Remote estimation of regional lake clarity with Landsat TM and MODIS satellite imagery

Water clarity is an ideal metric of regional water quality because clarity can be accurately and efficiently estimated remotely on a landscape scale. Remote sensing of water quality is useful in regions containing numerous lakes that are too expensive to monitor regularly using traditional field methods. Field-assessed lakes generally are easily accessible and may represent a non-random, spatially unbalanced sample. Remote sensing provides a more complete spatial perspective of regional water quality than existing, interest-based sampling. We developed a remote monitoring procedure for Maine lakes using Landsat Thematic Mapper (TM) and Moderate-Resolution Imaging Spectroradiometer (MODIS) satellite imagery. Similar Landsat-based procedures have been implemented for Minnesota and Wisconsin lakes, however, we modified existing methods by incorporating physical lake variables and landscape characteristics that affect water clarity on a landscape scale. No published studies exist using MODIS data for remote lake monitoring owing to the coarse spatial resolution (500 m) (Landsat = 30 m). We applied these methods to estimate lake water clarity during 1990-2010 using Landsat imagery (1,511 lakes) and during 2001-2010 using MODIS imagery (83 lakes). Landsat is useful for long-term monitoring of lakes > 8 ha and MODIS is applicable to annual and within-year monitoring of large lakes (> 400 ha).

An important application of remote lake monitoring is detection of spatial and temporal patterns in regional water quality. We applied the Landsat-based methods to examine trends in Maine water clarity during 1995-2010. Remote change detection of water clarity should be based on August and early September (late summer) imagery, so our analysis was restricted to years in which cloud-free late summer imagery were available. We divided Maine intro three lake regions (northeastern, south-central and western) to examine spatial patterns in lake clarity. I found an overall decrease in average statewide lake water clarity of 4.94-4.38 m during 1995-2010. Water clarity ranged 4-6 m during 1995-2010, but consistently decreased during 2005-2010. Clarity in both the northeastern and western regions has experienced declines from 5.22 m in 1995 to 4.36 and 4.21 m respectively in 2010, whereas clarity in the south-central region remained unchanged since 1995 (4.50 m).