Subsurface drainage is the primary water management approach in field cultivation in Nordic areas. Installing and improving a subsurface drainage system change water flow dynamics and routes in the soil, which affect nitrogen (N) load from the field. The role of soil properties, drainage system improvements and surrounding areas in the formation of water flow routes is not fully understood. The objective of this study was to quantify the effects of subsurface drainage on water flow and N transport using field monitoring data, statistical analysis and mathematical modeling. The performance of two drainage installation methods applied in the Sievi experimental field were investigated with statistical analysis. Differences in groundwater level occurred due to drainage installation method and soil type at the drain depth, but the absolute differences were small (0.1 m). A state-of-the-art process-based hydrological model was applied to investigate the effects of soil properties and drainage systems on water flow routes. Field subsurface drainage schemes were simulated with 3D, 2D and 1D model applications using data from a clayey field in southern Finland (Nummela). Model applications showed how field drainage can be described with models of different dimensions and scale (from drain spacing to field section scale). The 3D drain spacing simulations demonstrated the benefits of using detailed soil data in model parameterization as an alternative to model calibration. In the 2D long-term simulations, the 3D soil parameterization was up-scaled to field section scale. The short-term 3D model simulations showed the dominant nature of soil macropores over the drainage system description. Comparison of the long-term 2D model simulations revealed that the improved drainage installations changed the shares of all the water flow routes, including groundwater outflow. A generic solute transport component was developed and tailored to describe N cycle transport and processes in 1D model simulations. Autumn period simulations of a poorly and well drained field sections showed that nitrate N loading was mainly controlled by the initial soil N storages after harvest and the timing of the precipitation events, while the soil moisture content differences explained the magnitudes of gaseous N losses. Long-term monitoring data series, statistical analysis and process-based modeling showed that the practical effects of subsurface drainage are site specific and comprehensive view on the local water and nutrient management is needed when controlling the environmental impacts of field cultivation. The 1D, 2D and 3D model applications could all be used to replicate the measured drain discharge data, even though the drainage system description differed between the cases due to the differences in water flow directions and the boundaries of the simulated domain. The finding of the research suggests that field water management moves the N load from one path to another rather than affecting to the total amount of the water volume or N loading.
|Publication status||Published - 2019|
|MoE publication type||G5 Doctoral dissertation (article)|
- process-based modeling
- statistical analysis
- drainage installation
- drain trench
- envelope material