A Transfer Function Model of Solute Transport Through Soil: 1. Fundamental Concepts

The transfer function model (TFM) of solute transport through unsaturated soil initiated by W. A. Jury (1982) is generalized to describe the movement of a solute that may undergo physical, chemical, or biological transformations as it moves in a soil unit through which the water flow may vary in both space and time. The generalized TFM is shown to be related to the law of mass balance for a solute as interpreted in the context of probability theory. The principal conceptual entity in the TFM is the solute lifetime probability density function. Methodologies for determining this function experimentally are outlined and its relation to the solute travel time density function defined by Jury is shown. Since the generalized TFM is formulated independently of any particular mechanism of solute transport through soil, it is concluded that mechanistic models of solute movement consistent with the law of mass balance are also consistent with the TFM.