Janine de Wit’s PhD research contributes to solutions for freshwater availability

Drainage systems were widely installed in the Netherlands after the Second World War. These networks of pipes, located at a depth of around one metre below ground level, discharge excess water so that farmers can work their land; an effective measure for increasing food production. In her dissertation ‘Managing Water Excess and Deficit in Agriculture: Subsurface Drainage and Irrigation Systems‘, De Wit explores the question of whether these existing drainage systems can also be used for a very different purpose: retaining water for longer periods through controlled drainage and replenishing groundwater through subirrigation. To what extent can this contribute to climate-resilient and future-proof freshwater availability?

Janine de Wit’s defence at Wageningen University. Photography: Niko Wanders

The subsurface plays a key role

Using field trials at four different locations on the elevated sandy soils of eastern and southern Netherlands, De Wit investigated the hydrological opportunities and limitations of controlled drainage with subirrigation. This work fitted well, for example, within the KLIMAP-programma, which focuses on the climate-adaptive design of these areas. “ The subsurface plays a very important role” De Wit explains. “In America, a location in Limburg, there is a loam layer in the subsurface. This creates resistance, so that during infiltration the soil retains the water for longer. The groundwater level remains sufficiently high, making the water available to plants. With subirrigation, or ‘underground irrigation’, crop production there increased by around 15 to 20 percent in dry years. That is a very positive result. But in Stegeren, which is located in Overijssel, the soil is essentially one large sandpit. There, around 80 percent of the water introduced into the soil drains directly away into the deeper groundwater system. In itself, that does not have to be a poor result if the aim of the measure is groundwater recharge. But it has little effect in terms of improving water supply for crops.”

Substantial water demand

Another important outcome of De Wit’s research provides insight into the water use associated with the measure. On an annual basis, a drainage system for subirrigation may require as much as 500 to 1,000 millimetres of water at some locations, although De Wit’s research shows that this can be reduced through smarter control. Compared with the amount of precipitation in the Netherlands — around 800 millimetres per year — this remains a substantial volume. De Wit: “At local scale and with the right soil composition, controlled drainage with subirrigation could be a good intervention. But when this is scaled up to a regional level, there is often insufficient water available. That is why smart control of subirrigation is needed. Together with KnowH2O, we developed a prototype that uses weather forecasts and field conditions to indicate how much water is needed and at what level the system should be set. For farmers, this does mean additional technology and somewhat more work. You have to allow the water level in the control chamber to fluctuate, rather than setting it once and leaving it unchanged. But it does save a great deal of water, which makes the measure more sustainable.”

No holy grail for water authorities

De Wit indicates that the technique of controlled drainage with subirrigation is highly valuable, but only under the right conditions. “For water authorities, I think converting drainage into controlled drainage is a very good step. But the step towards subirrigation requires careful consideration: where should it be applied? And how can it be implemented in practice? Various functions need water, not only agriculture, but also nature, as well as citizens and industry. When I sit down with water authorities, I will not say that controlled drainage with subirrigation is suddenly the silver bullet. In addition to the water demand I mentioned earlier, some of the water introduced into the soil also drains into the ditch. That is undesirable and requires water authorities to adjust ditch water levels accordingly. They therefore need to be aware of what this means for their policies.”

A climate-resilient agricultural and water system

De Wit has also considered the application of this measure for farmers. She grew up in a farming family and can readily empathise with their position. This is evident even from the ease and perseverance with which she moves through the polder landscape. Armed with a spade and advanced measuring equipment, she collects the required data in all weather conditions. And when a mouse has gnawed through a sensor cable, leaving her computer screen blank, she does not let it discourage her. “At home, we talked about the weather every day,” she says. “And I always wanted to know how everything worked. With the changing climate, I realise that a transition is needed. We have to think in terms of both agriculture and nature. Together with my colleagues at KWR, I have investigated controlled drainage with subirrigation in depth. We can now provide sound advice on this piece of the puzzle and on how it can contribute to a climate-resilient agricultural and water system.”