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Simulating water balance and evapotranspiration in a subsurface drained clayey agricultural field in high-latitude conditions

Secondary drainage impact of groundwater outflow can affect drainage design and form a pathway for nutrient loading in agricultural areas. Holistic assessment of water balance and all outflow pathways can benefit design of sustainable drainage in a changing climate. In this study, three-dimensional, hydrological FLUSH model was applied to investigate a field-scale data set and to produce a closure of water balance throughout all seasons in a clayey subsurface drained agricultural field in high-latitude conditions. Description of evapotranspiration (ET)-groundwater interactions using a three-dimensional hydrological model provides a new approach for evaluating standard computational methods to estimate ET with limited crop data. Different ET estimates were tested in the context of total water balance, and the coupling of ET and groundwater outflow was assessed. Comparison of measured and simulated water balance components demonstrated that reference ET (Penman–Monteith method) overestimated ET in the cropped field in high latitude conditions. The FAO-56 single crop coefficient approach was also noted to overestimate ET in the studied conditions. A calibrated constant crop coefficient satisfactorily described ET in spring and in autumn, but underestimated it during summer periods. The results suggest that care should be taken when applying standard methods in high-latitude conditions. Groundwater outflow and ET were shown to be interlinked, but even a relatively high potential ET affected the amount of groundwater outflow only slightly. The results demonstrate that groundwater outflow can form an important component of the water balance in clayey subsurface drained fields. The strength of the 3D model was demonstrated in showing how ET had an impact on all outflow components of drained field sections. Such a modelling tool is useful for generating scenarios that show how changes in climate forcing and thereby ET can alter the partitioning of the field-scale water balance.