Frequency distributions of visual water clarity in 12 New Zealand rivers

Abstract

The visual clarity regime in 12 New Zealand rivers was investigated by measuring black disk visibility (sighting range) at 4‐weekly intervals for a year. In 11 of the rivers, black disk visibility (y_BD) was inversely related to flow (Q) and a simple power law function with a negative exponent: y_BD = αQ2^−β was an adequate model (average r = ‐0.84). The exponent β, which quantifies the shape of this expression, ranged from 0.12 to 1.38. When combined with flow‐frequency distributions, these power law relationships between clarity and flow provide a simple and cheap means to calculate frequency distributions for clarity which otherwise could only be obtained from a great many direct measurements. Clarity‐frequency distributions characterise the clarity regimes of rivers and indicate the proportion of time that sites are suitable for certain recreational uses. In this study, median clarity (estimated from clarity‐frequency distributions) ranged from 1.2 m in the relatively turbid Turakina River, which is seldom suitable for recreation, to 12.1 m in the very clear and visually attractive waters of the gorge site on the Motueka River.

Keywords:

• water clarity
• optical properties
• New Zealand rivers
• clarity‐frequency distributions