dual-porosity model to quantify macropore flow effects on nitrate leaching

The dual-porosity model MACRO was coupled with the N turnover and leaching model SOILN to quantify macropore flow effects on nitrate (NO3) leaching from structured soils. This was done by comparing model simulations performed with and without macropore flow with measurements of tile drain flow, NO3 contents in the soil profile, NO3 concentrations in drain discharge, and N in harvested grain made in an 8-yr field experiment on a clay soil. Macropore flow had only small effects on the soil hydrology, so that the MACRO model could give good predictions of the accumulated tile drain discharge for simulations both with and without macropore flow. By taking macropore flow into account, the SOILN model could satisfactorily depict the NO3 amounts in the soil and harvested grain and the leaching pattern, while simulations without macropore flow could not match all measured components of the N mass balance. In simulations with macropore flow, 62% of the simulated NO3-N concentrations in drain flow were within a factor of 1.5 of measured values, while simulating without macropore flow resulted in only 35% of concentrations within a factor of 1.5 of the measured values. A comparison of simulations with and without macropore flow showed that macropore flow reduced NO3-N leaching by 28% from June 1990 to July 1995, but with large variations found between different years (from 3-45%). However, for shorter periods, macropore flow increased leaching. The results demonstrate the importance of macropore flow for NO3 transport in structured soils.