Integrating Controlled Drainage and Sub-Irrigation demand: Insights from FLUSH simulations in a Nordic agricultural field

Nordic agricultural drainage systems have traditionally been designed for the removal of excess water from spring snowmelt and autumn rainfall; however, as the crops grow, increased evapotranspiration coupled with typical low precipitation in the early growing season can result in moisture shortages. Controlled drainage (CD) offers a more adaptive solution by regulating outflow and maintaining higher water table depths (WTDs), thereby enhancing soil moisture retention and reducing nutrient losses. While the hydrological benefits of CD are well established, its potential to quantify sub-irrigation, the additional water required to maintain optimal root-zone moisture, remains insufficiently explored. In this study, we applied FLUSH, a process-based two-dimensional hydrological model, to assess whether controlled subsurface drainage systems can be used to estimate sub-irrigation demand in a flat agricultural field in northern Finland. Three water-management scenarios were simulated: conventional drainage (CV), CD, and controlled subsurface drainage with sub-irrigation (CD-SI). Multi-year simulations were used to evaluate WTD dynamics, drain discharge, groundwater outflow, and upward water movement under different scenarios. Model results show that elevating drainage control levels increases water retention and can generate upward flow from drains during dry periods, partially meeting crop water demand. Scenario comparisons confirm that the CD-SI (sub-irrigation) scenario introduces a consistent subsurface inflow, while CV and CD present minimal upward fluxes. Evapotranspiration patterns are primarily climate-driven, with only moderate increases under CD and CD-SI due to improved soil moisture availability. Both controlled and sub-irrigated systems reduce cumulative and daily drain discharge, indicating enhanced infiltration and storage within the root zone. These findings also show that CD systems with continuous monitoring can provide valuable information for estimating sub-irrigation demand, especially in soils with high microporosity and hydraulic conductivity. Overall, the study highlights the potential of integrated drainage and sub-irrigation strategies to support climate-responsive water management in Nordic agriculture.