Hydrological consequences of controlled drainage with subirrigation
Authors: de Wit J.A. (Janine) , van Huijgevoort M.H.J. (Marjolein) , van Dam J.C. (Jos) , van den Eertwegh G.A.P.H. (Gé) , van Deijl D. (Dion) , Bartholomeus R.P. (Ruud) 
Published: February 2023

Sufficient freshwater is needed for several water dependent sectors. However, e.g. climate change, weather extremes, economic growth, urbanization and increased food production make it more complex to guarantee sufficient freshwater for all sectors, even in temperate climates like the Netherlands. The range of weather extremes from extremely dry to extremely wet is expected to increase and to occur more frequently. However, the current Dutch water management system is not designed to anticipate both weather extremes.Controlled drainage with subirrigation could be a viable measure to i) discharge water only when needed, ii) retain water and iii) recharge water using an external source. This system thus has the potential to 1) improve growing conditions for crops at field scale, 2) reduce peak discharges at regional scale, and 3) increase groundwater recharge on regional scale. Consequently, this system could anticipate both dry and wet extremes. However, the implementation of controlled drainage with subirrigation could significantly alter different water balance components.We show data and model output of five experimental sites where controlled drainage with subirrigation is applied. Field data were collected over the years 2017-2022, like external water supply, groundwater table and soil moisture content. Other water balance components, crop yield and configuration of the management of the system were modelled with SWAP (Soil-Water-Atmosphere-Plant model), using observations for calibration purposes.Results show that by subirrigation, water can be applied to the soil and will lead to increased water storage and higher groundwater tables. Groundwater tables were up to 0.7 m higher during the growing season, leading to both increased crop yields and larger groundwater recharge. Drought vulnerability decreased at the test sites. However, the water supply for subirrigation can be high (500 mm per year, on average). Additionally, effects of subirrigation on the water balance components are strongly site-dependent. For example, a resistant layer below the drainage/infiltration pipes is needed to ensure enough resistance to limit downward seepage and to raise the phreatic groundwater level. Furthermore, ditch levels surrounding agricultural fields need to be adjusted to the raised groundwater levels, as too deep ditch water levels result in (unfavorable) drainage and loss of water. Field experiments also show that proper management is important to prevent clogging of the drainage systems.Construction, topographical location, external water source and proper management are important for subirrigation to be successful. Responsible implementation of subirrigation in terms of the water balance at the regional scale is needed; freshwater availability to apply subirrigation is an issue. When these boundary conditions are met, controlled drainage with subirrigation could raise the groundwater level and improve the soil moisture availability for crops, while still having the option to discharge water when needed.