Variations in the concentrations of naturally occurring isotopes in rainfall provide a “continuous” signal that has the potential to generate more information on how weather conditions control solute transport than traditional field tracer experiments. An isotopically distinct rainfall event can be rapidly transferred through the system when preferential flow occurs. The objective of this study was to develop and test an approach for analyzing how rainfall and soil moisture control preferential transport to tile drains. An episodic solute transit time distributions model that accounts for these effects was developed. The model was tested on artificial data (daily values of tile drain discharge and 18O concentrations in the drainage water) generated by the MACRO model for soils with different potentials for preferential flow. The results showed that the transit time model gave an excellent fit to the artificial data for the soil with a high potential for preferential flow. The dynamics of the preferential flow events were well captured also for the soil with medium potential for preferential flow but the magnitude was sometimes poorly simulated. Once the model parameter values have been determined, the model can be used to calculate the amount and distribution with time of preferential flow to tile drains for all daily rainfall data. Model calculations integrate the soil's potential for preferential flow and the effects of weather conditions and are, therefore, highly relevant as site-specific indicators for the risk of preferential leaching of agrochemicals. The approach needs to be evaluated against real field data, however, before its usefulness can be established.

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