Transparency trends

One of the primary uses for water clarity data (also known as transparency data) is to determine water quality trends over time. The following results are for Minnesota lakes and streams using data collected by volunteer water monitors using Secchi disks (lakes) and Secchi tubes (streams) through 2019.

Trend analysis

To determine Secchi clarity trends, all available lake and stream Secchi data were extracted from EQuIS, the Minnesota Pollution Control Agency’s water quality database. The majority of water clarity data available were collected by volunteers as part of MPCA’s community water monitoring programs.

Secchi transparency trends: Lakes

Based on 47 years of data collected (1972-2019): 2019 Lake trend results by county (wq-s2-06)

For lakes, the trend analysis was performed with a Seasonal Mann Kendall test. This statistical test detects changes in water clarity over time by comparing months across years (Mays are compared to Mays, Junes to Junes, etc.) Lake water clarity must change more than half a foot per decade to be considered a detectable change, or trend.

Secchi transparency trends: Streams

Based on 22 years of data (1998-2019):  2019 Stream trends by county (wq-csm1-12)

For streams, we used a Tobit regression model, which detects changes in water clarity over time by comparing months across years (Mays are compared to Mays, Junes to Junes, etc.). This test also accounts for the limit of the Secchi tube to detect water clarity of 100 centimeters or less. Stream water clarity must change at least 2 cm per decade to be considered a detectable change, or trend.