The spatial heterogeneity in the soil hydraulic properties was considered here as the main cause of the spatial variability in soil water content, θ. The aim of the study was (i) to identify the critical mean soil water content, Θ_cr, below which the standard deviation of the soil water content, σ_θ, can only diminish, and (ii) to provide a mathematical tool capable of linking Θ_cr to inherent soil hydraulic properties. The relationships between σ_θ and the mean value of θ, Θ, were analyzed by a first‐order analysis, based on a stochastic continuum presentation of steady‐state flow in 3‐D, spatially hetrogeneous, unsaturated soils, using the Gardner‐Russo model to describe the constitutive relations locally. Novel results of the study suggested that the mean pressure head, H, associated with Θ_cr, H_cr = H (Θ_cr), is an inflection point of the mean soil water capacity function, C(H) = dΘ/dH. Results of numerical simulations of flow in 3‐D, spatially heterogeneous, unsaturated soils, using the van Genuchten (1980, https://doi.org/10.2136/sssaj1980.03615995004400050002x) model to describe the constitutive relations locally, suggested that the results of the first‐order analysis might be applicable to more realistic situations, involving transient, nonmonotonous flows and more complicated descriptions of the local constitutive relations.