4 Changes in Water Quality State (1989-2005)

A single snapshot of water quality state, such as that provided for 2005 here, can provide a wealth of useful information on areas where water quality is particularly good or bad. However, it does not tell us whether water quality is getting better or worse over time. In addition, because it does not incorporate information about inter-annual variability, a single year's data may provide a limited national-scale picture. In the figures below, water quality patterns over time have been summarised at the national scale. This is a relatively descriptive way to highlight changes in water quality over the 17-year period of the NRWQN's operation.

Annual median values were calculated using monthly data for 12 water quality parameters at each of the 77 NRWQN sites from 1989 to 2005. These annual site medians were then used to calculate the annual 5th, 20th, 50th, 80th and 95th percentile values across the NRWQN sites. The 50th percentile gives us a picture of what is happening in a national "average" river in terms of annual median water quality data. The 20th and 80th percentiles are included because management decisions often follow the 80:20 rule. The 5th and 95th percentiles tell us about state and changes over time in our "best" and "worst" rivers.

These summaries of moving annual state can be quickly and easily updated each year and could be used to provide early warning of emerging issues for water resource management. It is likely that more formal trend analyses (see Section 5) will occur less frequently (eg, every five years), so the summaries below provide a relatively quick and simple check on changes in water quality on a year-to-year basis. Note that it is possible the descriptive trends presented below may produce different results from a more formal site-by-site trend analysis (see Section 5). This could occur as a result of differing time periods being analysed, the presence of strong seasonal trends that might be obscured when reporting annual medians over time, or the influence of changing flows during the period (formal trend analysis is carried out on flow-adjusted data).

Note that the figure for flow is indicative only, as it involves spot measurements of flow on monthly occasions. More detailed analyses of continuous flow records are likely to produce a more useful summary of changes in flow over time.

Note: Lines, from bottom to top, correspond to values of the 5th, 20th, 50th, 80th and 95th percentiles for each year (n of sites = 77).

Figure 4.1: Summary data for flow and temperature from the NRWQN over a 17-year period (1989-2005)

See figure at its full size (including text description).

Figure 4.2: Summary data for conductivity and pH from the NRWQN over a 17-year period (1989-2005)

See figure at its full size (including text description).

Figure 4.3: Summary data for dissolved oxygen and visual clarity from the NRWQN over a 17-year period (1989-2005)

See figure at its full size (including text description).

Figure 4.4: Summary data for oxidised-N and ammoniacal-N from the NRWQN over a 17-year period (1989-2005)

See figure at its full size (including text description).

Figure 4.5: Summary data for total N and dissolved reactive P from the NRWQN over a 17-year period (1989-2005)

See figure at its full size (including text description).

Figure 4.6: Summary data for total P and biological oxygen demand from the NRWQN over a 17-year period (1989-2005)

See figure at its full size (including text description).

4.1 Trends over time

The data shown in the series of figures in the previous pages were analysed for trends over time using a Spearman Rank Correlation test. Significant trends are in bold. '*' = significant (P < 0.05); '**' = highly significant (P < 0.01); '***' = very highly significant (P < 0.001).

Table 4.1: Spearman rank correlation coefficients for trends over time (1989-2005) based on annual summary data from 77 NRWQN sites

	N (years)	5th	20th	50th	80th	95th
Flow	17	-0.047	-0.061	-0.350	-0.434	-0.260
Temperature	17	0.701**	0.321	0.333	0.302	0.277
Conductivity	17	0.301	0.213	0.483	-0.010	0.223
pH	17	-0.270	0.059	-0.114	-0.370	-0.637**
Dissolved oxygen	17	-0.088	0.112	-0.254	-0.269	-0.110
Visual clarity	17	0.127	0.488*	0.395	0.132	0.436
Oxidised nitrogen	17	-0.810***	-0.100	0.373	0.390	0.797***
Ammoniacal nitrogen	16	-0.963***	-0.948***	-0.939***	-0.611*	0.435
Total nitrogen	16	0.394	0.738**	0.585*	0.597*	0.708**
Dissolved reactive phosphorous	17	-0.256	-0.074	0.475	0.635**	0.047
Total phosphorous	17	-0.104	0.397	0.235	0.007	-0.368
Biochemical oxygen demand	14	-0.748**	-0.886***	-0.881***	-0.818***	-0.704**

4.2 Key results

Our coolest rivers (5th percentile water temperature) have shown a warming trend between 1989 and 2005. A regression line fitted to the data indicates a change of 1.6°C over 17 years. These rivers are unlikely to be strongly influenced by human activity (see map of 2005 temperature medians in Section 3.2).
Rivers with naturally high pH (95th percentile) have shown a negative trend (ie, rivers have become slightly less alkaline). The reason for this trend is unclear at present.
Clarity values tended to show weak positive correlations with time with a significant improving trend for the 20th percentile.
There has been a steady increase in concentrations of NO_x-N in rivers where this important plant nutrient was already high (ie, 95th percentile). A regression line fitted to this data indicates a slope equivalent to 17.9 mg m^-3 yr^-1. This suggests that rivers identified in Section 3.7 as having high NO_x-N concentrations (eg, Mataura, Oreti, Waingongoro and Waihou) may have become more enriched over the 1989-2005 period, although trends analysis for individual sites over that period would be needed to confirm this. In contrast, rivers with very low levels of nitrate have shown a decreasing trend during 1989-2005 (ie, nitrate levels are declining). These 5th percentile rivers are less likely to be influenced by human activity and changes may be associated with climate variability.
Levels of ammoniacal nitrogen have shown strong downward trends in most of our rivers, but rivers with high NH₄-N concentrations (ie, 95th percentile) have not shown this decreasing trend. Results in Section 3.8 indicate that sites on the lower reaches of Waingongoro, Manawatu, Waihou and Tarawera rivers are those with the highest NH₄-N concentrations and formal trend analysis at these sites should confirm whether deterioration with respect to ammoniacal nitrogen levels is occurring.
All except the least enriched rivers show an increasing trend in concentrations of total nitrogen. This is somewhat surprising given the strong decreasing trends in NH₄-N and inconsistency in directions and strength of trends in NO_x-N (ie, increasing concentrations in 95th percentile and decreasing concentrations in 5th percentile rivers), although it is supported by results from formal trend analysis for the 1989-2003 period (Section 5.8) which shows a national-scale increasing trend in TN concentrations.
DRP shows an increasing trend for the 80th percentile but this is not very strong. TP showed no statistically significant trends. There is a strong non-linear pattern in DRP concentrations for 95th percentile values over time. A second order polynomial fitted to the data produces an R2 value of 0.63. This suggests that DRP concentrations in our most enriched rivers peaked in the late 1990s and has subsequently trended downwards so that concentrations in 2005 are similar to those observed in 1989.
There has been a consistent decreasing trend in BOD₅ in all rivers across the country. BOD₅ is now only analysed in samples from three sites (Rangitopuni, and lower Tarawera and Manawatu rivers).

Previous|Next