Mika Turunen

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.

Development and application of a solute transport model to describe field-scale nitrogen processes during autumn rains

A new generic, three-dimensional, solute transport component was developed into FLUSH, which is a hydrological model developed for Nordic conditions. Water flow and solute transport descriptions in FLUSH follow the dual-permeability concept, which divides the total soil pore space into mobile soil matrix and macropore systems. The solute transport model was parameterized to simulate the main processes of nitrogen (N) cycle in clayey, subsurface-drained soils during autumn periods after the harvest. The model simulates transport of nitrate and ammonium N, as well as mineralization, nitrification, and denitrification. Reactions in soil are affected by temperature and moisture, as simulated by FLUSH. Ammonium can adsorb on soil particles in both pore systems, while organic N is described in simulations as an immobile solute in the soil matrix. One-dimensional version of the model was applied to two subdrained field sections (1.3 and 3.4 ha) in the Nummela experimental field in southern Finland during two autumn periods (2008 and 2011). The model was able to replicate the measured dynamics of nitrate N concentrations in drain discharge during both the periods. Concentrations were the most dependent on drain discharge dynamics and the rate of nitrification. Measured and simulated ammonium concentrations in drain discharge were about 10 times smaller than nitrate concentrations, even though the levels of N input with initial values and deposition for both inorganic fractions were similar. Successful solute transport simulation results further increase the confidence in the description of the water flow processes in FLUSH.