18 years of lake clarity, from space
The problem
Regional councils need to know whether lakes are getting clearer or murkier — but in-situ Secchi monitoring is patchy, and many lakes are sampled rarely or not at all. Trends drawn from sparse field data can be misleading.
The approach
Using the full Landsat archive (1999–2018), we derived empirical Secchi-depth relationships and applied them consistently across 23 lakes in the Rotorua district — producing a uniform, 18-year clarity record where field data alone could not.
- Atmospheric correction and consistent cross-sensor calibration
- Optical water-type screening so the right relationship is applied per lake
- Regional trend detection with uncertainty
The outcome
A defensible, district-wide picture of where clarity is improving, holding or declining — directly usable for state-and-trend reporting and prioritising management effort.
Published as Regional trends in remotely sensed water clarity over 18 years in the Rotorua Lakes, New Zealand — New Zealand Journal of Marine and Freshwater Research (2019).