Results
Figures 4-10 show the patterns of change in each water quality variable
over space and time. For the regression models, the Gaussian correlation
range parameter was estimated to be r = 3.4. The time scaling
factor in \(d^{\text{st}}\) was estimated to be τ = 0.21, indicating
that a distance of one kilometer is equivalent to a time span of about 5
days (0.21×5 days ≈ 1 km). In other words, two observations made one
kilometer apart (on the same day) are as correlated with each other as
two made five days apart (at the same site).
The estimated coefficient for the distance term in the regression models
indicates the direction and magnitude of the change in the water quality
variable per kilometer, and the corresponding t test indicates
whether the change was statistically significant. Table 1 shows the
estimated coefficient and t test result for each water quality
variable.
Thus BOD, orthophosphate, and ammonia-N, all increased statistically
significantly, by 0.049, 0.023, and 0.005 mg/L per kilometer,
respectively, and temperature increased by 0.077 oC
per kilometer. Nitrate-N and DO both decreased significantly, by 0.022
and 0.035 mg/L per kilometer, respectively. The decrease in nitrate-N
was due to the very high nitrate-N values observed at the 14 km and 19
km sites. The pH did not change significantly over the stretch of river
studied.
The seasonal (month) effect in the regression model was statistically
significant for BOD, nitrate-N, DO, and temperature (p = 0.000 in all
four cases, using a likelihood-ratio chi square test), but not for
orthophosphate, ammonia-N, or pH (p = 0.253, 0.979, and 0.801,
respectively). There was not a significant long term trend (year effect)
in any of the variables except temperature, which showed a significant
decrease over the three year study period (p = 0.158, 0.472, 0.419,
0.472, 0.760, and 0.118, respectively for BOD, nitrate-N,
orthophosphate, ammonia-N, DO, and pH, and p = 0.024 for temperature).
The control site shift coefficient b was negative and
significant, indicating lower values relative to the other sites, for
nitrate-N (b = -5.94, p = 0.000), pH (b = -0.54 and p =
0.000), and temperature (b = -2.09 and p = 0.001), but not
significant for BOD, orthophosphate, ammonia-N, or DO (p = 0.833, 0.225,
0.767, and 0.985, respectively). Thus, nitrate-N was 5.94 mg/L lower at
the control site relative to the other sites, pH was 0.54 units lower,
and temperature was 2.09 oC lower.