A surface density model based on experimental and calculated criticality data is developed for finite water-reflected arrays and results in semiempirical analytic expressions describing criticality. The relations provide information on the reactivity associated with such perturbations to arrays as changes in unit shapes, cell volumes, array shapes, and array reflectors. Equivalence between different fissile materials in a critical array is defined. The surface density and density analog models are shown to be in correspondence when applied to the same data. The density analog model is expressible as f(N) = g(m)p-2. The functions f(N) and g(m) are explicitly given, and the constant exponent has general applicability.