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Steady-State Funnel Flow: Its Characteristics and Impact on Modeling

Dep. of Agric. Engineering

K.-J. S. Kung^*

Dep. of Soil Science, Univ. of Wisconsin, Madison, WI 53706-1299

*Corresponding author (kung{at}calshp.cals.wisc.edu).

ABSTRACT

Preferential flow could dictate the leaching of contaminants in unsaturated soils. The objectives of this study were to: (i) numerically determine the characteristics of steady-state funnel-type preferential flow and how would it be influenced by infiltration rate, soil layer density, and textural combinations; and (ii) explore the validity of several basic assumptions used in different models to incorporate pesticide leaching through preferential flow paths. Water movement and pesticide transport in 12 two-dimensional hypothetical profiles were numerically simulated. Results showed that there was an induction zone where water was gradually congregated into preferential flow paths and its movement could be conceptualized as a network of tributaries merging into rivers. Beneath the induction zone, water moved through several distinct flow 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 increased. There existed a critical number beyond which any further increase in layer density would not influence the frequency distribution of normalized water fluxes. Frequency of water fluxes in the stationary region was not lognormally distributed. Pesticide leaching depended primarily on the last 20% of the water flux distribution. If a profile was made 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). Because 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.

Received for publication September 11, 1995.

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