The development of biofilms in unsaturated soils is likely to influence the hydraulic conductivity function. Despite its importance, this effect has received little attention. Mostafa and Van Geel (2007, Vadose Zone Journal, 6:175–185) proposed several hydraulic conductivity models that acount for the effect of bacteria in unsaturated soils. We have expanded these models by considering the change in biofilm pore-size distribution and its effect on the entire hydraulic conductivity function. Three scenarios were considered: (i) the biofilm fills the smallest pores first; (ii) a biofilm of uniform thickness coats all pore walls; and (iii) the biofilm coats the soil with a constant volume fraction of each pore. The results show that the pore-scale distribution of the biofilm has a significant effect on the hydraulic properties of the soil and therefore has to be accounted for when modeling flow and transport in biofilm-affected soils.

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