FREQUENCY-DISTRIBUTIONS OF VISUAL WATER CLARITY IN 12 NEW-ZEALAND RIVERS

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) = alpha-Q-beta was an adequate model (average r = -0.84). The exponent beta, 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.