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Abstract One common formulation of Richard's equation for variably saturated flows in porous media treats pressure head as the principal unknown and moisture content as a constitutive variable. Numerical approximations to this “head‐based” formulation often exhibit mass‐balance errors arising from inaccuracies in the temporal discretization. This article presents a finite‐element collocation scheme using a mass‐conserving formulation. The article also proposes a computable index of global mass balance.
Allen et al. (1985) studied this question.