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Consequently, some of the agrochemicals can bypass the soil matrix and its capacity to store, absorb, and transform the solute. Thus, agrochemicals and nutrients can be carried to great depths, where degradation is limited (Boesten et al., 2000), possibly increasing the total amounts leached to groundwater. The preferential processes usually take place in what is often called macropores (Beven and Germann, 1982, 2013; Jarvis, 2007). In the literature, the term macropores is arbitrarily used for soil structural components like cracks, fissures, root channels, and earthworm borrows (Beven and German, 1982, 2013). These generally occur in soils with well-developed structures, that is, soils with silt or clay texture (Beven, 2018; Jarvis, 2007). Jarvis (2007) distinguishes between biologically induced macropores (e.g., root channels and earthworm borrows) termed biopores and macropores as a result of soil aggregation (e.g., fissures and cracks). In this work, the term biopores will be used to describe root channels, and earthworm borrows in which capillary effects are negligible. Artificial drainage is a common agricultural practice, which allows a field to be cultivated and increases crop production, but simultaneously can create new pathways for leaching of nutrients and agrochemicals to surface water (

For large-scale hydrological modeling, the accuracy of the models used is a trade-off with the computational requirements. The models that perform well on the daily/meter scale may not perform well when applied at the yearly/kilometer scale. We compare two models of water flow and nitrate and bromide transport in a tile drained soil. The first model is based on a 2-D grid with an explicit drain node, here called the Dynamic Drainage Model (DDM). The second and less computationally expensive model is based on an 1-D vertical discretization where the horizontal flow is included as a sink term based on the Hooghoudt theory, here called the Hooghoudt Drainage Model (HDM). Both are based on Finite Volume Method solutions to Richard's equation and to the advection-dispersion equation (ADE), and embedded within the Daisy agroecological model, which includes the nitrogen cycle. The two models are run with 10 years of weather data and three different lower-boundary conditions. Losses of water, nitrogen, and bromide to both drain pipes and deep percolation/leaching are compared between the models, at daily and yearly time scales. In no case do we find the discrepancy large enough to warrant a rejection of the use of the faster HDM instead of DDM. For the daily time scale, we find in general a higher Nash-Sutcliffe efficiency coefficient for water (0.98-1.00) than for nitrate (0.97-1.00), and the lowest for bromide (0.95-1.00). The results are explained with a low concentration gradient along the water flow pathway toward the drain.

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