17 Dec 2018
The Secchi depth and the Forel-Ule colour have been recorded since the middle of the 19th Century in many lakes and oceans around the world.
The Secchi depth and the Forel-Ule colour have been recorded since the middle of the 19th Century in many lakes and oceans around the world. Pietro Angelo Secchi (1818-1878) developed the first standardised method for determining water clarity. This involved lowering a white disk into the water and measuring the depth at which it disappears from sight. This has become known as the Secchi depth. François Alphonse Forel (1841-1912) proposed a standard set of colours for determining water colour in 1890. This classification was designed to categorise blue and green waters. Wilhelm Ule (1861-1940) extended the colour scale proposed by Forel to include blue-green to brown waters. The combined scale became known as the Forel-Ule scale and has since been used to determine the colour of seas, lakes and rivers. If you would like to learn more of this history we recommend reading the works of Marcel Wernand and colleagues, in particular, Wernand (2010, http://dx.doi.org/10.2971/jeos.2010.10013s) and Wernand & van der Woerd (2010, doi: http://dx.doi.org/10.2971/jeos.2010.10014s).