VARIABILITY OF VARIOGRAMS AND SPATIAL ESTIMATES DUE TO SOIL SAMPLING - A CASE-STUDY

Measuring of electromagnetic soil conductivity (EMC) is used to follow the temporal evolution of soil salinity, because it is a rapid techni que with a portable instrument. A test site of 1.2 km by 2.4 km is sel ected as a reference. The suitability of sampling schemes for monitori ng soil salinity during the first years of irrigation is studied. An i nitial sampling consists of 17 rows and 33 columns of observation poin ts 75 m apart, i.e. 561 data points, regularly spaced on 288 ha; it al lows to determine the ''actual variogram'' of EMC on this site. This s tructure is complex, as is often the case in nature, with alternating strongly and weakly salted areas, due to small creeks across the site. The sample size required to get accurate estimates of soil salinity a nd to follow temporal changes is assessed by subsampling the data set. Simulation by repeated sub-sampling is preferred because of the compl exity of the data structure. Five series of 20 sub-samples are randoml y taken from the initial sample, with size 50, 75, 100, 150 and 200 da ta points, respectively. For each sample size, the analysis consists i n computing the mean squared error of the 20 sub-sample variograms rel ative to the ''actual sample variogram''', and similarly for the fitte d models. Confidence limits for the theoretical variogram were directl y estimated from the 20 fitted models. Finally, the effect of this unc ertainty is studied by comparing kriged estimates with observed values . The consistency of both experimental and fitted variograms increases with sample size. In this case, a choice of 150 data points appears t o be consistent. Despite a large variability in experimental variogram s, the fitted models and the kriged estimates are not so different as could be expected.