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from −1 to 1 where values near −1 indicate that like members of a group are evenly interspersed
across the field like the colored squares of a checkerboard, and values near 1 indicate that members
are grouped closely together in space. Moran's I was calculated using the Excel 97/2000 Visual
Basic routine written by Sawada (1999).
14.3 ReSUltS
Measurements at only 117 of 121 grid points were used because standing water in the southeast cor-
ner of the field prevented access to three locations and the fourth measurement was lost. The mean
K d for atrazine was 4.94 L kg −1 (Table 14.1) and ranged from 1.74 to 10.92 L kg −1 with the sample
population distribution being positively skewed (mean > median). SOC averaged 0.0280 kg kg −1 and
varied between 0.0123 and 0.0556 kg kg −1 , and the distribution was also positively skewed. Both K d
and SOC were better described by lognormal distributions than normal distributions as determined
by the Kolmogorov-Smirnov test and the D'Agostino-Pearson test (Cambardella et al., 1994). EC a
measurements ranged from 20.4 to 65.4 mS −1 and averaged 41.0 mS −1 . These values are relative,
however, and are not direct measures of the true soil electrical conductivity (Lesch et al., 1992).
EC a data were not described well by either normal or lognormal distributions, although the relative
difference between the mean and median was less for the untransformed data.
The spatial distribution of each parameter over the intensive-grid area is shown in Figure 14.2.
None of the parameters were randomly distributed across the area but were instead grouped into
areas of like values. Moran's I statistic for K d , SOC, and EC a was 0.52, 0.74, and 0.59, respectively,
indicating substantial spatial clustering. Each parameter showed lower values in areas of the field
mapped as well-drained Clarion loam and higher values in the area mapped as very poorly drained
Okoboji mucky silt loam. This agrees with our hypothesis that all three parameters should be cor-
related with drainage class.
In addition to similar spatial patterns, the spatial structures of the three parameters were almost
identical as determined by similar correlograms (not shown). The correlograms indicated a spatial
dependence between measurements for each parameter to distances of about 80 m. The similar
spatial distribution leads to a significant simple correlation statistic between each pair with the cor-
relation between EC a and SOC being the strongest (Table 14.1). Given the similar spatial patterns,
spatial structure, and positive correlation between the parameters, it appears likely that K d can be
successfully estimated from either SOC or EC a . K oc was calculated by regressing the K d values
versus the SOC values using linear least sum-of-squares. The resulting value for K oc was 0.171 L
kg −1 with a standard error of regression of 1.42 L kg −1 .
An analogous regression was performed between K d and EC a . However, because the K d values
were lognormally distributed and the EC a values were better described by a normal distribution, the
expression equivalent to K oc is ln K d = b EC a or K d = exp (b EC a ), where b was found by nonlinear
tAble 14.1
descriptive Statistics and Correlation Coefficients
for 117 Measurements of Apparent electrical
Conductivity (eC a ), Atrazine partition Coefficient
(k d ), and Mass fraction organic Carbon (SoC)
Correlation
SoC
Stan.
dev.
property
Mean
Median
K d
EC a (mS m −1 )
41.0
41.8
13.0
0.765
0.575
K d (L kg −1 )
4.94
4.61
1.86
0.686
SOC (kg kg −1 )
0.0280
0.0274
0.0100
 
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