Spatial Scale Dependence of In Situ Solute Transport

Abstract In field solute‐transport experiments, two spatial scales of the continuum of the dispersion process have been measured: local scale and field scale. The objective of this experiment was to develop a method for measuring in situ the transition from the local scale to the field scale during unsaturated flow conditions. The spatial variability of in situ solute dispersion was examined in two field sites. Soil‐solution samplers were installed in a transect at a 0.4‐m depth and 0.2‐m spacing in both a cultivated and never‐cultivated (forested) site. A pulse of KCl was applied to both sites under conditions of constant surface flux density of water, which was applied using a trickle irrigation system. The variance of solute travel time, V 2 ( t ), at different spatial scales was calculated from moment analysis of breakthrough curves (BTC) obtained by averaging local BTC across different spatial scales. The scale dependence of V 2 ( t ) indicated scales of at least 2.8 and 3.8 m were needed to reach an effective far field variance for the forested and cultivated sites, respectively. The larger scale in the cultivated site was due to an increase in horizontal correlation length scales of soil properties caused by tillage mixing. The scale dependence of V 2 ( t ) can be used to determine the minimum plot size necessary to include all major horizontal variations in solute travel time, which can then be compared with spatial distributions of soil properties affecting transport.