New Zealand Journal of Marine and Freshwater Research abstracts

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Use of the SHMAK clarity tube for measuring water clarity: comparison with the black disk method

Cathy Kilroy
Barry J. F. Biggs

National Institute of Water and Atmospheric
Research Limited
P.O. Box 8602, Riccarton
Christchurch, New Zealand
email: c.kilroy@niwa.cri.nz

Abstract   A metre-long transparent tube for measuring visual water clarity was developed as part of the New Zealand Stream Health Monitoring and Assessment Kit (SHMAK) for use by non-scientists. Water clarity measured using the tube (the horizontal sighting range of a black target) was compared with that measured using the standard “black disk” method (in which a black target is viewed horizontally underwater) to determine the feasibility of estimating black disk clarities from clarity tube readings. Readings using the two methods showed good agreement for waters with clarity c. 50 cm are lower than the black disk visibility and follow a logarithmic relationship with visibility, which enables useful readings to be made in clearer water. Independent testing of the relationship showed that black disk visibilities may be predicted with reasonable confidence in waters that contain very low concentrations of dissolved humic materials (yellow substance). In waters that are noticeably brown-coloured, the relationship is less consistent especially at black disk visibilities of more than 1.5 m. Recommendations are given for the use of the clarity tube.

Keywords   black disk; clarity tube; visual clarity; water quality

M01075 Received 13 August 2001; accepted 25 January 2002; published 17 September 2002
New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36: 519-527
0028-8330/02/3603-0519 $7.00 © The Royal Society of New Zealand 2002

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