Exploring the Spatially Varying Impact of Urbanization onWater Quality in Eastern Massachusetts Using GeographicallyWeighted Regression - Geotechnologies and the Environment - page 142

The watersheds with different levels of urbanization in the study area are good for
examining how the impact of urbanization on water quality varies across the urban-
ization gradient. Furthermore, the great spatial variability in urbanization indicators
suggests that the significant spatial non-stationarity might exist in the relationships
between urbanization and water quality variables over the study area, so GWR is
a more appropriate method than traditional regression techniques, which is already
confirmed by model evaluation (Tu and Xia 2008 ).

9.5.2 Relationships Between Developed Land and Water Quality

Because of space considerations, not all the maps of parameter estimates and
local R 2 values from the GWR models can be presented here. The ranges of local
parameter estimates and R 2 s are summarized in Table 9.1 . Parameter estimates for
an independent variable actually reflect the correlations between the independent

Table 9.1 Ranges of local parameter estimates and R 2 from GWR models

Local R 2

Local parameter estimates

Water quality
indicator

% developed
land

population
density

% developed
land

population
density

SC Min 0.200 -0.462 0.293 0.210

Max 0.374 2.022 0.644 0.973

NH 3 -N Min 0.596 1.188 0.272 0.414

Max 1.024 1.684 0.624 0.710

NO 2 -N Min 15.18 10.90 0.349 0.309

Max 25.75 88.96 0.655 0.928

KN Min − 0.040 − 0.040 0.344 0.099

Max 0.100 1.372 0.999 0.997

NO 3 -N +
NO 2 -N

Min 0.001 0.240 0.317 0.194

Max 0.032 0.980 0.630 0.791

P Min 0.006 − 0.410 0.130 0.110

Max 0.059 3.280 0.380 0.918

PO 4 -P Min − 2.25 − 17.68 0.140 0.077

Max 0.51 51.13 0.501 0.890

Ca Min 0.017 − 0.280 0.511 0.243

Max 0.122 1.690 0.996 0.935

Mg Min 0.014 0.100 0.364 0.312

Max 0.021 0.710 0.664 0.931

Na Min 0.011 0.100 0.172 0.438

Max 0.065 1.270 0.992 0.996

K Min − 0.007 0.131 0.243 0.370

Max 0.024 0.234 0.999 0.650

Cl Min 0.060 0.940 0.203 0.246

Max 0.270 4.400 0.972 0.841

SO 4 Min 0.006 0.040 0.345 0.345

Max 0.045 0.790 0.991 0.991

DS Min 0.0001 0.001 0.302 0.147

Max 0.0023 0.032 0.998 0.945