Water Research 36 (2002) 4330–4340 Application of Landsat imagery to regional-scale assessme of lake clarity Steven M. Kloiber a , Patrick L. Brezonik b, *, Marvin E. Bauer c a Metropolitan Council Environmental Services, 230 East Fifth Street, St. Paul, MN 55101, USA b Department of Civil Engineering, Water Resources Center, University of Minnesota, Minneapolis, 1985 Buford Avenue, St. P MN 55108, USA c Department of Forest Resources and Remote Sensing Laboratory, University of Minnesota, 1530 N Cleveland Avenue, St. P MN 55108, USA Received 1 June 2001; received in revised form 1 March 2002; accepted 26 March 2002 Abstract A procedure that uses Landsat imagery to estimate Secchi disk transparency (SDT) of lakes was developed and applied to B500 lakes with surface areas >10 ha in the seven-county metropolitan area of Minneapolis and St. MN, USA, to assess spatial patterns and temporal trends in lake clarity. Thirteen Landsat MSS and TM images o the period 1973–1998 were used for the analysis. Satellite brightness values from lake surfaces were calibrate available historical data on SDT ðn ¼ B20240Þ measured nearly contemporaneously with the acquisition date image. Calibration regression equations for the late-summer TM images had a range of r 2 from 0.72 to 0.93. Regression analysis for three late-summer MSS images yielded r 2 values ranging from 0.60 to 0.79. Results indicate that a single late-summer image yields a reliable estimate of regional lake clarity and reasonably accurate estimates of SDT for individual lakes. An analysis of seasonal patterns on a large lake water-quality database was used to develop a that adjustssynoptic satellite SDT estimates from differentdatesto a common reference, making them more comparable from year-to-year. Analysis of long-term trends shows that in spite of the large land-use changes w region over the study period, only 49 (about 10%) of assessed lakes in the region showed significant temporal SDT over the period, and more lakes had increasing SDT (34) than decreasing SDT (15). r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Landsat; Satellite; Secchi disk; Lake; Clarity; Trends 1. Introduction Broad-based, regional-scale monitoring is critical for making informed lakemanagement decisions.Data from monitoring programsare used frequentlyto estimate expected ranges in water quality for lakes in a region, examine regional differences, and investigate the relationships between landscape conditions and water quality. This information can be used to develop water- quality criteria and to guide management decisions. A properlydesigned monitoringprogram should have adequate representation across both space and time to provide the information needed for scientific or manage- mentpurposes. However,the design ofregional-scale monitoring programs frequently requires a compromise between spatial and temporal detail. Because of cost and logistical problems, ground-based monitoring programs usually sacrifice spatial coverage, monitoring fewer lakes in favor of more frequent sampling. Relatively few programs monitor many lakes across large area orstatesto conductregional-scale assess- ments.The GreatAmerican Secchi Dip-In [1], which *Corresponding author. Tel.: +1-612-624-9282; fax: +1-612-625-1263. E-mail address: brezo001@tc.umn.edu (P.L. Brezonik). 0043-1354/02/$ - see front matterr 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 1 4 6 - X