Of the remaining parameters, pH values varied the least, followed by COD, Na+, and salinity, with standard deviations equal to 0.3, 11.7, 8.8, and 23.4, respectively. An understanding of data variation is essential if meaningful comparisons between groups of data are to be made (Gomez & Gomez, 1976; Clewer & Scarisbrick, 2001).
 

3.1.1    Surface water pH

Surface water pH indicates the level of acidity of the surface water. The pH range measured for Ngwerere River water was 7.0–8.0, with a mean of 7.62 ± 0.05 (Table 1). Furthermore, pH measurements were highly uniform, with measurements remaining close to the mean (Fig. 2).
 

3.1.2    Surface water salinity

Salinity is a consistent component of freshwater ecosystems. The salinity values measured in this study ranged between 60–163 mg L-1, and moderately varied around the mean (Fig. 2). Estimation of the effects of salinity is complex as it contains different components that all contribute to a single salinity measure. The composition of individual components that contribute to salinity varies spatiotemporally, based on underlying geomorphology and geology.
 

3.1.3    Total dissolved solids

In this study, TDS measurements ranged between 258–567 mg L-1, with a mean value of 437.91 ± 14.35 mg L-1. The median value was in the third quartile, with significant extensions of the upper and lower whiskers, suggesting that some values widely varied from the mean (Fig. 2). TDS data had a relatively large spread.
 

3.1.4    Chemical oxygen demand

The range of COD recorded in this study was 4–36 mg L-1 O2, with an average COD of 17.45 ± 2.04 mg L-1 O2 (Table 1). The COD measurements had intermediate variation (Fig. 2), with a standard deviation of 11.72.
 

3.1.5    Sodium

Sodium concentration in Ngwerere River water ranged from 22–60 mg L-1 while average concentration was 45.46 ± 1.53 mg L-1 (Table 1). The data for Na+ had a comparatively low variation (Fig. 2). However, 60 mg L-1 Na+ was recorded within the watershed, indicating potential chemical pollution. Furthermore, an average of 45.46 ± 1.53 mg L-1 is within the upper margin of the safe sodium concentration range, indicating that Na+ levels will soon exceed the level expected in surface waters if no interventions are in place.
 

3.1.6    Total suspended solids

Total suspended solids data had the largest spread. Although most values were within the second and third quartiles, spread around the median was comparative wider than that of other parameters (Fig. 2). Measurements of TSS in this study ranged from 2–710 mg L-1, with a mean of 192.79 ± 46.90.
 

3.2       Analysis of variance (ANOVA)

Three ANOVA tests were conducted to determine differences in means of WQ parameters according to watershed position, site location, and the month of data collection within the NPW (Tables 2, 3, and 4). The outcomes of the ANOVA are discussed in the following sections.