Effect of Discretized Preferential Transport in the Assessment of N-Leaching Through Clay Field

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To be able to assess the risk of contaminants like nitrate and pesticides being transported through a variably-saturated macroporous clay till to artificially drain pipes, the conceptual understanding of the preferential water flow is crucial and hence the degree of water percolating rapidly through vertical macropores or slowly through the low permeable matrix. This study compared six conceptual models using the dual permeability module of the one-dimensional model DAISY, with incorporation of three different macropore settings and being exposed to two different groundwater tables set as lower boundary conditions (from piezometer P3 and P4). The three macropore settings included vertical macropores supplying water directly to

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i) field drain pipes,

ii) field drain pipes and matrix and

iii) field drain pipes and matrix added with fractures supplying water to the matrix in the saturated zone.

A water balance for the six different conceptual models was obtained based on 10-years of climate, drainage from field drain pipes, a concentration of N and Br- in the drainage water and groundwater data from an agricultural clay till field. The estimated drainage obtained with the six conceptual models were compared to the measured drainage. No large discrepancies between the estimated and measured drainage were identified. The macropore setting ii) exposed to groundwater fluctuations measured in piezometer P3 gave the best description of the drainage of the field. The estimated water balance of all six concepts revealed that 70% of the precipitation input to drainage was transported via macropores. 77% of this precipitation was transported by the vertical macropores, which were initiated in the plow layer.

The macropore setting ii) were evaluated further by comparing the simulated concentration of N and Br- in the drainage water, with the measured concentration. The simulated Br- and N concentration gradient show a good connection with the transported water via macropores. Differences between measured and simulated N are mainly related to crop behavior rather than hydrological causes.

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