How (Managed) Drainage Shapes Regional Hydrology: Model Based Insights for Flanders

Abstract. Artificial drainage is widely used in agricultural landscapes to improve trafficability and prevent waterlogging, but comes at the expense of groundwater storage and regional drought resilience. Through model scenarios, this study evaluates how large-scale drain-level adjustments influence groundwater dynamics, water balances, and streamflow extremes across different regions in Flanders, Belgium. A physically based, spatially distributed hydrological model coupling land-surface and groundwater processes was adapted to represent controlled drainage through adjustable weirs, by modifying drain levels and incorporating temporary water storage within drainage ditches behind control structures. Permanently raising drain levels from 1 m to 0.5 m below the surface increased average groundwater levels by 15 cm across Flanders, with increases exceeding 30 cm in actively drained areas. Higher groundwater availability generally resulted in modest increases in evapotranspiration, particularly in flat sandy regions. Simulations showed that raising drain levels shortly after winter (April) was more effective than implementing measures only during summer (June onward), when groundwater tables had often already fallen below the drainage depth. Drainage management also altered streamflow extremes. Under a permanent drain-level increase, low flows increased by 12.8 %. In contrast, summer-only management reduced low flows by up to 10.1 % in some drainage-dominated catchments, highlighting a trade-off between local water retention and downstream water availability. The effects on flood peaks were spatially variable. Peak discharges increased by up to 11.3 % in saturation-prone areas due to reduced unsaturated soil storage, whereas temporary storage behind control structures reduced peak flows by up to 26.5 % in regions dominated by infiltration-excess runoff. The results demonstrate that drainage management can be an effective tool for increasing groundwater storage and mitigating drought impact. Nevertheless, its impact on flood risk and downstream flow regimes needs to be assessed carefully, since it depends strongly on local hydrogeological conditions and management timing. Depending on the location, the effect can exacerbate or mitigate flooding risk due to peak flows. These findings highlight the need for spatially targeted and adaptive drainage-management strategies to support climate-resilient water management in heavily modified agricultural landscapes.