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.