Soil salinity and sodicity are among the oldest soil and groundwater pollution problems and are widespread across the globe. Where salinity affects crop water uptake and yield, sodicity may additionally cause poorly reversible soil structure degradation and a severely reduced hydraulic conductivity. We use the model HYDRUS-1D to simulate sodicity development in soils with shallow, Na-rich groundwater under a normal weather regime with distinct dry seasons. Attention is given to the impact of a sudden fresh water input on the formation of a sodic layer. The complex interplay between soil chemistry, soil physics, soil mechanics (as far as swell-shrink behavior is concerned), and fluctuating atmospheric conditions results in a remarkably regular relation between depth, location, and severity of a sodic layer that forms within the soil as a function of rainfall intensity. A threshold behavior is observed: sodic layer formation is absent at rainfall intensities below this threshold, whereas sodic layer thickness and hydraulic conductivity reduction increase rapidly with intensities exceeding this threshold. This is the case even for different soil types and groundwater depths. Field observations agree with our simulations: the properties of the layer with sodicity-induced structure degradation are more strongly developed, as this layer is situated at a shallower depth. The implementation of hydraulic conductivity reduction as a function of exchangeable Na percentage and ionic strength in HYDRUS-1D can be improved towards a smooth reduction function, changing soil physical parameters due to swelling and dispersion of clay and reconsideration of the reversibility of sodicity development.

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Artificial drainage systems are used worldwide to remove excess water or to ascertain sufficient leaching of salts in irrigated crop lands in (semi-)arid regions (van der Molen et al., 2007). Focusing on temperate climates, considerable areas of agricultural land (roughly 10% of the rain fed agriculture) are drained via surface or subsurface measures (Smedema et al., 2004). An often used method is subsurface installation of corrugated tile drains, for example in the US and Europe (e.g., Schilling et al., 2015;Sloan et al., 2016). These drainage systems lower the field scale groundwater level, which ensures sufficient oxygen supply to crop roots, trafficability and many other benefits for crop growth optimization (Skaggs, Fausey, & Evans, 2012;Sloan et al., 2016). Agricultural production has benefitted greatly from improved drainage (Smedema et al., 2004). However, the focus regarding tile drainage in the temperate zone has also shifted toward its negative side effects such as eutrophication of surface water with N and P due to shorter groundwater travel times, changing hydrological behavior on a catchment scale and changing (aquatic) ecosystems (Ross et al., 2016;Schilling et al., 2015;Sloan et al., 2016). Also over-drainage frequently occurs, if too much water is drained from the soil during wet periods. Soil water storage is then depleted much faster during the following growing season, which results in crop water shortage (Smedema et al., 2004). The problem of over-drainage will likely become more prominent in some of the mid-latitude regions with the tendency toward drier summers, in combination with more extreme and irregular precipitation events (Rowell, 2009;Spinoni et al., 2018).Changes to the drainage system can be adopted to limit adverse effects of artificial drainage. Instead of employing a fixed drainage base, which for a regular drainage system is the drain installation depth, the drainage base could be allowed to fluctuate by placing control structures at drainage system outlets. This allows to choose for a reduction or complete cessation of water and nutrient discharge via drains toward surface waters. This practice is referred to as controlled drainage (CD) or drain water management (DWM) (Ross et al., 2016;Skaggs, Fausey, & Evans, 2012), as opposed to free flowing, regular drainage (RD). Controlled drainage is increasingly being implemented in the US and Europe (Ross et al., 2016). An overview by Ross et al. (2016) shows that total drain water outflow is on average reduced by nearly 50% using controlled drainage (based on 17 studies in the US) as compared to regular drainage. This is in agreement with values mentioned by Skaggs, Fausey, and Evans (2012), that range from 18% to 85% reduction, as well as with values reported in several other studies (e.g.,

<p>Rewetting of drained peatlands is a proposed measure to reduce greenhouse gas (GHG) emissions. Worldwide, drained peatlands are responsible for 9–15 % of the total GHG emission and reducing these emissions therefore has a large potential to combat climate warming. In the Netherlands, almost all peatlands are drained and 85% are in agricultural use. The Dutch government has set the aim to reduce the yearly emission from peatlands with 1 Mton by 2030. Different measures are proposed to achieve this goal. There is, however, insufficient data to determine the magnitude of GHG emissions from Dutch peatlands and to validate the effects of mitigation measures. Therefore, in 2019, the National Research Program on Greenhouse Gas Emissions from Peatlands (NOBV) was initiated. In this program we use transparent automated flux chambers, eddy covariance and aircraft measurements, combined with a network of groundwater, soil and meteorological sensors, to perform long-term unattended measurements of soil-atmosphere GHG fluxes and relevant environmental variables on different dairy farms in the Netherlands. We aim to quantify emission magnitudes and monitor the effects of elevated summer water tables (using subsoil irrigation as mitigation measure) as well as develop models that predict GHG emissions and the effects of rewetting measures on a national scale.</p><p>In this presentation we will show the CO<sub>2</sub> flux results of the first two monitoring years of five drained peatlands. We will present the effects of elevating groundwater levels during the summer period with subsoil irrigation and discuss the differences between sites and years. In the wet year (2021) the mitigation effect was much less than in the dry year (2020), in some cases even negative, and mitigation effects strongly varied among locations. Aggregating data from all 5 sites shows that soil temperature and water table depth are important predictors for ecosystem respiration. However, overall, CO<sub>2</sub> fluxes did not show a clear relationship with water table depth after controlling for temperature. Only a water table depth < -20 cm showed clear potential for emission reduction.</p>