Geography Reference
In-Depth Information
A theoretical geostatistical or variogram model (Figure 4) can be defined
essentially by ‗sill' and ‗range'. ‗Sill' is the constant value on the y-axis
around which a variogram stabilizes after a large distance and ‗range' is the
value at x-axis at which the variogram becomes constant or nearly constant.
The sill value is usually very close to the variance of the variable (Matheron,
1965; Ahmed, 2006). In addition, the sudden apparent jump near the origin
that occurs in some cases is known as ‗nugget' effect. The shape of the
variogram between origin and the point of stabilization is different for
different variables, which entirely depends on its nature of variability
(Matheron, 1965). In order to understand spatial structure, experimental water
quality data are classified into lag distances with approximately the same
number of data and semi-variogram values are calculated for each class
(denoted by individual points shown in Figure 4) using geostatistics or GIS
software packages such as MathWorks, GSLib, GSTAT, GeoPack, ILWIS,
ArcGIS, IDRISI, etc.
(C) Fitting of Theoretical and Experimental Variograms
The experimental variogram calculated from the observed water quality
data using Equation (9) is usually an erratic curve (Kitanidis, 1997, 1999).
Experimental Variogram
Theoretical Variogram
1.6
1.2
Sill
0.8
0.4
Nugget
0
0
Range
25
50
75
100
Lag Distance (m)
Figure 4. Fitting of the theoretical variogram to an experimental variogram.
