Laboratory Experiments to Investigate Human Sensitivity to Changes in Water Clarity

Optical water quality legislation and standards often incorporate undefined, untested and subjective narrative statements for allowable change to natural waters as a consequence of, say, effluent discharges. We have examined the possible meaning of one such narrative statement, viz., the term “conspicuous change” which has been incorporated into New Zealandʹs Resource Management Act (1991) in the context of an allowable alteration to the clarity of natural waters. However, “conspicuousness” is a subjective impression that is generated within a human observer and is dependent not only upon the physical features of the water but also on the perceptual and visual responses to those features. By presenting laboratory-based observers with a series of water samples that varied in visual clarity, we have established the psychophysical function relating visual clarity differences to perceived differences. This function is logarithmic and the data can be fitted to a log-linear plot. The slope of this line appears to be dictated by the smallest clarity change that is noticed by observers. We argue that the form of this function will generally result in the need for large reductions in visual clarity before respondents will rate a change in water clarity as being “conspicuous”.