Controlled drainage with subirrigation as a water solution for agriculture

Drainage is very common in Dutch agriculture: for example, to make it possible to work fields or ensure that crops do not get too wet, water often needs to be drained away. Drains leading to the ditches alongside fields are a solution here. We have been draining water this way in the Netherlands for decades. But circumstances are changing. With climate change, we are seeing much heavier rainfall and recurring water shortages during dry periods. As a result, we need to drain water at some times but we also want to be able to bring water in at other times – and often precisely when everyone is clamouring for water.

Controlled drainage with subirrigation

In response to this changing problem, new ideas have been developed and implemented over the past decade or so. One answer, for example, could be a smarter drainage system that can be used in more ways. If you have perforated pipes underground that can drain water, you can surely also use them to supply water underground? If you add a collector drain and a control well to your system, you can not only lower the groundwater level or retain water but also raise the groundwater level. By adding water underground, you lose less water through direct evaporation. Some of the water you bring in goes not to the plant but to the replenishment of the deeper groundwater. Subirrigation via drains can also contribute to water availability for crops on the high-lying areas with sandy soils. The image below gives a reasonable idea of how that works.

A review of developments in drainage systems between 1950-2020. Agricultural fields started without drainage (I), followed by the construction of conventional drains (II) and controlled drainage (III), composite drainage regulated with a fixed mechanism (IV-1) or online control (IV-2). The most recent application is composite controlled drainage with subirrigation, in combination with a fixed mechanism or online control (V).

Field trials provide scientific basis

Accordingly, during the past nine years, a range of field trials have been conducted with subirrigation. It was very enjoyable to be involved and see that development. Starting in 2015, a field trial in Haaksbergen investigated the effects of subirrigation on the high-lying areas with sandy soils. That was followed by several projects with field trials such as Boer-Bier-Water, the Lumbricus Programme and the TKI project KLIMAP with field trials in Lieshout (2016), America (2017-present) and Stegeren (2018-2022). These field trials were conducted in a range of locations with different geohydrological characteristics in the high-lying areas with sandy soils. They established an important basis for applied scientific research looking at the operation and effects of controlled drainage with subirrigation from, among others, Gé van den [GS5] [JW6] Eertwegh and Dion van Deijl of KnowH2O and Ruud Bartholomeus and myself, Janine de Wit, of KWR. Developments in the field of drainage are also described in detail in this article. And in the meantime, both the field measurements and the results have appeared in scientific publications. We have learned so much in recent years about the uses of controlled drainage with subirrigation.

Water levels in the monitoring wells are measured both manually and automatically.

The automatic measurements are also checked manually once in a while.t.

Soil structure and geohydrology determine whether controlled drainage with subirrigation has a positive effect

The main conclusion is that controlled drainage with subirrigation can contribute positively to water retention in the soil-water system and to the replenishment of regional water stocks.

• In the growing season, controlled drainage with subirrigation can ensure that the groundwater level does not fall too much so that crops receive adequate water. If the water table stays high enough and the roots go deep enough, more water becomes available to the plants through subirrigation, and transpiration (crop evaporation) and dry matter production increase. 
• The field trials have shown that only a limited part (up to 25% in dry years) of the water brought in for subirrigation goes to the crop. The rest of the water seeps out of the plot into the deeper groundwater or runs off to adjacent ditches depending on water level management and the different characteristics of the area. There were differences in soil structure and geohydrological characteristics between the four field trials, and it was precisely the apparently small differences that determined the effect of subirrigation on the crop and how much water is needed to achieve that effect. Where there is more resistance to seepage, for example because there are layers of loam (which provide resistance) in the sandy subsoil, the infiltrated water is stopped from sinking rapidly into the deeper groundwater. And if the ditch level is kept higher in line with the higher groundwater level, there will be less water loss to the ditch. It is therefore important to adapt the management of the level of surface water so that subirrigation can work properly and to know whether the soil structure and geohydrological characteristics are suitable.

Subirrigation may require a lot of water. During the field trials, we wanted to maintain a high groundwater level continuously during the growing season. That required 500 to 1,000 mm annually. For comparison purposes, that is 75 to 150% of average annual precipitation in the Netherlands. Ongoing follow-up research has already shown that water demand can be reduced by tens of percent by automating incoming water flows and control levels in line with the current moisture conditions in the field and weather forecasts.

Think about water demand, management and maintenance in advance

In addition to geohydrology and soil structure, other important factors should be considered when deciding about controlled drainage with subirrigation:

• Crucially, the amounts of water required can be very high. This depends on geohydrological characteristics such as soil resistivity and shallow groundwater levels (< 2.5 m), and also on, for example, ditch levels in winter and summer and the rooting depth of the crop. In any case: there must be a suitable source of supply for the water required: surface water, groundwater or treated residual water, for example. Whatever the source, the quality of the water is very important and it cannot be used uncritically. For example, treated residual water and surface water often contain contaminants.
• In the field trials conducted so far, the ‘maximum’ amount of water was supplied. We are now working on follow-up studies to determine the effect of smarter water inflows. Can the water supply regime be tailored better over time to the water requirements of the crop? And is optimal water availability for a crop always necessary? Can we not also accept some drought stress in order to reduce the volume of incoming water? How much water do you want to supply as a maximum and how much drought stress can you accept? What ditch level can you maintain to avoid unnecessary losses of incoming water? And again: what are the soil characteristics? I am currently co-writing a scientific paper on this with other researchers. I can share the first results here: significant amounts of incoming water can be saved by actively managing in line with weather forecasts and accepting that crop conditions will be slightly suboptimal. We are going to conduct further research on what happens to water demand if several farmers in an area want to use subirrigation on their land – is there enough water available on the regional scale?
• Management and maintenance also constitute a very important success factor in controlled drainage with subirrigation. Subirrigation requires incoming supplies of water such as surface water, groundwater or treated residual water – sources with that vary widely in terms of composition. That has implications for the design, construction and management of subirrigation drainage systems: if you do not take the water composition into account, the drainage system may get blocked. We are also studying this factor, as can be seen in this report.

A drainage pipe blocked as a result of the precipitation of iron

With this research, we are contributing to a sound, scientific basis for how controlled drainage with subirrigation works and in which conditions it may or may not be an appropriate measure. On the basis of this knowledge, water managers can make better assessments of the different factors involved in their efforts to establish more climate-robust water systems.