Solute transport modeling using transfer functions

Modeling of solute movement through soil is important for predicting agricultural impacts on water quality. Application of the transfer function modeling approach to solute transport is investigated. In this approach, the solute output rate from a soil\u27s solute transport volume is related to the input rate weighted by the transfer function. The transfer function is equivalent to the probability density function of solute lifetimes in the solute transport volume. Methods of determining transfer functions given various input and output functions are described. Parameter-estimation techniques are presented for fitting Fickian, lognormal, gamma, normal, and Weibull probability density functions to solute outflow data. Procedures are detailed for measuring tracer outflow from laboratory soil columns with and without macropores. Outflow of chloride applied to the surface before water application and bromide added continuously for eight hours in sprinkler-applied water were measured. Continuously-applied bromide moved more rapidly through soil with macropores than through homogeneous soil columns, while the opposite was true for the single surface application of chloride. Intermittent water applications followed simulated chemigation and conventional surface applications of tracers. Little difference in solute transport due to soil treatment was observed for low-intensity applications. For high-intensity events, chemigation-applied bromide moved more rapidly through soil with macropores than did chloride applied before water application. For homogeneous soil columns, the opposite was the case. Different transfer functions resulted from impulse and step-change solute inputs to the same soil columns. This result indicates that the solute input rate to the region of active transport differs from the surface application rate. Theoretical chloride input rates to the solute transport volume are calculated using transfer functions derived from step-change inputs of bromide. Transfer functions resulting from intermittent water applications depend on both tracer and water input rate, especially where preferential flow occurs. The dispersion coefficient calculated from fitting a Fickian function to outflow rate data for use in convection-dispersion modeling depends not only on soil properties, but also on water and tracer application