Parameterization of Macropore Flow Using Dye-Tracer Infiltration Patterns in the SWAP Model

Preferential flow is known to influence infiltration, soil moisture content distribution, groundwater response, and runoff generation. Various model concepts are used to simulate preferential flow. Preferential flow parameters are often determined by indirect optimization using outflow or discharge measurements, thereby providing limited insight into model performance concerning soil moisture distribution. In this study, we used a physically based macropore concept, embedded in the SWAP model, in combination with dye infiltration patterns to parameterize macropore infiltration for three locations in a catchment: hilltop, hillslope, and valley bottom. The model with the calibrated macropore parameters was applied and validated under natural field conditions, using detailed data on soil moisture content, rainfall, and discharge. The results show that the macropore model parameters can be optimized well to reproduce the dye-tracer infiltration patterns. The simulations of the dye patterns show much better results when macropore flow is included. Using the tracer infiltration patterns, however, the optimized maximum depth of macropores depends completely on the maximum depth of the stained area, while the macropores are known to extend deeper into the soil. Therefore, for long-term simulations, the wetting of deeper layers is too slow for the simulations both with and without macropores. Runoff production was better simulated with macropores. For the simulations without macropores, a higher lumped saturated conductivity was used; despite the resulting increased infiltration into the soil matrix, runoff generation remained far too high.