How is water clarity measured and what is its significance?

Water quality levels are indicated by how well light transmits through water.

Field technician taking a water quality sample with a secchi disk, Lake Rotomanuka.

There are two main measurements of water quality:

  • visual clarity: the depth through water that is visible to the human eye
  • light penetration: the amount of light available underwater for algae and aquatic growth.

Light is critical for plant growth, and is usually an indicator of the health of a water body. Several methods are used to measure visual clarity:

  • taking the sighting distance of a black disk placed underwater using a periscope-type viewer
  • using a secchi disk, which takes vertical measurements through the water. (Secchi depth is typically about 25% greater than black disc visibility.)
  • with a device that measures the strength of a collimated beam of light.

Read about efforts to manage water quality and availability on the Ministry for Primary Industry's website