Water clarity of New Zealand rivers

Figure.NZ calculated the median, upper quartile, lower quartile, min and max clarity measures by land cover class. These calculations were undertaken to mirror the analysis undertaken for the Environmental Reporting Series (see http://stats.govt.nz/browse_for_stats/environment/environmental-reporting-series/environmental-indicators/Home/Fresh%20water/river-water-quality-clarity.aspx Figure 1)

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Water clarity is a measure of underwater visibility in rivers and streams. Fine particles like silt, mud, and organic material can reduce water clarity. Some water bodies naturally have low water clarity. Poor clarity affects the habitat and food supply of aquatic life, such as fish and aquatic birds, and the growth of aquatic plants. It can also impact on the aesthetic values and recreational use of rivers and streams.

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).
NIWA have measured monthly water clarity consistently at 77 sites along 35 major rivers between 1989 and 2013. These 35 rivers drain about 50 percent of New Zealand’s land area.

Sites are classified by dominant land cover in the upstream catchment. Higher values for water clarity are better than lower values.