Plotsize and sample number for neutron probe measurements in small field trials
1993
KAMGAR, A. | HOPMANS, J. W. | WALLENDER, W. W. | WENDROTH, O.
Soil water storage over a 2.85-m soil depth was measured from 200 aluminum access pipes, separated by 0.3 m in both directions, in a 1.2 × 15.0-m plot in dry and wet periods during 1988 and 1989. The objective was to determine the minimum plot size and number of soil water content measurements if measured with a neutron probe for small field trials. The minimum plot size representing the 15-m plot was found to be dependent on water storage variance and on the distance over which soil water storage measurements were spatially correlated. We concluded that a plot length of 5 m was needed to represent the mean and variance of the 15-m plot. Bootstrapping and temporal stability analysis were used to estimate the minimum number of observation tubes required to estimate the mean and variance of 1.2 × 5.0-m plots. Bootstrapping showed that at least 10 soil water storage measurements were required in the 5-m plot. Soil water storage distribution within the plot was found to be highly stable in time, especially for individual soil layers. Using temporal stability analysis, the number of required access pipes needed to estimate a plot-average soil water storage was further reduced to three. However, the variance of soil water storage was not conserved while reducing the number of measurement locations. We propose that a field study with small field trials should start with the maximum feasible plot size and number of measurement locations. In this initial phase, statistical techniques as proposed in this study can then be applied to reduce the required number of observations, using predetermined error limits.
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