Solute transport at the pedon and polypedon scales

Abstract

Soil solute transport parameters are usually measured at the pedon scale, but predictions of transport at the polypedon scale are often required. Our objectives were to compare pedon- and polypedon-scale convection dispersion equation (CDE) transport parameters measured in a field leaching experiment and to compare deterministic and stochastic predictions of polypedon-scale transport. The study was conducted near Watkinsville, Ga. on a 12.5 by 30.5 m plot. Time domain reflectometry (TDR) waveguides (30 and 60 cm in length) were installed vertically at 80 locations to measure water content and impedance. The plot was irrigated and a Cl pulse was used as a tracer. We found very good agreement between the average CDE parameters measured at the pedon scale and those estimated at the polypedon scale in the 0–30 and 0–60 cm depth intervals. Although the variability in average pore velocities between pedons was large, the variability in velocities within a pedon caused by hydrodynamic dispersion was greater. As a result, a deterministic approach based on the CDE more accurately predicted the estimated polypedon scale breakthrough curve (BTC) at 30 and 60 cm depths than a stochastic approach based on the convective lognormal transfer function (CLT). This indicates that the pedon serves as a representative elementary volume (REV) for solute transport in this soil.