New Zealand Journal of Marine and Freshwater Research abstracts
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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