3.2. Spatial structure of NO3-N concentrations in shallow groundwater
The experimental semivariogram r(h) was calculated for the NO3-N concentrations in 2002 and 2007, respectively, and a scatter plot of r(h) vs. h was generated. We then used different theoretical semivariance models to fit the calculated values and the model with the best goodness of fit and the smallest nugget value was selected (Goovaerts, 1997). The spatial correlation distances of NO3-N concentrations for 2002 and 2007 were 2.85 and 2.74 km, respectively, which indicated that the small-scale factors such as farmers practices and land use type would be the main the influence factors for the variability of NO3-N concentrations (Table 2).