Quantifying the Pore Size Spectrum of Macropore-Type Preferential Pathways under Transient Flow

It is well known that there is a spectrum of pores in a soil profile. The conventional use of a single lumped value of soil hydraulic conductivity to describe a spectrum of hydraulically active pores may have unintentionally impeded the development of field-scale chemical transport theory and perhaps indirectly hindered the development of management protocols for chemical application and waste disposal. In this study, three sets of four field-scale tracer mass flux breakthrough patterns measured under transient unsaturated flow conditions were used to evaluate the validity of an indirect method to quantify equivalent pore spectra of macropore-type preferential flow pathways. Results indicated that there were distinct trends in how pore spectra of macropore-type preferential flow pathways changed when a soil profile became wetter during a precipitation event. This suggests that the indirect method has predictive value and is perhaps a better alternative to the lumped soil hydraulic conductivity approach in accurately determining the impact of macropore-type preferential flow pathways on water movement and solute transport under transient unsaturated flow conditions.