STEADY-STATE FUNNEL FLOW - ITS CHARACTERISTICS AND IMPACT ON MODELING

Preferential Bow could dictate the leaching of contaminants in unsatur ated soils. The objectives of this study were to: (i) numerically dete rmine the characteristics of steady-state funnel-type preferential flo w and how would it be influenced by infiltration rate, soil layer dens ity, and textural combinations; and (ii) explore the validity of sever al basic assumptions used in different models to incorporate pesticide leaching through preferential Bow paths. Water movement and pesticide transport in 12 two-dimensional hypothetical profiles were numericall y simulated. Results showed that there was an induction zone where wat er was gradually congregated into preferential Bow paths and its movem ent could be conceptualized as a network of tributaries merging into r ivers. Beneath the induction zone, water moved through several distinc t Bow domains without lateral interaction. The frequency distribution of normalized water fluxes would spread out when the layering density increased or the pore discontinuity across a textural boundary increas ed. There existed a critical number beyond which any further increase in layer density would not influence the frequency distribution of nor malized water fluxes. Frequency of water fluxes in the stationary regi on was not lognormally distributed. Pesticide leaching depended primar ily on the last 20% of the water flux distribution. If a profile was m ade of regions with different layer densities, the water fluxes of the entire unsaturated profile might be neither spatially stationary nor vertically invariant. It was possible to numerically derive the worst- case scenario (i.e., the widest spreading of the water fluxes). Becaus e it is impossible to measure the frequency distribution of normalized water fluxes in a field, it is prudent to predict pesticide leaching according to this worst-case frequency distribution.