Quantifying Macropore Flow Effects on Nitrate and Pesticide Leaching in a Structured Clay Soil

Larsson, M. 1999. Quantifying macropore flow effects on nitrate and pesticide leaching in a structured clay soil. Field experiments and modelling with the MACRO and SOILN models. Doctoral thesis. ISSN 1401-6249, ISBN 91-576-5489-1 Rapid non-equilibrium flow of water in soil macropores is one significant, yet poorly understood, process controlling solute transport in soils. To protect surface waters and groundwater from pollution of agro-chemicals, it is important to improve our understanding of the effects of macropore flow on solute leaching. In this study, the effect of macropore flow was quantified for nitrate and pesticides on a structured clay soil in south-west Sweden, using two models, the dual-porosity, dual-permeability model MACRO and the nitrogen turnover model SOILN. This was done by first calibrating the MACRO model against extensive field measurements of tile drain flow, soil moisture contents, and Br tracer (drainage concentrations and amounts in the soil). Model simulations with MACRO and the coupled models MACRO-SOILN were then performed with and without macropore flow and compared with measurements of solute content in the soil profile and solute concentrations in drain discharge of NO3-N and a weaklysorbed herbicide, bentazone. Ten-year simulations with 60 hypothetical compounds were also conducted to evaluate the effect of macropore flow on pesticides with widely different sorption and degradation properties. The results showed that macropore flow reduced total leaching of bentazone to tile drains by as much as 50 % for the one-year experimental period. This is because much of the bentazone was stored in the micropores, moving at a ‘reduced’ velocity, ‘protected’ against bypass of water in the macropores. However, the scenario simulations indicated that macropore flow considerably increases leaching for less mobile pesticides, since this is the dominant leaching mechanism. The largest increase, up to five orders of magnitude, was found for moderately to strongly sorbed pesticides with half-lives <10 days. For most pesticides, macropore flow also significantly reduced the influence of compound properties on leaching. For nitrate, the simulated effect of macropore flow was a reduction in leaching by 28 % from June 1990 to July 1995, but due to the influence of climatic conditions, large variations were found between years (from 3 to 45 %). Without taking macropore flow into consideration, the model could not depict the observed leaching pattern of bromide and bentazone. It was also impossible to match all the measured components of the nitrogen mass balance without accounting for macropore flow. Even though there are many uncertainties in the modelling approach, it is concluded that macropore flow models could be useful in the evaluation of alternative soil and crop management practices to minimize adverse impacts of non-point source pollution on water quality.