Spatial variability of solute leaching : experimental validation of a quantitative parameterization

Soil heterogeneity, soil structure, and fingered flow affect solute leaching from the vadose zone to the ground water. Recently, the spatial variation of cumulative solute fluxes at a given depth was characterized by fitting the two-parameter beta distribution to sorted amounts of solute leaching at different sampling points. We tested this parameterization on data from a chloride tracer experiment performed on a monolith lysimeter, below which drainage was collected from 300 compartments with a combined area of 0.75 m2. The effect of total sampling area, sample size, and the number of samples and their spatial distribution (random locations vs. clustered) on the fitted parameters was examined. Sixteen or more sampling locations of 25 cm2 each (5% of the total area) resulted in adequate representation of parameter values. Increasing the sample size underestimated the degree of heterogeneity. We therefore recommend that the fitted parameter values of the beta distribution be reported together with the sample size. In solute-transport experiments, collecting many small samples will give more accurate results than taking fewer but larger samples. | Soil heterogeneity, soil structure, and fingered flow affect solute leaching from the vadose zone to the ground water. Recently, the spatial variation of cumulative solute fluxes at a given depth was characterized by fitting the two-parameter beta distribution to sorted amounts of solute leaching at different sampling points. We tested this parameterization on data from a chloride tracer experiment performed on a monolith lysimeter, below which drainage was collected from 300 compartments with a combined area of 0.75 m2. The effect of total sampling area, sample size, and the number of samples and their spatial distribution (random locations vs. clustered) on the fitted parameters was examined. Sixteen or more sampling locations of 25 cm2 each (5% of the total area) resulted in adequate representation of parameter values. Increasing the sample size underestimated the degree of heterogeneity. We therefore recommend that the fitted parameter values of the beta distribution be reported together with the sample size. In solute-transport experiments, collecting many small samples will give more accurate results than taking fewer but larger samples.