Water clarity, 1989–2013

Source:
NIWA; regional councils

Method:
"Water clarity is measured using a black disc the size of a soccer ball. The disc is placed in the water, and viewed through an underwater viewing box at increasing distances until the black disc disappears from sight. This provides a consistent measure of the greatest distance an object can be seen through the water (Davies–Colley, 1988).
Estimates of median clarity across New Zealand is based on water clarity measurements from 377 river sites monitored by the 16 regional councils and 77 sites along 35 major rivers measured by the National Institute of Water and Atmospheric Research (NIWA).
There is considerable natural variability in water clarity throughout New Zealand. ‘Trigger values’ help determine the point when further investigation is needed to see if there is an ecosystem health problem related to water quality. Lowland river measurement sites have been given a trigger value of 0.6 metres. Upland river measurement sites have been given a trigger value of 0.8 metres (ANZECC 2000).
National water clarity is inferred from the predominant land cover in a catchment and the surrounding land–scape characteristics, such as, climate, elevation, and geology.
The accuracy of the data source is of medium quality.



Reference:
Davies–Colley, RJ (1988). Measuring water clarity with a black disc. Limnology and Oceanography, 33(4), 616–623. Accessed 18 August 2015 from www.horizons.govt.nz. "