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Northeast Thailandfield texture—Class 1Semivariance0.250.200.150.000 1 2 3 4 5 6 7Class 2Class 30.250.070.060.200.050.150.040.030.100.000.008 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8Distance (km)Fig. 7. The indicator variograms for three finger-estimated soil texture classes (light, medium,heavy).Table 5. Effective range a (given in m) and the relative nugget effect b of the exponential modelsand pure nugget variation that were fitted to the indicator variograms c : depth of clay, silt, andsand of topsoil and subsoil.T 0.1 T 0.2 T 0.3 T 0.4 T 0.5 T 0.6 T 0.7 T 0.8 T 0.9ClayEffective range 613 613 759 846 847 1,372 2,015 2,803 1,518Relative nugget 0.25 0.49 0.39 0.55 0.55 0.34 0.40 0.44 0.38effectSiltEffective range 2,131 2,453 2,803 993 788 934 1,693 1,168 –Relative nugget 0.20 0.27 0.37 0.31 0.28 0.31 0.31 0.25 1effectSandEffective range – 993 701 1,226 701 1,401 4,088 3,737 4,380Relative nugget 1 0.40 0.26 0.26 0.27 0.49 0.47 0.24 0.20effecta For practical purposes, the effective range, i.e., three times the distance parameter of the exponential model,is given to indicate the limit of spatial dependence. b The relative nugget effect was calculated as the ratio ofnugget variance (discontinuity at the beginning of the variogram caused by sampling error and short-scalevariability not resolved by the sampling scheme) to sill (value where the variogram reaches a plateau). c Indicatorvariograms were calculated with values for thresholds (T) corresponding to the nine deciles (0.1–0.9) of theconditional cumulative distribution function.The SIK map confirms the short-range variation in soil texture that is indicatedby variograms of the texture classes (Fig. 7). The variograms display mainly shortrangestructures, indicating a change of texture classes over very short distances.Variography results listed in Table 5 reveal that low clay values change rapidly in thetopsoil over short distances. No spatial dependence was detected beyond 1 km forvalues corresponding to indicator thresholds < 0.5 of the conditional cumulative distributionfunction (corresponding to 7.2% clay content). Large contents of topsoilclay are spatially dependent over longer distances of up to 2.8 km. Low silt values are92 Oberthür and Kam