own land. This drainage water then infiltrates the subsurface through the vertical wells at a depth of 9 to 22 metres below ground level. During the growing season, this water can be extracted from a depth of 9 to 15 metres below ground level and delivered to the shallow soil through the drainage system (sub-irrigation). The two-part system was installed and it went into operation in 2020.

Dry launch

It was a difficult beginning since the completion of the system coincided with the end of a dry spring. The crops urgently needed irrigation. It was decided to skip the infiltration phase and start with sub-irrigation. But the soil was so dry that a lot of water had to be delivered to raise the groundwater level to the drain level. In the summer of 2020, a total of 42,000 m3 of groundwater was extracted for delivery through sub-irrigation. As a result, it was late before the carrots growing in the field benefited from the better moisture situation in the soil, even though the summer was generally not very dry. The effect of sub-irrigation on yields was not clearly positive. However, the moisture level in the soil on the surface was higher. So the system worked but the benefit for the crop came too late.

Benefits of sub-irrigation

In the winter of 2020/21, it was possible to demonstrate how the drainage system performed. It ‘harvested’ 15,000 m3 of rainwater and infiltrated it to the deeper subsurface, replenishing the fresh groundwater stocks. In net terms, this would not seem to be a particularly sustainable outcome: with 42,000 m3 of water extraction and infiltration from the previous stages, the water stocks as a whole would have been depleted by 27,000 m3. But the situation in practice is different. After the entire cycle, the water stocks had not changed in a measurable way, as was demonstrated by the monitoring of the fresh groundwater stocks around the infiltration and extraction wells. Only a small proportion of the 42,000 m3 of water that was delivered was used by the plants; the rest ended up in the deeper subsurface, replenishing the fresh groundwater stocks. This shows that water circulates primarily between deep and shallow layers, with no net loss. If conventional irrigation had been used, there would have been losses due to high levels of evaporation. This demonstrates the benefits of sub-irrigation.

‘Conceptual representation of the DrainStore concept, in which level-controlled drainage is linked to an aquifer storage and recovery system. A: Situation with infiltration during the wet season, B: situation with abstraction of groundwater and subirrigation during the growing season.’

Higher groundwater level

What lessons have we learned from the years that DrainStore has been used in practice? The conclusion is that the system of drainage and sub-irrigation certainly gives farms the possibility to improve their own freshwater situation. It is important, however, for the moisture levels in the soil to be good as soon as the crop germinates. This means that the groundwater level already has to be at the drain level at that time. The moisture levels in the shallow soil were monitored in the second year of operation (2021). This shows that sub-irrigation allows for a higher groundwater level provided it is applied in a timely and sustained way. Although pumping water obviously costs energy, it does deliver the desired growing conditions in the field. Crop growth may therefore improve as a result.

Well box containing infiltration and extraction well filters, with a piezometer alongside in the red tube.

DrainStore merits a place among other freshwater configurations

The conclusion to emerge from the field trial is that DrainStore certainly merits a place among the other freshwater projects in Zeeland. What emerged in particular was that local conditions such as the subsurface structure, existing freshwater stocks (particularly in the subsurface) and specific grower requirements are important to consider in the decision about which technology should be used. This is all a question of tailored solutions based on sound research and monitoring.

Field impression of the drainage wells and water facility between the carrots on Meulwaeter’s land.