ABSTRACT
This chapter reviews the development and application of models to predict the impact of physical nonequilibrium processes on agricultural nonpoint source pollution. Classical theory of water flow and solute transport is based on continuum physics, with the underlying assumption that unique values of soil water pressure and solute concentration can be defined for a representative elementary volume of soil. Physical nonequilibrium occurs when soil heterogeneities result in the generation of lateral differences either in water pressures or solute concentrations, or both. Finger flows can be initiated by small- and large-scale heterogeneities within the soil and/or by flow concentration at the surface either due to interception by vegetation and stem flow, or water repellency. Nonequilibrium conditions occur in field soils due to a variety of physical causes and must be considered the rule rather than the exception.
