Modeling water balance and effects of different subsurface drainage methods on water outflow components in a clayey agricultural field in boreal conditions

Proper drainage practices to remove excess water are crucial for crop cultivation in the humid climatic conditions of the boreal areas. The objectives of this study were to close the water balance, to quantify the amount of groundwater outflow, to identify the effects of topography on drain discharge, and to determine the effects of different subsurface drain installation methods and spacing of drainage lines on water outflow components in a clayey agricultural field. Hydro-meteorological, soil and topographic data were available from paired field sections in southern Finland including two control sections and two sections where different subsurface drainage methods were applied. A 3D hydrological model (FLUSH) was applied to the whole field area for snow- and frost-free periods in three measurement years to decipher the hydrological effects of the drainage improvements. The simulated field area was 14ha. Simulation results revealed that a steep slope outside of the field decreased drain discharge with 40% and increased groundwater outflow, which was quantified to be a major component of the water balance, approximately 9–15% of the precipitation. The model simulations demonstrated and quantified how drainage improvements in a treatment section affected the hydrology of an adjacent control section. This revealed that the sections shared a hydrological connection through subsurface flow processes. Such connection is typically neglected in the experimental comparison of measurement results from paired field sections. According to the simulations trenchless drain installation changed soil hydraulic properties by decreasing the volumetric fraction of connected soil macropores and by increasing the rate of water exchange between soil matrix and macropores. This affected more the dynamics than the absolute amount of drain discharge. The 3D model was useful in closing the water balance although the limitations were lack of data outside monitored sections and exclusion of snow and frost processes.