Prediction uncertainty of plume characteristics derived from a small number of measuring points

A small number of measuring points may inflict a bias on the characterisati on of now and transport based on field experiments in the unsaturated zone. Simulation of pure advective transport of a Gaussian plume through a setup of 30 regularly placed measuring points revealed regular temporal fluctuat ions about the real spatial moments. An irregular setup predicted both irre gular fluctuations and larger discrepancies from the real value. From these considerations, a regular setup is recommended. Spatial moments were sensi tive to the plume size relative to the distance between individual measurin g points. To reduce prediction errors of the variance, the distance between the measuring points should be less than twice the standard deviation of t he examined plume. The total size of the setup should cover several standar d deviations of the plume to avoid mass being lost from the monitored area. Numerical simulations of a dispersing plume (comparing calculations based on 9000 nodes with 30 measuring points) revealed that vertical and horizont al centres of mass were predicted well at all degrees of heterogeneity, and the same was the case for horizontal variances. Vertical variances were mo re susceptible to prediction errors, but estimates were of the same order o f magnitude as the real values.