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.