The relationship between fingering flow fraction and water flux in unsaturated soil at the laboratory scale

Abstract

Preferential flow is a widespread process in unsaturated soil and it is important to incorporate preferential flow in mathematical and numerical models to accurately predict subsurface flow and solute transport. In this study we conduct experiments to relate the fraction of fingering flow (f) with water flux (qs) and average active water saturation (Se∗) which is a key constitutive relation in modeling subsurface preferential flow. Infiltration experiments were conducted in a two-dimensional transparent slab chamber, with different soil grain sizes and initial water saturation. Available experimental data from the literature are also compiled and compared with our new data. Results show that the f-qs relationship is independent on grain size of the soil and whether the sand is initially dry or wet. The exponent of the f-qs relationship (a0) varies in a small range (0.39–0.48) and the exponent of the f-Se∗ relationship (γ) can be predicted from a0 and soil grain size. The Richards equation was combined with the f-qs relationship to model the preferential flow experiments. The results show that the constitutive relationship successfully predicts preferential flow velocity in unsaturated soil.